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1. REGI STER PARAMETER DESCRI PTION OF CONTENT TYPE FORMAT NUMBER OF OF DATA 0 Modbus address Sensor address in Modbus R W U16 network default 1 1 Serial communication Sets the serial R W U16 settings communication default 5 2 3 Irradiance Temperature compensated R 32 temperature signal in x 0 01 W m compensated signal 4 5 rradiance Uncompensated signal R 32 uncompensated signal in x 0 01 W m 6 Sensor body In x 0 01 C R S16 temperature 7 Sensor electrical Inx 0 1 Q R U16 resistance 8 Scaling factor irradiance Default 100 R U16 9 Scaling factor Default 100 R U16 temperature 10 11 Sensor voltage output Inx 10 V R 32 12 to 31 Factory use only Register 0 Modbus address contains the Modbus address of the sensor This allows the Modbus master to detect the slave SR20 D2 in its network The address can be changed the value of the address must be between 1 and 247 The default Modbus address is 1 Note The sensor needs to be restarted before changes become effective Register 1 Serial communication settings is used to enter the settings for baud rate and the framing of the serial data transfer Default setting is setting number 5 19200 baud 8 data bits even parity and 1 stop bit Setting options are shown in the table below Note The sensor needs to be restarted before changes become effective SR20 D2 manual v1507 36 71 Hukseflux Thermal Sens
2. 5 5 to 40 VDC 12 VDC recommended Figure 5 5 2 Electrical diagram of the connection of SR20 D2 to a typical voltmeter or datalogger with the capacity to measure voltage signals Usually a 100 Q shunt resistor R is used to convert the current to a voltage SR20 D2 operates on a supply voltage of 5 to 30 VDC In addition 5 5 to 40 VDC is needed for the 4 20 mA function SR20 D2 manual v1507 25 71 Hukseflux Thermal Sensors 5 6 Connecting to an RS 485 network SR20 D2 is designed for a two wire half duplex RS 485 network In such a network SR20 D2 acts as a slave receiving data requests from the master An example of the topology of an RS 485 two wire network is shown in the figure below SR20 D2 is powered from 5 to 30 VDC The power supply is not shown in the figure The VDC power supply ground must be connected to the common line of the network SR20 D2 Slavel i Slaven Figure 5 6 1 Typical topology of a two wire RS 485 network figure adapted from Modbus over serial line specification and implementation guide V1 02 www modbus org The power supply is not shown in this figure After the last nodes in the network on both sides line termination resistors LT are required to eliminate reflections in the network According to the EIA TIA 485 standard these LT have a typical value of 120 to 150 Q Never place more than two LT on the network and never place the LT on a derivation cable To minimise noise on the net
3. Change settings function opens the Change serial communication settings window as shown in the figure below pe Change Serial Communication Settings Change settings SR20 D2 2602 Serial communication settings Modbus address 2 1 247 BAUD rate 19200 symb s Parity Even d Data amp stop bits a databits 1 stopbit WARNING without these settings the sensor cannot be connected The user is strongly advised to carefully note the new settings as Change settings Cancel Figure 6 1 6 1 Change serial communication settings window in the Sensor Manager SR20 D2 manual v1507 33 71 Hukseflux Thermal Sensors When new communication settings or a new Modbus address are entered these need to be confirmed by clicking Change settings The instrument will then automatically restart In case the Change settings function is not activated the original settings remain valid If the Modbus address is changed the Sensor Manager will automatically reconnect with the instrument using the new address after restart 6 1 7 Sensor Manager adjustment of the sensitivity by power users The Sensor Manager does not allow a standard user to change any settings that have a direct impact on the instrument output i e the irradiance in W m However in case the instrument is recalibrated it is common practice that the sensitivity is adjusted and that the latest result is adde
4. 1 for each SR20 D2 register number depending on processing by the network master this offset applies to coils as well Consult the manual of the device acting as the local master Table 9 11 3 Coils COILS COIL PARAMETER DESCRI PTI ON TYPE OF OBJ ECT TYPE 0 Restart Restart the sensor W Single bit 1 Reserved 2 Check Measure sensor W Single bit electrical resistance SR20 D2 manual v1507 68 71 Hukseflux Thermal Sensors 9 12 EC declaration of conformity We Hukseflux Thermal Sensors B V Delftechpark 31 2628 XJ Delft The Netherlands in accordance with the requirements of the following directive 2004 108 EC The Electromagnetic Compatibility Directive hereby declare under our sole responsibility that Product model SR20 D2 Type Pyranometer has been designed to comply and is in conformity with the relevant sections and applicable requirements of the following standards Emission EN 61326 1 2013 Immunity EN 61326 1 2013 Eric HOEKSEMA Director Delft June 07 2015 SR20 D2 manual v1507 69 71 2015 Hukseflux Thermal Sensors B V www hukseflux com Hukseflux Thermal Sensors B V reserves the right to change specifications without notice
5. 99 99 of the power of the terrestrial radiation has a wavelength longer than 3 000 x 10 m and about 99 per cent longer than 5 000 x 10 m For lower temperatures the spectrum shifts to longer wavelengths ref WMO World measurement standard representing the SI unit of irradiance with an uncertainty Radiometric of less than 0 3 see the WMO Guide to Meteorological Instruments and Reference Methods of Observation 1983 subclause 9 1 3 The reference was adopted by WRR the World Meteorological Organization WMO and has been in effect since 1 July 1980 ref ISO 9060 Albedo ratio of reflected and incoming solar radiation Dimensionless number that varies between 0 and 1 Typical albedo values are lt 0 1 for water from 0 1 for wet soils to 0 5 for dry sand from 0 1 to 0 4 for vegetation up to 0 9 for fresh snow Angle of angle of radiation relative to the sensor measured from normal incidence varies incidence from 0 to 90 Zenith angle angle of incidence of radiation relative to zenith Equals angle of incidence for horizontally mounted instruments Azimuth angle angle of incidence of radiation projected in the plane of the sensor surface Varies from 0 to 360 0 is by definition the cable exit direction also called north east is 90 ASTM G113 09 Sunshine duration sunshine duration during a given period is defined as the sum of that sub period for which the direct solar irradiance e
6. hemispherical solar radiation SR20 D2 offers irradiance in W m as a digital output and as a 4 20 mA output It must be used in combination with suitable power supply and a data acquisition system which uses the Modbus communication protocol over RS 485 or one that is capable of handling a 4 20 mA current loop signal The instrument is classified according to ISO 9060 and should be used in accordance with the recommended practices of ISO IEC WMO and ASTM Table 3 1 1 Specifications of SR20 D2 continued on next pages SR20 D2 MEASUREMENT SPECIFICATIONS LIST OF CLASSIFICATION CRITERIA OF ISO 9060 ISO classification ISO 9060 1990 secondary standard pyranometer WMO performance level WMO No 8 high quality pyranometer seventh edition 2008 Response time 95 35 Zero offset a response to 200 W m 5 W m unventilated net thermal radiation 2 5 W m ventilated Zero offset b response to 5 K h lt 2 W m change in ambient temperature Non stability lt 0 5 change per year Non linearity lt 0 2 100 to 1000 W m Directional response lt 10 W m Directional response test of individual report included instrument Spectral selectivity lt 3 0 35 to 1 5 x 10 m Temperature response lt 0 4 30 to 50 C Temperature response test of report included individual instrument Tilt response lt 0 2 0 to 90 at 1000 W m For the exact definition of
7. 01 C R S16 temperature 7 Sensor electrical Inx 0 1 Q R U16 resistance 8 Scaling factor irradiance Default 100 R U16 9 Scaling factor Default 100 R U16 temperature 10 11 Sensor voltage output Inx 10 V R 32 12 to 31 Factory use only 32 to 35 Sensor model Part one of sensor description R String 36 to 39 Sensor model Part two of sensor description R String SR20 D2 manual v1507 66 71 Hukseflux Thermal Sensors MODBUS REGISTERS 0 99 continued REGISTER PARAMETER DESCRIPTION OF CONTENT TYPE FORMAT NUMBER OF OF DATA 32 to 35 Sensor model Part one of sensor description R String 36 to 39 Sensor model Part two of sensor description R String 40 Sensor serial number R U16 41 42 Sensor sensitivity In x 10 V W m7 R Float 43 Response time Inx0O 1s R U16 44 Sensor resistance Inx 0 1 Q R U16 45 Reserved Always 0 R U16 46 47 Sensor calibration date Calibration date of the sensor R U32 in YYYYMMDD 48 to 60 Factory use 61 Firmware version R U16 62 Hardware version R U16 63 64 Sensor sensitivity In x 10 V W m R Float history 1 Default value is 0 65 66 Calibration date history 1 Former calibration date of the R U32 sensor in YYYYMMDD Default value is 0 67 68 Sensor sensitivity See register 63 64 R Float history 2 69 70 Calibration date history 2 See register 65 66 R U32 71 72 Sensor sen
8. address decimal equivalent 64 03 Modbus function 08 Number of bytes returned by the sensor 8 bytes transmitted by the sensor 00 40 Register 0 Modbus address SR20 D2 manual v1507 43 71 Hukseflux Thermal Sensors 00 05 Register 1 Serial settings 19200 baud 8 data bits even parity bit 1 stop bit 00 01 Register 2 Temperature compensated signal Most Significant Word MSW Decimal equivalent 1 7C 4F Register 3 Temperature compensated signal Least Significant Word LSW Decimal equivalent 31823 79 DA CRC the checksum of the transmitted data Together register 2 and 3 are representing the temperature compensated solar radiation output measured by the SR20 D2 The MSW is in register 2 and the LSW in 3 The output has to be calculated by the formula MSW x 216 LSW 100 In this example the result is 2 x 1 31823 100 973 59 W m Request for body temperature register 6 Master Request 40 03 00 06 00 01 6B 1A 40 Modbus Slave address 03 Modbus function 00 06 Start register 00 01 Number of registers 6B 1A CRC Sensor response 40 03 02 08 B1 43 FF 40 Modbus Slave address 03 Modbus function 02 Number of bytes 08 B1 Content of register 7 decimal equivalent 2225 43 FF CRC Temperature Register 7 x 0 01 2225 x 0 01 22 25 C Register 6 represents the sensors body temperature The rece
9. entire rated operating temperature range SR20 D2 offers two types of outputs digital output via Modbus RTU over 2 wire RS 485 and analogue 4 20 mA output current loop For communication between a PC and SR20 D2 the Hukseflux Sensor Manager software is included It allows the user to plot and export data and change the SR20 D2 Modbus address and its communication settings E Hukseflux Sensor Manager Sa x Hukseflux Thermal Sensors Scan for sensor Connected sensors pdate me ents Manually gt Now coma z 1 SR20 D2 2602 com bus address 2 135W m2 25 11 C Update ports 2 SR20 D2 2603 A1 Modbus address 2 23 W m 24 64 C 1 5 Find Fd Al SR20 D2 2604 M d j 4 3 19 W m 24 58 C Serial communication setting A wa Jn Lid tivechart window Sar Figure 0 2 User interface of the Sensor Manager SR20 D2 is designed for use in SCADA Supervisory Control And Data Acquisition systems supporting Modbus RTU Remote Terminal Unit protocol over RS 485 In these networks the sensor operates as a slave SCADA systems are often implemented in photovoltaic solar energy PV systems and meteorological networks Using SR20 D2 ina network is easy Once it has the correct Modbus address and communication settings and is connected to a power supply the instrument can be used in RS 485 networks A typical network will request the irradiance registers 2 3 and temperature data register 6 every 1 second and eve
10. fixation of sun screen thumb screw 3 inner dome 4 thermal sensor with black coating 5 outer dome 6 sun screen 7 humidity indicator 8 desiccant holder 9 levelling feet 10 bubble level 11 connector SR20 D2 manual v1507 11 71 Hukseflux Thermal Sensors SR20 D2 s scientific name is pyranometer A pyranometer measures the solar radiation received by a plane surface from a 180 field of view angle This quantity expressed in W m7 is called hemispherical solar radiation The solar radiation spectrum extends roughly from 285 to 3000 x 10 m By definition a pyranometer should cover that spectral range with a spectral selectivity that is as flat as possible In an irradiance measurement by definition the response to beam radiation varies with the cosine of the angle of incidence i e it should have full response when the solar radiation hits the sensor perpendicularly normal to the surface sun at zenith 0 angle of incidence zero response when the sun is at the horizon 90 angle of incidence 90 zenith angle and 50 of full response at 60 angle of incidence A pyranometer should have a so called directional response older documents mention cosine response that is as close as possible to the ideal cosine characteristic In order to attain the proper directional and spectral characteristics a pyranometer s main components are e athermal sensor with black coat
11. main supply lt 75x 10 Wat 12 VDC Communication protocol Modbus over 2 wire RS 485 half duplex Transmission mode RTU System requirements for use with PC Windows XP and later USB or RS 232 COM port and connector RS 485 USB converter or RS 485 RS 232 converter Software requirements for use with PC Java Runtime Environment software available free of charge at http www java com User interface on PC Hukseflux Sensor Manager software supplied with the instrument on a USB flash drive for available software updates please check http www hukseflux com page downloads 4 TO 20 mA 4 to 20 mA output irradiance in W m Transmitted range O to 1600 W m Output signal 4 to 20 x 107A Standard setting see options 4 x 107A at 0 W m and 20 x 10 A at 1600 W m SR20 D2 manual v1507 16 71 Hukseflux Thermal Sensors Table 3 1 1 Specifications of SR20 D2 started on previous pages Principle of 4 to 20 mA output 2 wire current loop note 2 additional wires are needed for the main supply of the sensor Rated operating voltage range of 4to 20 5 5 to 40 VDC mA output Power consumption of main supply lt 75 x 10 W at 12 VDC Power consumption of 4 to 20 mA lt 40 x 10 W at 12 VDC with recommended 100 Q current loop shunt resistor see chapter on using SR20 D2 s 4 20 mA output BACKWARDS COMPATIBILITY SR20 D2 and SR20 D1
