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1. 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 removal spanner size 20 mm tooling required for wire fixation and removal internal wiring inside SR25 body screwdriver blade width 2 mm 8 4 Appendix on spare parts for SR25 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 SR25 cable with connector specify length in multiples of 5 m e O ring SR25 NOTE Outer dome level and sensor of SR25 cannot be supplied as spare parts In SR25 manual v1508 case of possible damage to the SR25 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 37 47 Hukseflux Thermal Sensors 8 5 Appendix on standards for classification and calibration Both ISO and ASTM have standards on instrument classification and methods of calibration The World Meteorological Organisation WMO has largely adopted the ISO classification system Table 8 5 1 Pyranometer standardis
2. 0 SR25 heated SR25 unheated SR20 ventilated SR20 unheated Figure 8 1 3 2 Average reduction of data availability due to dew in minutes per day for various sensor configurations SR25 with heating had no reduction in data availability due to dew SR25 manual v1508 35 47 Hukseflux Thermal Sensors 8 2 Appendix on cable extension replacement The sensor cable of SR25 is equipped with a M16 straight connector In case of cable 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 himself Please note that Hukseflux does not provide support for DIY connector and cable assembly SR25 is equipped with one cable Keep the distance between data logger or amplifier and sensor as short as possible Cables act as a source of distortion by picking up capacitive noise In an electrically quiet environment the SR25 cable can be extended without problem to 100 metres If done properly the sensor signal although small will not significantly degrade because the sensor resistance is very low so good immunity to external sources and because there is no current flowing so no resistive losses Connector cable and cable connection specifications are summarised below Table 8
3. EN 61000 3 3 1995 Al 2001 A2 2005 jo o O lt I N Kees VAN DEN BOS Director Delft May 04 2015 SR25 manual v1508 45 47 2015 Hukseflux Thermal Sensors B V www hukseflux com Hukseflux Thermal Sensors B V reserves the right to change specifications without notice
4. Table 8 9 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 percentage change in sensitivity per year The dependence of ISO change per sensitivity resulting from ageing effects which is a measure of the 9060 y
5. 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 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 lowe
6. 2 1 Preferred specifications for SR25 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 Cable 8 wire shielded with copper conductors at Hukseflux 8 wire shielded cable is used of which 2 wires are used for signal transmission 2 for heating and 2 to 4 for the temperature sensor Conductor resistance lt 0 1 Q m Length cables should be kept as short as possible in any case the total cable length should be less than 100 m Outer sheath with specifications for outdoor use for good stability in outdoor applications SR25 manual v1508 36 47 Hukseflux Thermal Sensors 8 3 Appendix on tools for SR25 Table 8 3 1 Specifications of tools for SR25
7. 5 0 to 90 at 1000 W m ADDITI ONAL WMO SPECIFICATI ONS WMO CLASS HIGH OUALITY 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 category of instrument SR25 manual v1508 41 47 Hukseflux Thermal Sensors 8 9 Appendix on definition of pyranometer specifications
8. C Sensor resistance range 100 to 200 Q Required sensor power zero passive sensor 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 3x 10 m Chassis connector M16 panel connector male thread 10 pole 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 hum
9. 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 guality 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 6 lifetime judge if the instrument should be reliable for another 2 years assessment or if it should be replaced 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 standar
10. e Longer cable in multiples of 5 m Specify total cable length e Internal temperature sensor This can be either a Pt100 or a 10 kQ thermistor Specify respectively T1 or T2 e Five silica gel bags in an air thight bag for SR25 desiccant holder Specify order number DCO1 Supply of products is subject to Hukseflux General Conditions of Sale The product warranty involving repair or replacement without charge for product or working hours is 24 months Hukseflux does not accept any liability for losses or damages related to use of the supplied products See the appendix and Hukseflux General Conditions of Sale for detailed statements on warranty and liability 1 2 Included items Arriving at the customer the delivery should include e pyranometer SR25 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 and directional response test e any other options as ordered Please store the certificates in a safe place SR25 manual v1508 8 47 Hukseflux Thermal Sensors 1 3 Quick instrument check A quick test of the instrument can be done by using a simple hand held multimeter and a lamp 1 Check the electrical resistance of the sensor between the green and white wire Use a multimeter at the 1000 Q range Measure the sensor resistance first with one polarity than
11. 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 possibly 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 instru
12. local climate and system efficiency between different sites These data can also be compared to measurements by local meteorological stations 6 3 Speed of repair and maintenance instrument lifetime 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 Factory warranty granting free of charge repair for defects that are clearly traceable to errors in production is 2 years Hukseflux pyranometers are designed to be suitable for the intended use for at least 5 years under normal meteorological conditions Hukseflux quality management s interpretation of service records is that the Mean Time Between Failure MTBF of Hukseflux solar and infra red radiation sensors is larger than 6 years only defects and large repairs are counted as failures T
13. 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 SR25 manual v1508 27 47 Hukseflux Thermal Sensors 7 Maintenance and trouble shooting 7 1 Recommended maintenance and quality assurance SR25 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 7 1 1 Recommended maintenance of SR25 If possible the data analysis and cleaning 1 and 2 should be done on a daily basis MINIMUM RECOMMENDED PYRANOMETER MAINTENANCE INTERVAL SUBJECT 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 Analyse 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
14. perform division by the sensitivity to calculate the solar irradiance E U S Formula 0 1 Data acquisition input resistance gt 1x10 Q Open circuit detection WARNING open circuit detection should not be used unless this is done separately from the normal measurement by more than 5 times the sensor response time and with a small current only Thermopile sensors are sensitive to the current that is used during open circuit detection The current will generate heat which is measured and will appear as an offset SR25 manual v1508 22 47 Hukseflux Thermal Sensors 6 Making a dependable measurement 6 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 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
15. suppress dew and