12. master If the SR20 D2 is not already defined as a standard sensor type on the network contact the supplier of the network equipment to see if a library file for the SR20 D2 is available Typical operation requires the master to make a request of irradiance data in registers 2 3 sensor temperature in register 6 and the sensor serial number in register 40 every 1 second and store the 60 second averages The data format of register 2 3 isa signed 32 bit integer and the temperature in register 6 is a signed 16 bit integer Up to five 16 bit registers can be requested in one request In case six or more registers are requested in just one request SR20 D2 will not respond If requesting six or more registers multiple requests should be used SR20 D2 will respond as expected SR20 D2 manual v1507 42 71 Hukseflux Thermal Sensors 6 3 1 Adapting Modbus address and communication settings Setting the instrument address and baud rate can be done in different ways e by connecting the sensor to the PC and using the Sensor Manager e by connecting the sensor to the PC and using another Modbus testing tool There are links to different solutions available at www modbus org e by using the available network user interface software The Modbus address is stored in register 0 and has a default value of 1 A user may change the address to a value in the range of 1 to 247 The address value must be unique in the network The communication sett
13. of the thermopile After the measurement a new value will be written into register 7 Requesting to write this coil with a high repetition rate will result in irregular behaviour of the sensor the check must be executed as an exceptional diagnostics routine only SR20 D2 manual v1507 41 71 Hukseflux Thermal Sensors 6 3 Network communication getting started Once it has the correct Modbus address and communication settings SR20 D2 can be connected directly to an RS 485 network and a power supply How to physically connect a sensor as a Slave in a Modbus network is shown in the figure below In such a connection the sensor is powered via an external power supply of 5 to 30 VDC When the sensor is screwed onto a grounded mounting plate which is usually the case the shield is not connected to ground at the cable end pink _ 4 to 20 mA 5 5 to 40 VDC grey _ e 4 to 20 mA 5 5 to 40 VDC brown not connected yellow e not connected black shield red 5 to 30 VDC blue 5 to 30 VDC i common white data RS 485 B B green data RS 485 A A RS 485 network SR20 D2 wire RS 485 network Figure 6 3 1 Connecting SR20 D2 to a typical RS 485 network Installing a SR20 D2 in the network also requires configuring the communication for this new Modbus device This usually consists of defining a request that can be broadcast by the
14. offsets directional response in U voltage readout errors or in S tilt error temperature dependence calibration uncertainty 5 In uncertainty analysis for pyranometers the location and date of interest is entered The course of the sun is then calculated and the direct and diffuse components are estimated based on a model the angle of incidence of direct radiation is a major factor in the uncertainty 6 In uncertainty analysis for modern pyrheliometers tilt dependence often is so low that one single typical observation may be sufficient 7 In case of special measurement conditions typical specification values are chosen These should for instance account for the measurement conditions shaded unshaded ventilated unventilated horizontal tilted and environmental conditions clear sky cloudy working temperature range 8 Among the various sources of uncertainty some are correlated i e present during the entire measurement process and not cancelling or converging to zero when averaged over time the off diagonal elements of the covariance matrix are not zero Paragraph 5 2 of GUM 9 Among the various sources of uncertainty some are uncorrelated cancelling or converging to zero when averaged over time the off diagonal elements of the covariance matrix are zero Paragraph 5 1 of GUM 10 Among the various sources of uncertainty some are not included in analysis this applies for instance to non linear
15. pyranometer ISO 9060 specifications see the appendix SR20 D2 manual v1507 14 71 Hukseflux Thermal Sensors Table 3 1 1 Specifications of SR20 D2 continued SR20 D2 ADDITIONAL SPECI FI CATI ONS Measurand hemispherical solar radiation Measurand in SI radiometry units irradiance in W m Optional measurand sunshine duration Field of view angle 180 Output definition running average over 4 measurements refreshed every 0 15 Recommended data request interval 1 s storing 60 s averages Measurement range 400 to 4000 W m Zero offset steady state lt 0 5 W m at 20 C lt 0 8 W m 40 to 80 C Zero offset dynamic during power up lt 10 W m nominal Measurement function optional programming for sunshine duration programming according to WMO guide paragraph 8 2 2 Internal temperature sensor Analog Devices ADT7310 digital SPI temperature sensor Rated operating temperature range 40 to 80 C Spectral range 20 transmission points 285 to 3000 x 10 m Standard governing use of the instrument ISO TR 9901 1990 Solar energy Field pyranometers Recommended practice for use ASTM G183 05 Standard Practice for Field Use of Pyranometers Pyrheliometers and UV Radiometers Standard cable length see options 5m Cable diameter 5 3 x 107m Chassis connector M16 panel connector male thread 10 po
16. radiation measurement General use for sunshine duration measurement Specific use for outdoor PV system performance testing Specific use in meteorology and climatology Installation of SR20 D2 Site selection and installation Installation of the sun screen Electrical connection of SR20 D2 wiring diagram Grounding and use of the shield Using SR20 D2 s 4 to 20 mA output Connecting to an RS 485 network Connecting to a PC Communication with SR20 D2 PC communication Sensor Manager software Network communication function codes registers coils Network communication getting started Network communication example master request to SR20 D2 Making a dependable measurement The concept of dependability Reliability of the measurement Speed of repair and maintenance Uncertainty evaluation Maintenance and trouble shooting Recommended maintenance and quality assurance Trouble shooting Calibration and checks in the field Data quality assurance Appendices Appendix on cable extension replacement Appendix on tools for SR20 D2 Appendix on spare parts for SR20 D2 Appendix on standards for classification and calibration Appendix on calibration hierarchy Appendix on meteorological radiation quantities Appendix on ISO and WMO classification tables SR20 D2 manual v1507 3 71 Hukseflux Thermal Sensors Appendix on definition of pyranometer specifications Appendix on terminology glossary Appendix on floating point format conve
17. the cable The outer In case there is a minor layer of moisture that is hardly visible replace the dome shows desiccant and wait a few days to see if the situation improves internal In case of condensation of droplets disassemble the instrument and dry out the condensation parts The inner dome shows internal condensation Arrange to send the sensor back to Hukseflux for diagnosis 3 3 Calibration and checks in the field Recalibration of field pyranometers is typically done by comparison in the field to a reference pyranometer The applicable standard is ISO 9847 International Standard Solar Energy calibration of field pyranometers by comparison to a reference pyranometer At Hukseflux an indoor calibration according to the same standard is used Hukseflux recommendation for re calibration if possible perform calibration indoor by comparison to an identical reference instrument under normal incidence conditions The recommended calibration interval of pyranometers is 2 years The registers containing the applied sensitivity and the calibration history of SR20 D2 are accessible for users This allows the user to choose his own local calibration service The same feature may be used for remotely controlled re calibration of pyranometers in the field Ask Hukseflux for information on ISO and ASTM standardised procedures for field calibration Request power user status and a password at the factory permittin
18. the certificates in a safe place SR20 D2 manual v1507 9 71 Hukseflux Thermal Sensors 1 3 Quick instrument check A quick test of the instrument can be done by connecting it to a PC and installing the Sensor Manager software See the chapters on installation and PC communication for directions 1 At power up the signal may have a temporary output level different from zero an offset Let this offset settle down 2 Check if the sensor reacts to light expose the sensor to a strong light source for instance a 100 W light bulb at 0 1 m distance The signal should read gt 100 W m now Darken the sensor either by putting something over it or switching off the light The instrument irradiance output should go down and within one minute approach 0 W m 3 Remove the sun screen see chapter on installation of the sun screen Inspect the bubble level 4 Inspect the instrument for any damage 5 Inspect if the humidity indicator is blue Blue indicates dryness The colour pink indicates it is humid in the latter case replace the desiccant see chapter on maintenance 6 Check the instrument serial number as indicated by the software against the label on the instrument and against the certificates provided with the instrument SR20 D2 manual v1507 10 71 f Hukseflux Thermal Sensors 2 Instrument principle and theory Figure 2 1 Overview of SR20 D2 1 cable standard length 5 metres optional longer cable 2
19. 0 winter mid latitude 11 4 8 1 9 9 7 4 2 Calibration uncertainty New calibration procedures were developed in close cooperation with PMOD World Radiation Center in Davos Switzerland The latest calibration method results in an uncertainty of the sensitivity of less than 1 2 compared to typical uncertainties of higher than 1 7 for this pyranometer class See the appendix for detailed information on calibration hierarchy SR20 D2 manual v1507 50 71 Hukseflux Thermal Sensors 8 Maintenance and trouble shooting 8 1 Recommended maintenance and quality assurance SR20 D2 can measure reliably at a low level of maintenance in most locations Usually unreliable measurements will be detected as unreasonably large or small measured values As a general rule this means that regular visual inspection combined with a critical review of the measured data preferably checking against other measurements is the preferred way to obtain a reliable measurement Table 8 1 1 Recommended maintenance of SR20 D2 If possible the data analysis and cleaning 1 and 2 should be done on a daily basis continued on next page MINIMUM RECOMMENDED PYRANOMETER MAINTENANCE INTERVAL SUBJ ECT ACTION 1 1 week data analysis compare measured data to maximum possible maximum expected irradiance and to other measurements nearby redundant instruments Also historical seasonal records can be used as a source for expected values Analys
20. 0 D2 irradiance plot in the Sensor Manager 6 1 5 Sensor Manager information about the instrument The main window shows the Show details button giving access to the Sensor details window This window displays calibration results and calibration history temperature coefficients and other properties of the selected instrument as shown on the next page The sensor serial number and all calibration information should match the information on the instrument label and on the product certificate SR20 D2 manual v1507 32 71 Hukseflux Thermal Sensors g paii j Sensor details Sensor details SR20 D2 2602 Calibration information Sensor properties Sensitivity 7 99 pV W m 2 Serial number 2602 Calibration date 12 05 2015 Sensor type SR20 D2 Firmware version 1 012 Show calibration history Hardware version 1 01 Temperature characterisation Live measurement 2015 07 13 10 11 56 Coefficient a 1 5006E 5 Coefficient b 6 0237E 4 Modbus address 2 BAUD rate 19200 Data amp stop bits 8 data 1 stop Coefficient c 9 9400E 1 0 7 1 Irradiance W m Serial communication settings EER 2 5 19 e Parity Even Auto update measurement 1sec Change settings Export sensor details Close window Figure 6 1 5 1 Sensor details window in the Sensor Manager 6 1 6 Sensor Manager changing Modbus address and communication settings In the Sensor details window the
21. 0 to 1 500 x 10 m 3 5 10 WMO 300 to 3 000 x 10 m Temperature response interval of 50 K 2 4 8 Tilt response 0 5 2 5 0 to 90 at 1000 W m ADDITI ONAL WMO SPECI FI CATI ONS WMO CLASS HIGH QUALITY GOOD QUALITY MODERATE QUALITY WMO achievable accuracy for daily sums 2 5 10 WMO achievable accuracy for hourly sums 3 8 20 WMO achievable accuracy for minute sums not specified not specified not specified WMO resolution 1 W m 5 W m 10 W m smallest detectable change CONFORMITY TESTI NG ISO 9060 individual group group instrument only compliance compliance all specs must comply WMO 7 2 1 The estimated uncertainties are based on the following assumptions a instruments are well maintained correctly aligned and clean b 1 min and 1 h figures are for clear sky irradiances at solar noon c daily exposure values are for clear days at mid latitudes WMO 7 3 2 5 Table 7 5 lists the expected maximum deviation from the true value excluding calibration errors At Hukseflux the expression 1 is used instead of a range of 2 an instrument is subject to conformity testing of its specifications Depending on the classification conformity compliance can be proven either by group or individual compliance A specification is fulfilled if the mean value of the respective test result does not exceed the corresponding limiting value of the specification for the specific ca
22. 0x03 Read Holding Registers 0x04 Read Input Register 0x05 Write Single Coil 0x06 Write Single Holding Register OxOF Write Multiple Coils 0x10 Write Multiple Registers SR20 D2 manual v1507 34 71 Hukseflux Thermal Sensors Table 6 2 2 Modbus data model MODBUS DATA MODEL PRIMARY TABLES OBJ ECT TYPE TYPE OF Discrete input Single bit R Coil Single bit R W Input register 16 bit word R Holding register 16 bit word R W R read only W write only R W read write The instrument does not distinguish between discrete input and coil neither between input register and holding register Table 6 2 3 Format of data FORMAT OF DATA DESCRI PTI ON U16 Unsigned 16 bit integer S16 Signed 16 bit integer U32 Unsigned 32 bit integer 32 Signed 32 bit integer Float IEEE 754 32 bit floating point format String A string of ASCII characters The data format includes signed and unsigned integers The difference between these types is that a signed integer passes on negative values which reduces the range of the integer by half Up to five 16 bit registers can be requested in one request if requesting six or more registers multiple requests should be used If the format of data is a signed or an unsigned 32 bit integer the first register received is the most significant word MSW and the second register is the least significant word LSW This way two 16 bit registers are
23. 65 66 R U32 Register 63 to 82 Only accessible for writing by Sensor Manager power users power users can write calibration history to registers 63 to 82 If default values are returned no re calibration has been written Last calibration sensitivity and calibration date are available in register 41 42 and 46 47 respectively Table 6 2 8 Modbus registers 83 to 85 directional response characterisation data MODBUS REGISTERS 83 85 REGISTER PARAMETER DESCRIPTION OF CONTENT TYPE FORMAT NUMBER OF OF DATA 83 84 Directional response Directional response R U32 measurement date measurement date in YYYYMMDD 85 Directional response R U16 measurement employee Register 83 to 85 these registers are for reference purposes SR20 D2 manual v1507 39 71 Hukseflux Thermal Sensors Table 6 2 9 Modbus registers 86 to 95 temperature response characterisation data MODBUS REGISTERS 86 95 REGISTER PARAMETER DESCRIPTION OF CONTENT TYPE FORMAT NUMBER OF OF DATA 86 Temperature response In x 0 01 R S16 87 88 Polynomial temperature R Float coefficient a 89 90 Polynomial temperature R Float coefficient b 91 92 Polynomial temperature R Float coefficient c 93 94 Temperature response Temperature response R U32 characterisation characterisation measurement date measurement date of the sensor in YYYYMMDD 95 Temperature response R U16 characterisation measurement e
24. SO and ASTM have standards on instrument classification and methods of calibration The World Meteorological Organisation WMO has largely adopted the ISO classification system Table 9 4 1 Pyranometer standardisation in ISO and ASTM STANDARDS ON INSTRUMENT CLASSIFICATION AND CALI BRATION ISO STANDARD EQUIVALENT ASTM STANDARD ISO 9060 1990 Solar energy Specification and classification of instruments for measuring hemispherical solar and direct solar radiation not available Comment work is in progress on a new ASTM equivalent standard Comment a standard Solar energy Methods for testing pyranometer and pyrheliometer characteristics has been announced in ISO 9060 but is not yet implemented not available ISO 9846 1993 Solar energy Calibration of a pyranometer using a pyrheliometer ASTM G167 05 Standard Test Method for Calibration of a Pyranometer Using a Pyrheliometer ISO 9847 1992 Solar energy Calibration of field pyranometers by comparison to a reference pyranometer ASTM E 824 10 Standard Test Method for Transfer of Calibration from Reference to Field Radiometers ASTM G207 11 Standard Test Method for Indoor Transfer of Calibration from Reference to Field Pyranometers ISO 9059 1990 Solar energy Calibration of field pyrheliometers by comparison to a reference pyrheliometer ASTM E 816 Standard Test Method for Calibration of Pyrheliometers by Comparison to Referen
25. SR20 D2 is the successor of both model SR20 D1 and model SR20 TR SR20 D2 is completely backwards compatible with SR20 D1 SR20 D1 users can use SR20 D2 without the need to change settings or wiring VERSIONS OPTIONS Adapted transmitted range 4 to 20 mA can be adjusted at the factory upon request Longer cable in multiples of 5 m option code total cable length ACCESSORIES Ventilation unit VU01 Bags of silica gel for desiccant set of 5 bags in an air tight bag option code DCO1 SR20 D2 manual v1507 17 71 Thermal Sensors f Hukseflux 3 2 Dimensions of SR20 D2 M5 2x H M6 85 Figure 3 2 1 Dimensions of SR20 D2 in x 10 m SR20 D2 manual v1507 18 71 Hukseflux Thermal Sensors 4 Standards and recommended practices for use Pyranometers are classified according to the ISO 9060 standard and the WMO No 8 Guide In any application the instrument should be used in accordance with the recommended practices of ISO IEC WMO and or ASTM 4 1 Classification standard Table 4 1 1 Standards for pyranometer classification See the appendix for definitions of pyranometer specifications and a table listing the specification limits STANDARDS FOR INSTRUMENT CLASSIFICATION ISO STANDARD EQUIVALENT WMO ASTM STANDARD ISO 9060 1990 Not available WMO No 8 Guide to Solar energy specification and Meteorological Instruments classification of instr
26. Thermal Sensors Hukseflux USER MANUAL SR20 D2 Digital secondary standard pyranometer with Modbus RTU and 4 20 mA output Copyright by Hukseflux manual v1507 www hukseflux com info hukseflux com Thermal Sensors f Hukseflux Warning statements A gt Pe amp Putting more than 30 Volt across the sensor wiring of the main power supply can lead to permanent damage to the sensor Putting more than 40 Volt across the sensor wiring of the current loop 4 to 20 mA can lead to permanent damage to the sensor For proper instrument grounding use SR20 D2 with its original factory made SR20 D2 cable Using the same Modbus address for more than one device will lead to irregular behaviour of the entire network Your data request may need an offset of 1 for each SR20 D2 register number depending on processing by the network master Consult the manual of the device acting as the local master SR20 D2 manual v1507 2 71 Hukseflux Thermal Sensors Contents Warning statements Contents List of symbols Introduction 1 OBWNH NR W Ne NOUBWNEH ea ee a eee oe ee E OTO V U ee ee ee E BWNP BWNHP BWNE NOUBWNEH Ordering and checking at delivery Ordering SR20 D2 Included items Quick instrument check Instrument principle and theory Specifications of SR20 D2 Specifications of SR20 D2 Dimensions of SR20 D2 Standards and recommended practices for use Classification standard General use for solar
27. UN system s authoritative voice on the state and behaviour of the earth s atmosphere and climate WMO publishes WMO No 8 Guide to Meteorological Instruments and Methods of Observation in which a table is included on level of performance of pyranometers Nowadays WMO conforms itself to the ISO classification system SR20 D2 manual v1507 20 71 Hukseflux Thermal Sensors 5 Installation of SR20 D2 5 1 Site selection and installation Table 5 1 1 Recommendations for installation of pyranometers Location the situation that shadows are cast on the instruments is usually not desirable The horizon should be as free from obstacles as possible Ideally there should be no objects between the course of the sun and the instrument Mechanical mounting thermal insulation preferably use connection by bolts to the bottom plate of the instrument A pyranometer is sensitive to thermal shocks Do not mount the instrument with the body in direct thermal contact to the mounting plate so always use the levelling feet also if the mounting is not horizontal do not mount the instrument on objects that become very hot black coated metal plates Instrument mounting with 2 bolts 2 x M5 bolt at 65 x 10 m centre to centre distance on north south axis connection from below under the bottom plate of the instrument Instrument mounting with one bolt 1 x M6 bolt at the centre of the instrument connection from below unde
28. ability can be improved by maintenance support Important aspects are e dome fouling by deposition of dust dew rain or snow Fouling results in undefined measurement uncertainty sensitivity and directional error are no longer defined This should be solved by regular inspection and cleaning e sensor instability Maximum expected sensor aging is specified per instrument as its non stability in change year In case the sensor is not recalibrated the uncertainty of the sensitivity gradually will increase This is solved by regular recalibration e moisture condensing under pyranometer domes resulting in a slow change of sensitivity within specifications This is solved by regular replacement of desiccant or by maintenance drying the entire sensor in case the sensor allows this For non serviceable sensors like most second class pyranometers this may slowly develop into a defect For first class and secondary standard models for instance model SR11 first class pyranometer and SR20 D2 digital secondary standard pyranometer extra desiccant in a set of 5 bags in an air tight bag is available SR20 D2 manual v1507 47 71 Hukseflux Thermal Sensors Another way to improve measurement reliability is to introduce redundant sensors e the use of redundant instruments allows remote checks of one instrument using the other as a reference which leads to a higher measurement reliability e in PV system performance monitoring in addit
29. again SR20 D2 manual v1507 29 71 Hukseflux Thermal Sensors 6 1 3 Sensor Manager main window r Hukseflux Sensor Manager Thermal Sensors Scan for sensor Connected sensors update measurements Manually a Getting started Update ports lt Select a serial COM port Modbus address range a 7 bbls address rang lt Simply hit Find or Find all 1 through 5 Not finding the sensor s you are Ic Try selecting a di nt seria Try extending the ID range for scanning Check the Serial connection settings with the sensor manual Serial communication settings BAUD rate 19200 X Parity Even X Data amp stop bits 8 databits 1 stopbit Load defaults Manually connect Modbus address s with selected Show details Disconnect with checked Plot on Live Chart Figure 6 1 3 1 Main window of the Sensor Manager When the Sensor Manager is started and a SR20 D2 is connected to the PC the user can communicate with the instrument If the instrument address and communication settings are known the serial connection settings and the Modbus address can be entered directly Clicking Connect will establish contact If the instrument address and communication settings are not known the instrument is found by using the Find or Find All function The Sensor Manager scans the specified range of Modbus addresses however only using the Serial connecti
30. anometer selection guide WMO has approved the pyranometric method to calculate sunshine duration from pyranometer measurements in WMO No 8 Guide to Meteorological Instruments and Methods of Observation This implies that SR20 D2 may be used in combination with appropriate software to estimate sunshine duration This is much more cost effective than using a dedicated sunshine duration sensor Ask for our application note SR20 D2 manual v1507 8 71 Hukseflux Thermal Sensors 1 Ordering and checking at delivery 1 1 Ordering SR20 D2 The standard configuration of SR20 D2 is with 5 metres cable Common options are e Longer cable in multiples of 5 m Specify total cable length e Five silica gel bags in an air tight bag for SR20 D2 desiccant holder Specify order number DCO1 e Adapted transmitted range for 4 20 mA output Standard setting is 4 mA at 0 W m and 20 mA at 1600 W m Specify preferred range setting e VUO1 ventilation unit 2 1 2 Included items Arriving at the customer the delivery should include e pyranometer SR20 D2 e sun screen e cable of the length as ordered e calibration certificate matching the instrument serial number e product certificate matching the instrument serial number including temperature response test report and directional response test report for the individual instrument e Hukseflux Sensor Manager software on a USB flash drive e any other options as ordered Please store
31. asurements using a PC The Sensor Manager s most common use is for initial functionality testing and modification of the SR20 D2 Modbus address and communication settings It is not intended for long term continuous measurement purposes The Sensor Manager software is supplied with the instrument on a USB flash drive For available software updates of the Sensor Manager please check www hukseflux com page downloads 6 1 1 Installing the Sensor Manager Running the Sensor Manager requires installation of the latest version of Java Runtime Environment software Java Runtime Environment may be obtained free of charge from www java com The SR20 D2 specifications overview Table 3 1 1 shows the system and software requirements for using a PC to communicate with SR20 D2 The Sensor Manager is supplied on a USB flash drive with the instrument 1 Insert the USB flash drive and copy the folder Hukseflux Sensor Manager to a folder on a PC For proper installation the user should have administrator rights for the PC 2 Double click Hukseflux_Sensor_Manager jar in the folder Hukseflux Sensor Manager This will start up the Sensor Manager 6 1 2 Trouble shooting during Sensor Manager installation e When Java Runtime Environment software is not installed a Windows message comes up displaying the file Hukseflux_Sensor_Manager jar could not be opened The solution is to install Java Runtime Environment on the PC and try
32. be connected directly to commonly used datalogging systems The irradiance E in W m is calculated by measuring the SR20 D2 output a small current subtracting 4 x 107A from it and then multiplying by the transmitted range r The transmitted range is provided with SR20 D2 on its product certificate By convention 0 W m irradiance corresponds with 4 x 10 A transmitter output current The transmitted range which is the irradiance at output current of 20 x 10 A and is typically 1600 W m7 The transmitted range can be adjusted at the factory upon request The central equation governing SR20 D2 is E r l 4x 10 16 x 10 Formula 5 5 1 SR20 D2 s low temperature dependence makes it an ideal candidate for use under very cold and very hot conditions The temperature dependence of every individual instrument is tested and supplied as a second degree polynomial The irradiance output provided by SR20 D2 is temperature corrected All temperature corrections are applied internally by the instrument The temperature coefficients a b and c can be found on the product certificate of each instrument Table 5 5 1 Requirements for data acquisition and amplification equipment Capability to SR20 D2 has a 4 20 mA output There are several measure 4 20 mA or possibilities to handle this signal It is important to realise measure currents or that the signal wires not only act to transmit the signal but measure voltages also act as p
33. ce Pyrheliometers SR20 D2 manual v1507 59 71 Hukseflux Thermal Sensors 9 5 Appendix on calibration hierarchy The World Radiometric Reference WRR is the measurement standard representing the SI unit of irradiance Use of WRR is mandatory when working according to the standards of both WMO and ISO 1S09874 states under paragraph 1 3 the methods of calibration specified are traceable to the WRR The WMO manual states under paragraph 7 1 2 2 the WRR is accepted as representing the physical units of total irradiance The worldwide homogeneity of the meteorological radiation measurements is guaranteed by the World Radiation Center in Davos Switzerland by maintaining the World Standard Group WSG which materialises the World Radiometric Reference See www pmodwrc ch The Hukseflux standard is traceable to an outdoor WRR calibration Some small corrections are made to transfer this calibration to the Hukseflux standard conditions sun at zenith and 1000 W m irradiance level During the outdoor calibration the sun is typically at 20 to 40 zenith angle and the total irradiance at a 700 W m level Table 9 5 1 Calibration hierarchy for pyranometers WORKING STANDARD CALIBRATION AT PMOD WRC DAVOS Calibration of working standard pyranometers Method ISO 9846 type 1 outdoor This working standard has an uncertainty uncertainty of standard The working standard has been calibrated under certain test conditions of
34. centage change in sensitivity per year The dependence of ISO change per sensitivity resulting from ageing effects which is a measure of the 9060 year long term stability 1990 Non linearity percentage deviation from the sensitivity at 500 W m due to the ISO 100 to 1000 change in irradiance within the range of 100 W m to 1000 W m 9060 W m Non linearity has an overlap with directional response and 1990 therefore should be handled with care in uncertainty evaluation Directional the range of errors caused by assuming that the normal incidence ISO response sensitivity is valid for all directions when measuring from any 9060 direction a beam radiation whose normal incidence irradiance is 1990 1000 W m Directional response is a measure of the deviations from the ideal cosine behaviour and its azimuthal variation Spectral percentage deviation of the product of spectral absorptance and ISO selectivity 350 spectral transmittance from the corresponding mean within 350 x 9060 to 1500 x10 m 10 m to 1500 x 10 m and the spectral distribution of irradiance 1990 WMO 300 to Spectral selectivity is a measure of the spectral selectivity of the 3000 x 10 m sensitivity Temperature percentage deviation of the sensitivity due to change in ambient ISO response temperature within an interval of 50 K the temperature of the 9060 interval of 50 K pyranometer body 1990 Tilt response percentage deviation from the sensitivity at 0 tilt hor
35. connection of the shield typically not connected at the network side Inspect the connection of the sensor power supply typically the negative is connected to the network common Prepare for indoor testing Install the Sensor Manager software on a PC Equip the PC with RS 485 communication Put DC voltage power to the sensor and establish communication with the sensor At power up the signal may have a temporary output level different from zero an offset Let this offset settle down The sensor Check if the sensor reacts to light expose the sensor to a strong light source for does not give instance a 100 W light bulb at 0 1 m distance The signal should read gt 100 W m any signal now Darken the sensor either by putting something over it or switching off the light The instrument voltage output should go down and within one minute approach 0 W m Check the data acquisition by replacing the sensor with a spare sensor with the same address Not able to Check all physical connections to the sensor and try connecting to the sensor communicate again If communicating is not possible try to figure out if the address and with the communication settings are correct Analyse the cable performance by measuring sensor resistance from pins to cable ends The electrical resistance should be lt 10 Q In case of doubt try a new cable Connect sensor to a PC and perform the Find and Find all operation with the Sensor Manag
36. d to the calibration history records This can be done after obtaining a password and becoming a power user Please contact the factory to obtain the password and to get directions to become a power user Example During a calibration experiment the result might be that SR20 D2 has an irradiance output in W m that is 990 whereas the standard indicates it should be 970 The SR20 D2 output is in this example 2 06 too high The original sensitivity of 16 15 x 10 V W m ought to be changed to 16 48 using registers 41 42 The old calibration result is recorded in the calibration history file In case there are still older results these are moved over to higher register numbers 63 to 81 6 2 Network communication function codes registers coils Warning Using the same Modbus address for more than one device will lead to irregular behaviour of the entire network This chapter describes function codes data model and registers used in the SR20 D2 firmware Communication is organised according to the specifications provided by the Modbus Organization These specifications are explained in the documents Modbus application protocol v1 1b and Modbus over serial line v1 02 These documents can be acquired free of charge at www modbus org Table 6 2 1 Supported Modbus function codes SUPPORTED MODBUS FUNCTION CODES FUNCTION CODE HEX DESCRIPTION 0x01 Read Coils 0x02 Read Discrete Inputs
37. e night time signals These signals may be negative down to 5 W m on clear windless nights due to zero offset a In case of use with PV systems compare daytime measurements to PV system output Look for any patterns and events that deviate from what is normal or expected 2 2 weeks cleaning use a soft cloth to clean the dome of the instrument persistent stains can be treated with soapy water or alcohol 3 6 months inspection inspect cable quality inspect connectors inspect mounting position inspect cable clean instrument clean cable inspect levelling change instrument tilt in case this is out of specification inspect mounting connection inspect interior of dome for condensation 4 desiccant desiccant replacement if applicable Change in case the blue replacement colour of the 40 humidity indicator turns pink indicating humidity then replace desiccant Coat the rubber of the cartridge with silicone grease or vaseline Desiccant regeneration heating in an oven at 70 C for 1 to 2 hours Humidity indicator regeneration heating until blue at 70 C 5 2 years recalibration recalibration by side by side comparison to a higher standard instrument in the field according to ISO 9847 request power user status and a password at the factory permitting to write to registers holding the sensitivity and the calibration history data via the Sensor Manager 6 lifetime judge if the instrument should be reliable for a
38. ector end of the cable the shield is connected to the connector housing and also to pin 9 5 4 Grounding and use of the shield Grounding and shield use are the responsibility of the user The cable shield called shield in the wiring diagram is connected to the aluminium instrument body via the connector In most situations the instrument will be screwed on a mounting platform that is locally grounded In these cases the shield at the cable end should not be connected at all When a ground connection is not obtained through the instrument body for instance in laboratory experiments the shield should be connected to the local ground at the cable end This is typically the ground or low voltage of the power supply or the common of the network In exceptional cases for instance when both the instrument and a datalogger are connected to a small size mast the local ground at the mounting platform is the same as the network ground In such cases ground connection may be made both to the instrument body and to the shield at the cable end SR20 D2 manual v1507 23 71 Hukseflux Thermal Sensors 5 5 Using SR20 D2 s 4 to 20 mA output SR20 D2 gives users the option to use 4 to 20 mA output instead of its digital output When using 4 to 20 mA output please read this chapter first When opting solely for SR20 D2 s digital output please continue with the next chapter Using the 4 to 20 mA output provided by SR20 D2 is easy The instrument can
39. er to locate the sensor and verify the communication settings If all physical connections are correct and the sensor still cannot be found please contact the factory to send the sensor to the manufacturer for diagnosis and service SR20 D2 manual v1507 52 71 Hukseflux Thermal Sensors SR20 D2 does not respond to a request for 6 or more It is not possible to request more than five 16 bit registers in one request In case of requesting six or more registers in just one request the sensor will not respond If requesting six or more registers use multiple requests the sensor will respond as expected registers The sensor Note that night time signals may be negative down to 5 W m on clear windless signal is nights due to zero offset a unrealistically Check if the pyranometer has clean domes high or low Check the location of the pyranometer are there any obstructions that could explain the measurement result Check the orientation levelling of the pyranometer Check the cable condition looking for cable breaks Check the condition of the connectors on chassis as well as the cable The sensor Check the presence of strong sources of electromagnetic radiation radar radio signal shows Check the condition and connection of the shield unexpected Check the condition of the sensor cable variations Check if the cable is not moving during the measurement Check the condition of the connectors on chassis as well as
40. ermal Sensors Table 9 1 1 Preferred specifications for SR20 D2 cable replacement and extension General replacement please order a new cable with connector at Hukseflux or choose for a DIY approach In case of DIY replacement by the user see connector specifications below and ask for the DIY connector assembly guide General cable extension please order a new cable with connector at Hukseflux or solder the new cable conductors and shield to the original sensor cable and make a connection using adhesive lined heat shrink tubing with specifications for outdoor use Always connect shield Connectors used chassis M16 panel connector male thread 10 pole HUMMEL AG 7 840 200 000 panel connector front mounting short version cable M16 straight connector female thread 10 pole HUMMEL AG 7 810 300 00M straight connector female thread for cable 3 to 6 x 103 m special version The shield is electrically connected to the connector Cable 8 wire shielded with copper conductors at Hukseflux 8 wire shielded cable is used of which 2 wires are used for signal transmission and 2 for power supply Conductor resistance lt 0 1 Q m Length Cables should be kept as short as possible In daisy chain topologies cable length to main data line should be less than 12 metres In point to point topologies cable length should not exceed RS 485 specifications of maximum 1200 metres Outer sheath with specificati
41. g to write to registers holding the sensitivity and the calibration history data via the Sensor Manager In case of field comparison ISO recommends field calibration to a higher class pyranometer Hukseflux suggests also allowing use of sensors of the same model and SR20 D2 manual v1507 53 71 Hukseflux Thermal Sensors class because intercomparisons of similar instruments have the advantage that they suffer from the same offsets It is therefore just as good to compare to pyranometers of the same brand and type as to compare to an instrument of a higher class ISO recommends to perform field calibration during several days 2 to 3 days under cloudless conditions 10 days under cloudy conditions In general this is not achievable In order to shorten the calibration process Hukseflux suggests to allow calibration at normal incidence using hourly totals near solar noon Hukseflux main recommendations for field intercomparisons are 1 to take normal incidence as a reference and not the entire day 2 to take a reference of the same brand and type as the field pyranometer or a pyranometer of a higher class and 3 to connect both to the same electronics so that electronics errors also offsets are eliminated 4 to mount all instruments on the same platform so that they have the same body temperature 5 assuming that the electronics are independently calibrated to analyse radiation values at normal incidence radiation possib
42. gle 6 relative to horizontal 6 relative to a tilted surface g global long wave t tilted h horizontal distinction horizontal and tilted from Hukseflux T symbols introduced by Hukseflux contributions of Eg and E are Eg andE t both corrected for the tilt angle of the surface SR20 D2 manual v1507 61 71 Hukseflux Thermal Sensors 9 7 Appendix on ISO and WMO classification tables Table 9 7 1 Classification table for pyranometers per ISO 9060 and WMO NOTE WMO specification of spectral selectivity is different from that of ISO Hukseflux conforms to the ISO limits WMO also specifies expected accuracies ISO finds this not to be a part of the classification system because it also involves calibration Please note that WMO achievable accuracies are for clear days at mid latitudes and that the uncertainty estimate does not include uncertainty due to calibration ISO CLASSI FI CATI ON TABLE ISO CLASS SECONDARY FIRST CLASS SECOND STANDARD CLASS Specification limit Response time 95 15s 30s 60s Zero offset a response to 200 W m net 7 W m 15 W m 30 W m thermal radiation Zero offset b response to 5 K h in ambient 2 W m 4 W m 8 W m temperature Non stability change per year 0 8 1 5 3 Non linearity 100 to 1000 W m 0 5 1 3 Directional response 10 W m 20 W m 30 W m Spectral selectivity 35
43. ibration according to ISO 9847 Type IIc Calibration uncertainty lt 1 2 k 2 Recommended recalibration interval 2 years Reference conditions 20 C normal incidence solar radiation horizontal mounting irradiance level 1000 W m Validity of calibration based on experience the instrument sensitivity will not change during storage During use under exposure to solar radiation the instrument non stability specification is applicable Adjustment after re calibration via a PC as power user with the Sensor Manager software Request power user status at the factory for sensitivity adjustment and for writing the calibration history data MEASUREMENT ACCURACY AND RESOLUTION Uncertainty of the measurement statements about the overall measurement uncertainty can only be made on an individual basis see the chapter on uncertainty evaluation WMO estimate on achievable accuracy 2 for daily sums see appendix for a definition of the measurement conditions WMO estimate on achievable accuracy 3 for hourly sums see appendix for a definition of the measurement conditions Irradiance resolution 0 05 W m Instrument body temperature resolution _7 8 x 10 3 C Instrument body temperature accuracy 0 5 C DIGITAL Digital output irradiance in W m instrument body temperature in C Rated operating voltage range 5 to 30 VDC Power consumption
44. ing It has a flat spectrum covering the 200 to 50000 x 10 m range and has a near perfect directional response The coating absorbs all solar radiation and at the moment of absorption converts it to heat The heat flows through the sensor to the sensor body The thermopile sensor generates a voltage output signal that is proportional to the solar irradiance e incase of SR20 D2 the analogue thermopile voltage is converted by the instrument electronics to a digital signal In this process also the temperature dependence of the thermopile is compensated SR20 D2 uses a high end 24 bit A D converter e a glass dome This dome limits the spectral range from 285 to 3000 x 10 m cutting off the part above 3000 x 10 m while preserving the 180 field of view angle Another function of the dome is that it shields the thermopile sensor from the environment convection rain e asecond inner glass dome For a secondary standard pyranometer two domes are used and not one single dome This construction provides an additional radiation shield resulting in a better thermal equilibrium between the sensor and inner dome compared to using a single dome The effect of having a second dome is a strong reduction of instrument offsets Pyranometers can be manufactured to different specifications and with different levels of verification and characterisation during production The ISO 9060 1990 standard Solar energy specification and clas
45. ings are stored in register 1 The default setting is setting number 5 representing a communication with 19200 baud even parity bit 8 data bits and 1 stop bit After a new address or communication setting is written the sensor must be restarted This can be done by writing OXFFOO to coil 0 6 4 Network communication example master request to SR20 D2 Normal sensor operation consists of requesting the output of registers 2 3 the temperature compensated solar radiation For quality assurance also the sensor serial number register 40 and the temperature in register 6 are useful In this example a SR20 D2 has address 64 The example requests the solar radiation temperature compensated register 2 3 sensor serial number register 40 and the temperature of the instrument register 6 The values are represented in hexadecimals Note 32 bit data are represented in 2 registers MSW and LSW should be read together in one request Request for solar radiation register 2 3 Master Request 40 03 00 00 00 04 4B 18 40 Modbus slave address decimal equivalent 64 03 Modbus function 03 Read holding registers 00 00 Starting register the master requests data starting from register 0 00 04 Length the number of registers the master wants to read 4 registers 4B 18 CRC the checksum of the transmitted data Sensor response 40 03 08 00 40 00 05 00 01 7C 4F 79 DA 40 Modbus slave
46. ion to instruments measuring in the plane of array horizontally placed instruments are used for the measurement of global radiation Global irradiance data enable the user to compare the local climate and system efficiency between different sites These data can also be compared to measurements by local meteorological stations 7 3 Speed of repair and maintenance Dependability is not only a matter of reliability but also involves the reaction to problems if the processing time of service and repairs is short this contributes to the dependability Hukseflux pyranometers are designed to allow easy maintenance and repair The main maintenance actions are e replacement of desiccant e replacement of cabling For optimisation of dependability a user should e estimate the expected lifetime of the instrument e design a schedule of regular maintenance e design a schedule of repair or replacement in case of defects When operating multiple instruments in a network Hukseflux recommends keeping procedures simple and having a few spare instruments to act as replacements during service recalibrations and repair 7 4 Uncertainty evaluation The uncertainty of a measurement under outdoor or indoor conditions depends on many factors see paragraph 1 of this chapter It is not possible to give one figure for pyranometer measurement uncertainty The work on uncertainty evaluation is in progress There are several groups around the world partici
47. ity for pyranometers because it is already included in the directional error and the spectral response for pyranometers and pyrheliometers because it is already taken into account in the calibration process SR20 D2 manual v1507 49 71 Hukseflux Thermal Sensors Table 7 4 1 1 Preliminary estimates of achievable uncertainties of measurements with Hukseflux pyranometers The estimates are based on typical pyranometer properties and calibration uncertainty for sunny clear sky days and well maintained stations without uncertainty loss due to lack of maintenance and due to instrument fouling The table specifies expanded uncertainties with a coverage factor of 2 and confidence level of 95 Estimates are based on 1 s sampling IMPORTANT NOTE there is no international consensus on uncertainty evaluation of pyranometer measurements so this table should not be used as a formal reference Pyranometer season latitude uncertainty uncertainty uncertainty class minute totals hourly totals daily totals ISO 9060 at solar noon at solar noon secondary summer mid latitude 2 7 2 0 1 9 standard SR20 D2 equator 2 6 1 9 1 7 pole 7 9 5 6 4 5 winter mid latitude 3 4 2 5 2 17 first class summer mid latitude 4 7 3 3 3 4 equator 4 4 3 1 2 9 pole 16 1 11 4 9 2 winter mid latitude 6 5 4 5 5 2 second class summer mid latitude 8 4 5 9 6 2 equator 7 8 5 5 5 3 pole 29 5 21 6 18
48. ived data needs to be divided by 100 to represent the correct outcome In this example the result is 2225 x 0 01 22 25 C Request for serial number register 40 Master Request 40 03 00 28 00 01 0B 13 40 Modbus slave address 03 Modbus function SR20 D2 manual v1507 44 71 Hukseflux Thermal Sensors 00 28 Start register 00 01 Number of registers 0B 13 CRC Sensor response 40 03 02 0A 29 43 35 40 Modbus Slave address 03 Modbus function 02 Number of bytes OA 29 Content of register 40 decimal equivalent 2601 43 35 CRC Register 40 represents the sensors serial number In this example the serial number is 2601 SR20 D2 manual v1507 45 71 Hukseflux Thermal Sensors 7 Making a dependable measurement 7 1 The concept of dependability A measurement with a pyranometer is called dependable if it is reliable i e measuring within required uncertainty limits for most of the time and if problems once they occur can be solved quickly The requirements for a measurement with a pyranometer may be expressed by the user as e required uncertainty of the measurement see following paragraphs e requirements for maintenance and repairs possibilities for maintenance and repair including effort to be made and processing time e arequirement to the expected instrument lifetime until it is no longer feasible to repair It is important to realise
49. izontal due ISO 0 to 90 at to change in tilt from 0 to 90 at 1000 W m irradiance Tilt 9060 1000 W m response describes changes of the sensitivity due to changes of 1990 the tilt angle of the receiving surface Sensitivity the change in the response of a measuring instrument divided by WMO the corresponding change in the stimulus 1 6 3 Spectral range the spectral range of radiation to which the instrument is Hukseflux sensitive For a normal pyranometer this should be in the 0 3 to 3 x 10 m range Some pyranometers with coloured glass domes have a limited spectral range SR20 D2 manual v1507 63 71 Hukseflux Thermal Sensors 9 9 Appendix on terminology glossary Table 9 9 1 Definitions and references of used terms TERM DEFINITION REFERENCE Solar energy or solar radiation solar energy is the electromagnetic energy emitted by the sun Solar energy is also called solar radiation and shortwave radiation The solar radiation incident on the top of the terrestrial atmosphere is called extra terrestrial solar radiation 97 of which is confined to the spectral range of 290 to 3 000 x 10 m Part of the extra terrestrial solar radiation penetrates the atmosphere and directly reaches the earth s surface while part of it is scattered and or absorbed by the gas molecules aerosol particles cloud droplets and cloud crystals in the atmosphere The former is the direct component the latter is the diffu
50. le Chassis connector type HUMMEL AG 7 840 200 000 panel connector front mounting short version Cable connector M16 straight connector female thread 10 pole Cable connector type HUMMEL AG 7 810 300 00M straight connector female thread for cable 3 to 6 x 10 m special version Connector protection class IP 67 IP 69 K per EN 60 529 connected Cable replacement replacement cables with connector can be ordered separately from Hukseflux Mounting 2 x M5 bolt at 65 x 10 m centre to centre distance on north south axis or 1 x M6 bolt at the centre of the instrument connection from below under the bottom plate of the instrument Levelling bubble level and adjustable levelling feet are included Levelling accuracy lt 0 1 bubble entirely in ring Desiccant two bags of silica gel 0 5 g 35 x 20 mm Humidity indicator blue when dry pink when humid IP protection class IP 67 Gross weight including 5 m cable 1 2 kg Net weight including 5 m cable 0 85 kg Packaging box of 200 x 135 x 225 mm HEATING Heater no heating SR20 D2 manual v1507 15 71 Hukseflux Thermal Sensors Table 3 1 1 Specifications of SR20 D2 started on previous pages CALI BRATION Calibration traceability to WRR Calibration hierarchy from WRR through ISO 9846 and ISO 9847 applying a correction to reference conditions Calibration method indoor cal
51. ly tilting the radiometers to approximately normal incidence if this is not possible to compare 1 hour totals around solar noon for horizontally mounted instruments 6 for second class radiometers to correct deviations of more than 10 Lower deviations should be interpreted as acceptable and should not lead to a revised sensitivity 7 for first class pyranometers to correct deviations of more than 5 Lower deviations should be interpreted as acceptable and should not lead to a revised sensitivity 8 for secondary standard instruments to correct deviations of more than 3 Lower deviations should be interpreted as acceptable and should not lead to a revised sensitivity 8 4 Data quality assurance Quality assurance can be done by e analysing trends in solar irradiance signal e plotting the measured irradiance against mathematically generated expected values e comparing irradiance measurements between sites e analysis of night time signals The main idea is that one should look out for any unrealistic values There are programs on the market that can semi automatically perform data screening See for more information on such a program www dqms com SR20 D2 manual v1507 54 71 Hukseflux Thermal Sensors SR20 D2 manual v1507 55 71 Hukseflux Thermal Sensors 9 Appendices 9 1 Appendix on cable extension replacement The sensor cable of SR20 D2 is equipped with a M16 straight connector In case of cable
52. mployee Register 86 to 95 these registers are for reference purposes A master Table 6 2 10 Modbus registers 96 to 99 humidity sensor information Please note that if your data request needs an offset of 1 for each SR20 D2 register number depending on processing by the network master this offset applies to coils as well Consult the manual of the device acting as the local MODBUS REGI STERS 96 99 REGI STER PARAMETER DESCRI PTION OF CONTENT TYPE FORMAT NUMBER OF OF DATA 96 97 Factory use 98 Humidity In x 0 01 R U16 99 Humidity temperature In x 0 01 C R S16 Register 98 Humidity provides the relative humidity within the instrument The value must be divided by 100 to get the value in Register 99 Humidity temperature the temperature measured by the humidity sensor The value must be divided by 100 to get the value in C SR20 D2 manual v1507 40 71 Hukseflux Thermal Sensors Table 6 2 11 Coils COILS COIL PARAMETER DESCRIPTION TYPE OF OBJ ECT TYPE 0 Restart Restart the sensor W Single bit 1 Reserved 2 Check Measure sensor W Single bit electrical resistance Coil 0 Restart when OxFFOO is written to this coil the sensor will restart If applied a new Modbus address or new serial settings will become effective Coil 2 Check when OxFFOO is written to this coil the internal electronics will measure the electrical resistance
53. nother 2 years assessment or if it should be replaced SR20 D2 manual v1507 51 71 Hukseflux Thermal Sensors MINIMUM RECOMMENDED PYRANOMETER MAINTENANCE continued 7 6 years parts if applicable necessary replace the parts that are most replacement exposed to weathering cable connector desiccant holder sun screen NOTE use Hukseflux approved parts only 8 internal if applicable open instrument and inspect replace O rings inspection dry internal cavity around the circuit board 9 recalibration recalibration by side by side comparison to a higher standard instrument indoors according to ISO 9847 or outdoors according to SO9846 8 2 Trouble shooting Table 8 2 1 Trouble shooting for SR20 D2 continued on next page General Inspect the instrument for any damage Inspect if the humidity indicator is blue Blue indicates dryness The colour pink indicates it is humid in the latter case replace the desiccant see chapter on maintenance Inspect if the connector is properly attached Check the condition of the connectors on chassis as well as the cable Inspect if the sensor receives DC voltage power in the range of 5 to 30 VDC In case 4 20 mA output is used inspect if the sensor receives DC voltage power in the range of 5 to 30 VDC via the main supply and if the current loop receives DC voltage power in the range of 5 5 to 40 VDC Do not use the same power supply for these voltages Inspect the
54. nsor as measured in the factory in x 0 1 s The value must be divided by 10 to get the value in s Register 44 Sensor electrical resistance returns the electrical resistance measured during the sensor calibration The resistance is in x 0 1 Q and must be divided by 10 to get the value in Q Register 46 47 Sensor calibration date last sensor calibration date from which the sensitivity in register 41 and 42 was found in YYYYMMDD Register 61 Firmware version Register 62 Hardware version SR20 D2 manual v1507 38 71 Hukseflux Thermal Sensors Table 6 2 7 Modbus registers 63 to 81 calibration history MODBUS REGISTERS 63 81 REGISTER PARAMETER DESCRIPTION OF TYPE FORMAT NUMBER CONTENT OF OF DATA 63 64 Sensor sensitivity history 1 In x 10 V W m7 R Float Default value is 0 65 66 Calibration date history 1 Former calibration date of R U32 the sensor in YYYYMMDD Default value is 0 67 68 Sensor sensitivity history 2 See register 63 64 R Float 69 70 Calibration date history 2 See register 65 66 R U32 71 72 Sensor sensitivity history 3 See register 63 64 R Float 73 74 Calibration date history 3 See register 65 66 R U32 75 76 Sensor sensitivity history 4 See register 63 64 R Float 77 78 Calibration date history 4 See register 65 66 R U32 79 80 Sensor sensitivity history 5 See register 63 64 R Float 81 82 Calibration date history 5 See register
55. ntually store the averages every 60 seconds How to issue a request process the register content and convert it to useful data is described in the paragraphs about network communication The user should have sound knowledge of the Modbus communication protocol when installing sensors in a network SR20 D2 manual v1507 7 71 Hukseflux Thermal Sensors The instrument should be used in accordance with the recommended practices of ISO WMO and ASTM The recommended calibration interval of pyranometers is 2 years The registers containing the applied sensitivity and the calibration history of SR20 D2 are fully accessible for users This allows the user to choose his own local calibration service The same feature may be used for remotely controlled re calibration of pyranometers in the field Ask Hukseflux for information on this feature and on ISO and ASTM standardised procedures for field calibration Suggested use for SR20 D2 e PV system performance monitoring e all networks with regular instrument exchange e scientific meteorological observations e reference instrument for comparison e extreme climates tropical polar The ASTM E2848 Standard Test Method for Reporting Photovoltaic Non Concentrator System Performance issued end 2011 confirms that a pyranometer is the preferred instrument for PV system performance monitoring SR20 D2 pyranometer complies with the requirements of this standard For more information see our pyr
56. nverter is usually powered via the USB interface in this case no external power is needed to feed the converter If an RS 485 to RS 232 converter is used this converter should be powered by an external source This may be the same supply used for the SR20 D2 pink _ e 4 to 20 mA 5 5 to 40 VDC grey _ e 4 to 20 mA 5 5 to 40 VDC brown not connected yellow e not connected black e shield red 5 to 30 VDC blue 5 to 30 VDC 7 common white data KESAH green data SR20 D2 wire RS 485 USB converter USB to PC Figure 5 7 1 Connecting SR20 D2 to an RS 485 to USB converter and a PC SR20 D2 manual v1507 28 71 Hukseflux Thermal Sensors 6 Communication with SR20 D2 6 1 PC communication Sensor Manager software SR20 D2 can be accessed via a PC In that case the communication with the sensor is done via the user interface offered by the Sensor Manager software or by another Modbus testing tool The Sensor Manager is supplied with the instrument on a USB flash drive There are links to testing tools paid or freeware available at www modbus org This chapter describes the functionality of the Sensor Manager only The Hukseflux Sensor Manager software provides a user interface for communication between a PC and SR20 D2 It allows the user to locate configure and test one or more SR20 D2 s and to perform simple laboratory me
57. oad defaults Manually connect Modbus address 64 Connect with selected Show details Disconnect with checked Plot on Live Chart Figure 6 1 3 2 Sensor Manager main window with three connected SR20 D2 s When an instrument is found temperature and irradiance data are displayed Updates are done manually or automatically Automatic updates can be made every second every 5 seconds or every minute 6 1 4 Sensor Manager plotting data When the Plot on Live Chart button in the lower right corner is clicked the Plot window opens A live graph is shown of the measurement with the selected instrument SR20 D2 manual v1507 31 71 Hukseflux Thermal Sensors The x axis time is scaled automatically to display data of the complete measurement period After checking the box Show tail only only the last minutes of measured data are displayed When the update interval is 1 second the Show tail only function is available after around 10 minutes of data collection The y axis displays the measured irradiance in W m The Y axis automatically scales to display the full measured range D LiveChart window carm n 00 00 01 089 00 00 07 000 00 00 13 000 00 00 19 000 Time settings Signal settings i cane Update interval Quantity amp units as Export data iw V Average over interval Irradiance W m F Show tail only Figure 6 1 4 1 Example of a SR2
58. on settings as indicated on screen When only one sensor is connected using Find is suggested because the operation stops when a sensor is found Find all will continue a scan of the complete range of Modbus addresses and may take extra time SR20 D2 manual v1507 30 71 Hukseflux Thermal Sensors If the Find or Find all operation does not find instruments a dialog box opens asking to confirm a scan of the address range using all possible communication settings The time this operation takes depends on the address range to be scanned To complete a scan of 247 addresses will take over 15 minutes When an instrument is found a dialog box opens providing its serial number Modbus address and communication settings Communicating with the instrument is possible after changing the communication settings and Modbus address in the main window to the values of the instrument and then clicking Connect E Hukseflux Sensor Manager Sa Hukseflux Thermal Sensors Scan for sensor Connected sensors update measurements Manually X Now Serial port CoML x 1 SR20 D2 2602 comi Modbus address 2 135W m2 25 11 C Update ports Modbus address range 2 SR20 D2 2603 comi Modbus address 3 2 23 W m 24 64 C 1 through 5 Find gt 3 SR20 D2 2604 comi Modbus address 4 3 19 W m2 24 58 C Serial communication settings BAUD rate 19200 v Data amp stop bits 8 databits 1 stopbit L
59. ons for outdoor use for good stability in outdoor applications 9 2 Appendix on tools for SR20 D2 Table 9 2 1 Specifications of tools for SR20 D2 tooling required for sun screen fixation and removal by hand tooling required for bottom plate fixation and removal hex key 2 5 mm tooling required for desiccant holder fixation and spanner size 20 mm removal tooling required for wire fixation and removal internal screwdriver blade width 2 mm wiring inside SR20 D2 body SR20 D2 manual v1507 57 71 Hukseflux Thermal Sensors 9 3 Appendix on spare parts for SR20 D2 e Desiccant holder with glass window and rubber ring e Desiccant set of 5 bags in air tight bag e Humidity indicator e Levelling feet set of 2 e Static foot e Sun screen with metal ring and thumb screw e SR20 D2 cable with connector specify length in multiples of 5 m e O ring SR20 D2 NOTE Outer dome level and sensor of SR20 D2 cannot be supplied as spare parts In case of possible damage to the SR20 D2 after repair the instrument must be tested to verify performance within specification limits This is required by ISO 9060 Testing involves verification of the directional response after dome thermal sensor and level replacement and verification of the temperature response after thermal sensor replacement SR20 D2 manual v1507 58 71 Hukseflux Thermal Sensors 9 4 Appendix on standards for classification and calibration Both I
60. ors Table 6 2 5 Setting options of register 1 SETTING OPTIONS SETTING BAUD RATE DATABITS STOPBITS PARITY NUMBER 1 9600 8 1 none 2 9600 8 1 even 3 9600 8 1 odd 4 19200 8 1 none 5 default 19200 8 1 even 6 19200 8 1 odd 10 38400 8 1 none 11 38400 8 1 even 12 38400 8 1 odd 16 115200 8 1 none 17 115200 8 1 even 18 115200 8 1 odd Register 2 3 Irradiance temperature compensated signal provides the temperature compensated solar radiation output in 0 01 W m The value given must be divided by 100 to get the value in W m Hukseflux recommends using this data to achieve the highest accuracy MSW and LSW should be read together in one request Register 4 5 Irradiance uncompensated signal Use for comparison purposes only Provides the sensor output in 0 01 W m not compensated for temperature dependence The data must be divided by 100 to get the value in W m Hukseflux recommends not to use this data MSW and LSW should be read together in one request Register 6 Instrument body temperature provides the temperature of the instrument body in 0 01 C The data must be divided by 100 to achieve the value in C Register 7 Sensor electrical resistance sensor resistance in 0 1 Q The data needs to be divided by 10 to get the value in Q This register returns a 0 by default To read the resistance first a measurement has to be performed This can be done by writing O
61. ower supply for the 4 20 mA current loop circuit SR20 D2 operates on a supply voltage of 5 to 30 VDC In addition 5 5 to 40 VDC is needed for the 4 20 mA output Do not use the same power supply for these voltages Some dataloggers have a 4 20 mA input In that case SR20 D2 can be corrected directly to the datalogger Some dataloggers have the capability to measure currents In some cases the datalogger accepts a voltage input Usually a 100 Q precision resistor is used to convert the current to a voltage this will then be in the 0 4 to 2 VDC range This resistor must be put in the wire of the sensor See next page and chapter 5 3 for electrical connections SR20 D2 manual v1507 24 71 Hukseflux Thermal Sensors See chapter 5 3 and the diagrams below for electrical connections to am and voltmeters when using SR20 D2 s 4 to 20 mA output D2 sensor pink grey ammeter I 4 to 20 mA 5 5 to 40 VDC 12 VDC recommended red 5 to 30 VDC 12 VDC recommended blue 5 to 30 VDC 12 VDC recommended Figure 5 5 1 Electrical diagram of the connection of SR20 D2 to a typical ammeter or datalogger with capacity to measure current signals SR20 D2 operates on a supply voltage of 5 to 30 VDC In addition 5 5 to 40 VDC is needed for the 4 20 mA function red 5 to 30 VDC 12 VDC recommended blue 5 to 30 VDC 12 VDC recommended pink grey voltmeter R U R I 4 to 20 mA
62. pating in standardisation of the method of calculation The effort aims to work according to the guidelines for uncertainty evaluation according to the Guide to Expression of Uncertainty in Measurement or GUM SR20 D2 manual v1507 48 71 Hukseflux Thermal Sensors 7 4 1 Evaluation of measurement uncertainty under outdoor conditions Hukseflux actively participates in the discussions about pyranometer measurement uncertainty we also provide spreadsheets reflecting the latest state of the art to assist our users in making their own evaluation The input to the assessment is summarised 1 The formal evaluation of uncertainty should be performed in accordance with ISO 98 3 Guide to the Expression of Uncertainty in Measurement GUM 2 The specifications of the instrument according to the list of ISO 9060 classification of pyranometers and pyrheliometers are entered as limiting values of possible errors to be analysed as type B evaluation of standard uncertainty per paragraph 4 3 7 of GUM A priori distributions are chosen as rectangular 3 A separate estimate has to be entered to allow for estimated uncertainty due to the instrument maintenance level 4 The calibration uncertainty has to be entered Please note that Hukseflux calibration uncertainties are lower than those of alternative equipment These uncertainties are entered in measurement equation equation is usually Formula 0 1 E U S either as an uncertainty in E zero
63. r the bottom plate of the instrument Performing a representative measurement the pyranometer measures the solar radiation in the plane of the sensor This may require installation in a tilted or inverted position The black sensor surface sensor bottom plate should be mounted parallel to the plane of interest In case a pyranometer is not mounted horizontally or in case the horizon is obstructed the representativeness of the location becomes an important element of the measurement See the chapter on uncertainty evaluation Levelling in case of horizontal mounting only use the bubble level and levelling feet For inspection of the bubble level the sun screen must be removed Instrument orientation by convention with the cable exit pointing to the nearest pole so the cable exit should point north in the northern hemisphere south in the southern hemisphere Installation height in case of inverted installation WMO recommends a distance of 1 5 m between soil surface and sensor reducing the effect of shadows and in order to obtain good spatial averaging SR20 D2 manual v1507 21 71 Hukseflux Thermal Sensors 5 2 Installation of the sun screen SR20 D2 s sun screen can be installed and removed by using the dedicated thumb screw See item 2 of the drawing below The thumb screw can be turned without tools for fixation or loosening of the sun screen as visualised below Once the thumb screw has
64. recommendations for instrument use in sunshine duration measurement STANDARDS FOR INSTRUMENT USE FOR SUNSHINE DURATION WMO WMO No 8 Guide to Meteorological Instruments and Methods of Observation chapter 8 measurement of sunshine duration 8 2 2 Pyranometric Method 4 4 Specific use for outdoor PV system performance testing SR20 D2 is very well applicable in outdoor PV system performance testing See also Hukseflux model SR12 first class pyranometer for solar energy test applications Table 4 4 1 Standards with recommendations for instrument use in PV system performance testing STANDARDS ON PV SYSTEM PERFORMANCE TESTING IEC ISO STANDARD EQUIVALENT ASTM STANDARD IEC 61724 Photovoltaic system performance ASTM 2848 11 Standard Test Method for monitoring guidelines for measurement data Reporting Photovoltaic Non Concentrator exchange and analysis System Performance COMMENT Allows pyranometers or reference COMMENT confirms that a pyranometer is the cells according to IEC 60904 2 and 6 preferred instrument for outdoor PV testing Pyranometer reading required accuracy better Specifically recommends a first class than 5 of reading Par 4 1 pyranometer paragraph A 1 2 1 COMMENT equals JISC 8906 Japanese Industrial Standards Committee 4 5 Specific use in meteorology and climatology The World Meteorological Organization WMO is a specialised agency of the United Nations It is the
65. replacement it is recommended to purchase a new cable with connector at Hukseflux An alternative is to choose for a Do it yourself DIY approach please ask for the DIY connector assembly guide In case of cable extension the user may choose purchasing a new cable with connector at Hukseflux or extending the existing cable by himself Please note that Hukseflux does not provide support for DIY connector and cable assembly SR20 D2 is equipped with one cable Maximum length of the sensor cable depends on the RS 485 network topology applied in the field In practice daisy chain topologies or point to point PtP topologies are used The length of the sensor cable should be as short as possible to avoid signal reflections on the line When the sensor is used in a traditional daisy chain bus topology the sensor cable length is the distance covered from the sensor to the cable trunk of the main data line This line is often called the stub Stub length and thus cable length has to be shorter than one tenth of the sensor driver s output rise time and a factor for signal velocity in the sensor cable For SR20 D2 in daisy chain configurations maximum cable length is around 12 metres In point to point configurations cable lengths can in theory be much longer RS 485 is specified for cable lengths up to 1200 metres Connector cable and cable connection specifications are summarised on the next page SR20 D2 manual v1507 56 71 Hukseflux Th
66. reserved for a 32 bit integer If the format of data is float it is a 32 bit floating point operator and two 16 bit registers are reserved as well Most network managing programs have standard menus performing this type of conversion In case manual conversion is required see the appendix on conversion of a floating point number to a decimal number MSW and LSW should be read together in one request This is necessary to make sure both registers contain the data of one internal voltage measurement Reading out the registers with two different instructions may lead to the combination of LSW and MSW of two measurements at different points in time An Unsigned 32 bit integer can be calculated by the formula MSW x 27 LSW U32 An example of such a calculation is available in the paragraph Network communication example master request to SR20 D2 SR20 D2 manual v1507 35 71 Hukseflux Thermal Sensors A manual of the device acting as the local master Your data request may need an offset of 1 for each SR20 D2 register number depending on processing by the network master Example SR20 D2 register number 7 master offset 7 1 master register number 8 Consult the Table 6 2 4 Modbus registers 0 to 11 measurements For basic operation Hukseflux recommends to read out registers 2 3 for solar radiation register 6 for instrument body temperature and register 40 for the sensor serial number MODBUS REGI STERS 0 11
67. rsion Appendix on function codes register and coil overview EC declaration of conformity 00000 EO Nr SR20 D2 manual v1507 4 71 Hukseflux Thermal Sensors List of symbols Quantities Voltage output Sensitivity Temperature Solar irradiance Solar radiant exposure Time in hours Temperature coefficient Temperature coefficient Temperature coefficient Output of 4 20 mA current loop Transmitted range of 4 20 mA output see also appendix 9 6 on meteorological quantities Subscripts Not applicable SR20 D2 manual v1507 Symbol DIMAUC ow Unit V V W m de W m W h m h 1 C 1 C A W m 5 71 Hukseflux Thermal Sensors Introduction SR20 D2 is a solar radiation sensor of the highest category in the ISO 9060 classification system secondary standard SR20 D2 is designed for the solar PV industry offering two types of commonly used irradiance outputs digital via Modbus RTU over RS 485 and analogue 4 20 mA current loop These industry standards allow for easy data acquisition easy read out and error free instrument exchange when using SR20 D2 SR20 D2 measures the solar radiation received by a plane surface in W m from a 180 field of view angle It is employed where the highest measurement accuracy is required This user manual covers SR20 D2 use Specifications of SR20 D2 differ from those of model SR20 For SR20 use consult the SR20 user manual Individually tes
68. se component of the solar radiation ref WMO Hukseflux Hemispherical solar radiation solar radiation received by a plane surface from a 180 field of view angle solid angle of 2 n sr ref ISO 9060 Global solar radiation the solar radiation received from a 180 field of view angle on a horizontal surface is referred to as global radiation Also called GHI This includes radiation received directly from the solid angle of the sun s disc as well as diffuse sky radiation that has been scattered in traversing the atmosphere ref WMO Hemispherical solar radiation received by a horizontal plane surface ref ISO 9060 Plane of array also POA hemispherical solar irradiance in the plane of a PV array irradiance ref ASTM E2848 11 IEC 61724 Direct solar radiation received from a small solid angle centred on the sun s disc on a given radiation plane ref ISO 9060 Terrestrial or radiation not of solar origin but of terrestrial and atmospheric origin and having Longwave longer wavelengths 3 000 to 100 000 x 10 m In case of downwelling E also radiation the background radiation from the universe is involved passing through the atmospheric window In case of upwelling E t composed of long wave electromagnetic energy emitted by the earth s surface and by the gases aerosols and clouds of the atmosphere it is also partly absorbed within the atmosphere For a temperature of 300 K
69. sification of instruments for measuring hemispherical solar and direct solar radiation distinguishes between 3 classes secondary standard highest accuracy first class second highest accuracy and second class third highest accuracy From second class to first class and from first class to secondary standard the achievable accuracy improves by a factor 2 SR20 D2 manual v1507 12 71 Hukseflux Thermal Sensors 1 2 0 wt ee ce 1 solar radiation Ys C S a 0 8 a 5 lt pyranometer geg 0 6 response v 6 ow 9 Y 2 0 4 BS gg 0 2 0 100 1000 10000 wavelength x 10 9 m Figure 2 2 Spectral response of the pyranometer compared to the solar spectrum The pyranometer only cuts off a negligible part of the total solar spectrum 4 3 m North se aie 7 p 7 gt 7 2 o r 2 pe East oe oO anes DE ee ee Se eee a 0 3 South 0 Z 80 GE 9 Se eee o 5 i Se West u s a a 2 z t N 3 Ya ISO secondary a N standard ri directional response limit 4 zenith angle Figure 2 3 Directional response of a SR20 D2 pyranometer of 4 azimuth angles compared to secondary standard limits SR20 D2 manual v1507 13 71 Hukseflux Thermal Sensors 3 Specifications of SR20 D2 3 1 Specifications of SR20 D2 SR20 D2 measures the solar radiation received by a plane surface from a 180 field of view angle This quantity expressed in W m is called
70. sitivity See register 63 64 R Float history 3 73 74 Calibration date history 3 See register 65 66 R U32 75 76 Sensor sensitivity See register 63 64 R Float history 4 77 78 Calibration date history 4 See register 65 66 R U32 79 80 Sensor sensitivity See register 63 64 R Float history 5 81 82 Calibration date history 5 See register 65 66 R U32 83 84 Directional response Directional response R U32 measurement date measurement date in YYYYMMDD 85 Directional response R U16 measurement employee 86 Temperature response In x 0 01 R S16 87 88 Polynomial temperature R Float coefficient a 89 90 Polynomial temperature R Float coefficient b 91 92 Polynomial temperature R Float coefficient c SR20 D2 manual v1507 67 71 Hukseflux Thermal Sensors MODBUS REGISTERS 0 99 continued REGISTER PARAMETER DESCRIPTION OF CONTENT TYPE FORMAT NUMBER OF OF DATA 93 94 Temperature response Temperature response R U32 characterisation characterisation measurement date measurement date of the sensor in YYYYMMDD 95 Temperature response R U16 characterisation measurement employee 96 97 Factory use only 98 Humidity In x 0 01 R U16 99 Humidity temperature In x 0 01 C R S16 Note 1 Up to five 16 bit registers can be requested in one request If requesting six A or more registers use multiple requests Please note that if your data request needs an offset of
71. ssa 1 8925 Calculation of floating point float sign bit x mantissa x 2 P e 1 x 1 8925 x 2 15 14 so floating point 15 14 SR20 D2 manual v1507 65 71 Hukseflux Thermal Sensors 9 11 Appendix on function codes register and coil overview Table 9 11 1 Supported Modbus function codes SUPPORTED MODBUS FUNCTION CODES FUNCTION CODE HEX DESCRI PTI ON 0x01 Read Coils 0x02 Read Discrete Inputs 0x03 Read Holding Registers 0x04 Read Input Register 0x05 Write Single Coil 0x06 Write Single Holding Register OxOF Write Multiple Coils 0x10 Write Multiple Registers A manual of the device acting as the local master Table 9 11 2 Modbus registers 0 to 99 Your data request may need an offset of 1 for each SR20 D2 register number depending on processing by the network master Example SR20 D2 register number 7 master offset 7 1 master register number 8 Consult the MODBUS REGISTERS 0 99 REGISTER PARAMETER DESCRIPTION OF CONTENT TYPE FORMAT NUMBER OF OF DATA 0 Modbus address Sensor address in Modbus R W U16 network default 1 1 Serial communication Sets the serial R W U16 settings communication default 5 2 3 Irradiance Temperature compensated R 32 temperature signal in x 0 01 W m compensated signal 4 5 rradiance Uncompensated signal R 32 uncompensated signal in x 0 01 W m 6 Sensor body In x 0
72. ted for temperature and directional response SR20 D2 is the most accurate digital secondary standard pyranometer available Its benefits Digital output easy implementation and servicing Best in class temperature response lt 0 4 30 to 50 C best zero offset a and best calibration uncertainty Included in delivery as required by ISO 9060 test certificates for temperature response and directional response e Re calibration registers fully accessible to users In order to improve overall measurement accuracy Hukseflux effectively targeted two major sources of measurement uncertainty calibration and zero offset a In addition SR20 D2 has a negligible temperature response All are best in class The temperature response of every individual instrument is tested and corrected onboard by the instrument electronics using a second degree polynomial SR20 D2 s low temperature dependence makes it the ideal candidate for use under very cold and very hot conditions Figure 0 1 SR20 D2 digital secondary standard pyranometer SR20 D2 manual v1507 6 71 Hukseflux Thermal Sensors SR20 D2 pyranometer employs a state of the art thermopile sensor with black coated surface two domes and an anodised aluminium body The connector desiccant holder and sun screen fixation are very robust and designed for long term industrial use SR20 D2 uses a high end 24 bit A D converter All parts are specified for use across SR20 D2 s
73. tegory of instrument SR20 D2 manual v1507 62 71 Hukseflux Thermal Sensors 9 8 Appendix on definition of pyranometer specifications Table 9 8 1 Definition of pyranometer specifications SPECIFICATION DEFINITION SOURCE Response time time for 95 response The time interval between the instant ISO 95 when a stimulus is subjected to a specified abrupt change and the 9060 instant when the response reaches and remains within specified 1990 limits around its final steady value The response time is a measure WMO of the thermal inertia inherent in the stabilization period for a final 1 6 3 reading Zero offset a response to 200 W m net thermal radiation ventilated ISO 200 W m net Hukseflux assumes that unventilated instruments have to specify 9060 thermal the zero offset in unventilated worst case conditions 1990 radiation Zero offsets are a measure of the stability of the zero point Zero offset a is visible at night as a negative offset the instrument dome irradiates in the far infra red to the relatively cold sky This causes the dome to cool down The pyranometer sensor irradiates to the relatively cool dome causing a negative offset Zero offset a is also assumed to be present during daytime Zero offset b response to 5 K h change in ambient temperature ISO 5 K hin ambient Zero offsets are a measure of the stability of the zero point 9060 temperature 1990 Non stability per
74. tegrated total is called radiant exposure In solar energy radiant exposure is often given in W h m Table 9 6 1 Meteorological radiation quantities as recommended by WMO additional symbols by Hukseflux Thermal Sensor POA stands for Plane of Array irradiance The term originates from ASTM and IEC standards SYMBOL DESCRIPTION CALCULATION UNITS ALTERNATIVE EXPRESSION El downward irradiance EJ E E W m H downward radiant exposure H Hg Hi J J m for a specified time interval Et upward irradiance ET E t E t W m Ht upward radiant exposure Ht Hg HT J m W h m Change of for a specified time interval units E direct solar irradiance W m DNI Direct normal to the apparent Normal solar zenith angle Irradiance Eo solar constant W m aie global irradiance Ey E cos On W m GHI Global hemispherical irradiance on Eg Horizontal a specified in this case Irradiance horizontal surface Egi global irradiance E Ecos 0 W m POA Plane of hemispherical irradiance on Egl Erte Array a specified in this case tilted surface Eal downward diffuse solar W m DHI Diffuse radiation Horizontal Irradiance E t Ei upward downward long W m wave irradiance E reflected solar irradiance W m E net irradiance E E Ef W m TI apparent surface Cork temperature Tt apparent sky Cork temperature SD sunshine duration h 8 is the apparent solar zenith an
75. that the uncertainty of the measurement is not only determined by the instrument but also by the way it is used See also ISO 9060 note 5 In case of pyranometers the measurement uncertainty as obtained during outdoor measurements is a function of e the instrument class e the calibration procedure uncertainty e the duration of instrument employment under natural sunlight involving the instrument stability specification e the measurement conditions such as tilting ventilation shading instrument temperature e maintenance mainly fouling e the environmental conditions Therefore ISO 9060 says statements about the overall measurement uncertainty under outdoor conditions can only be made on an individual basis taking all these factors into account defined at Hukseflux as all factors outside the instrument that are relevant to the measurement such as the cloud cover presence or absence of direct radiation sun position the local horizon which may be obstructed or condition of the ground when tilted The environmental conditions also involve the question whether or not the measurement at the location of measurement is representative of the quantity that should be measured SR20 D2 manual v1507 46 71 Hukseflux Thermal Sensors 7 2 Reliability of the measurement A measurement is reliable if it measures within required uncertainty limits for most of the time We distinguish between two causes of unreliabilit
76. the standard The working standard has traceability to WRR world radiometric reference CORRECTION OF WORKING STANDARD CALIBRATION TO STANDARDISED REFERENCE CONDITIONS Correction from test conditions of the standard to reference conditions i e to normal incidence and 20 C Using known working standard pyranometer properties directional non linearity offsets temperature dependence This correction has an uncertainty uncertainty of correction At Hukseflux we also call the working standard pyranometer standard INDOOR PRODUCT CALIBRATION Calibration of products i e pyranometers Method according to ISO 9847 Type IIc which is an indoor calibration This calibration has an uncertainty associated with the method In some cases like the BSRN network the product calibration is with a different method for example again type 1 outdoor CALI BRATION UNCERTAINTY CALCULATION ISO 98 3 Guide to the Expression of Uncertainty in Measurement GUM Determination of combined expanded uncertainty of calibration of the product including uncertainty of the working standard uncertainty of correction uncertainty of the method transfer error The coverage factor must be determined at Hukseflux we work with a coverage factor k 2 SR20 D2 manual v1507 60 71 Hukseflux Thermal Sensors 9 6 Appendix on meteorological radiation quantities A pyranometer measures irradiance The time in
77. turned the sun screen loose the screen can be lifted off manually After removal the user may inspect the bubble level item 10 of the drawing and remove the cable connector item 11 Figure 5 2 1 Installation and removal of SR20 D2 s sun screen SR20 D2 manual v1507 22 71 Hukseflux Thermal Sensors 5 3 Electrical connection of SR20 D2 wiring diagram The instrument must be powered by an external power supply providing an operating voltage in the range from 5 to 30 VDC This is the main power supply for the sensor using the red and blue wires Do not put more than 30 Volt across these wires In addition when using the 4 to 20 mA output current loop 5 5 to 40 VDC must be supplied to the designated pink and grey wires Do not put more than 40 Volt across these wires See chapter 5 5 for using SR20 D2 s 4 to 20 mA output Table 5 3 1 Wiring diagram of SR20 D2 PIN WIRE SR20 D2 VDC main power supply 4 to 20 mA VDC main power supply ground Grey 4 to 20 mA e O Ww N not connected not connected shield to instrument body White RS 485 B B u N O fF Note 1 pin 9 is the cable shield which shields the signal wires and is connected to the instrument body The body is typically connected to the mounting platform which should be locally connected to ground The shield is not the main power supply ground which is at pin 6 VDC Note 2 at the conn
78. uments for and Methods of Observation measuring hemispherical solar and chapter 7 measurement of direct solar radiation radiation 7 3 measurement of global and diffuse solar radiation 4 2 General use for solar radiation measurement Table 4 2 1 Standards with recommendations for instrument use in solar radiation measurement STANDARDS FOR INSTRUMENT USE FOR HEMISPHERICAL SOLAR RADIATION ISO STANDARD EQUIVALENT WMO ASTM STANDARD ISO TR 9901 1990 ASTM G183 05 WMO No 8 Guide to Solar energy Field Standard Practice for Field Meteorological Instruments pyranometers Recommended Use of Pyranometers and Methods of Observation practice for use Pyrheliometers and UV chapter 7 measurement of Radiometers radiation 7 3 measurement of global and diffuse solar radiation 4 3 General use for sunshine duration measurement According to the World Meteorological Organization WMO 2003 sunshine duration during a given period is defined as the sum of that sub period for which the direct solar irradiance exceeds 120 W m SR20 D2 manual v1507 19 71 Hukseflux Thermal Sensors WMO has approved the pyranometric method to estimate sunshine duration from pyranometer measurements Chapter 8 of the WMO Guide to Instruments and Observation 2008 This implies that a pyranometer may be used in combination with appropriate software to estimate sunshine duration Ask for our application note Table 4 3 1 Standards with
79. work when no transmission is occurring a pull up and pull down resistor are required Typical values for both resistors are in the range from 650 to 850 Q SR20 D2 manual v1507 26 71 Hukseflux Thermal Sensors pink _ e 4 to 20 mA 5 5 to 40 VDC grey _ e 4 to 20 mA 5 5 to 40 VDC brown not connected yellow not connected black shield red e 5 to 30 VDC om g oon blue 5 to 30 VDC 7 x common g D white data RS 485 B B Cc ita green data RS 485 A A SR20 D2 wire RS 485 network Figure 5 6 2 Connection of SR20 D2 to an RS 485 network SR20 D2 is powered by an external power supply of 5 to 30 VDC SR20 D2 manual v1507 27 71 Hukseflux Thermal Sensors 5 7 Connecting to a PC SR20 D2 can be accessed via a PC In that case communication with the sensor is done via the user interface offered by the Sensor Manager software see the next chapters or by another Modbus testing tool Depending on the available ports on the PC either an RS 485 to USB converter or an RS 485 to RS 232 converter is used The figure below shows how connections are made The converter must have galvanic isolation between signal input and output to prevent static electricity or other high voltage surges to enter the data lines An external power supply is required to power the SR20 D2 5 to 30 VDC An RS 485 to USB co
80. xFFOO to coil 2 Hukseflux recommends to use this function only when necessary for diagnostics in case of sensor failure Register 8 Scaling factor irradiance default scaling factor is 100 Register 9 Scaling factor temperature default scaling factor is 100 Register 10 Sensor voltage output sensor voltage output signal of the thermopile in x 10 V SR20 D2 manual v1507 37 71 Hukseflux Thermal Sensors Table 6 2 6 Modbus registers 32 to 62 sensor and calibration information MODBUS REGISTERS 32 62 REGISTER PARAMETER DESCRIPTION OF CONTENT TYPE FORMAT NUMBER OF OF DATA 32 to 35 Sensor model Part one of sensor description R String 36 to 39 Sensor model Part two of sensor description R String 40 Sensor serial number R U16 41 42 Sensor sensitivity In x 10 V W m72 R Float 43 Response time Inx0O 1s R U16 44 Sensor resistance Inx 0 1 Q R U16 45 Reserved Always 0 R U16 46 47 Sensor calibration date Calibration date of the sensor R U32 in YYYYMMDD 48 to 60 Factory use 61 Firmware version R U16 62 Hardware version R U16 Register 32 to 39 Sensor model String of 8 registers This register will return 8 numbers which correspond with ASCII characters Register 40 Sensor serial number Register 41 42 Sensor sensitivity the sensitivity of the sensor in x 10 V W mz2 Format of data is float Register 43 Response time the response time of the se
81. xceeds 120 W m ref WMO SR20 D2 manual v1507 64 71 Hukseflux Thermal Sensors 9 10 Appendix on floating point format conversion For efficient use of microcontroller capacity some registers in the SR20 D2 contain data in a float or floating point format In fact a floating point is an approximation of a real number represented by a number of significant digits mantissa and an exponent For implementation of the floating point numbers Hukseflux follows the IEEE 754 standard In this example the floating point of register 41 and 42 is converted to the decimal value it represents In the Sensor Manager software and other Modbus tools floating point data will be converted to decimal data automatically Example of the calculation of register 41 42 representing a floating point for the sensitivity of the sensor which is 15 14 Data in register 41 16754 MSW Data in register 42 15729 LSW Double word MSW x 27 LSW so 16754 x 2 15729 1098005873 According to IEEE 754 Sign bit 1098005873 lt 2147483647 so sign bit 1 The number 2147483647 is defined by IEEE 754 Exponent 1098005873 2 130 digits after the decimal point are ignored 130 127 3 so exponent 3 The number 127 is a constant defined by IEEE 754 Mantissa 130 x 2 1090519040 1098005873 1090519040 7486833 7486833 2 0 8925 According to IEEE 754 1 has to be added to get mantissa 0 8925 1 1 8925 so manti
82. y of the measurement e related to the reliability of the pyranometer and its design manufacturing calibration hardware reliability e related to the reliability of the measurement uncertainty measurement reliability which involves hardware reliability as well as condition of use Most of the hardware reliability is the responsibility of the instrument manufacturer The reliability of the measurement however is a joint responsibility of instrument manufacturer and user As a function of user requirements taking into account measurement conditions and environmental conditions the user will select an instrument of a certain class and define maintenance support procedures In many situations there is a limit to a realistically attainable accuracy level This is due to conditions that are beyond control once the measurement system is in place Typical limiting conditions are e the measurement conditions for instance when working at extreme temperatures when the instrument temperature is at the extreme limits of the rated temperature range e the environmental conditions for instance when installed at a sub optimal measurement location with obstacles in the path of the sun e other environmental conditions for instance when assessing PV system performance and the system contains panels at different tilt angles the pyranometer measurement may not be representative of irradiance received by the entire PV system The measurement reli

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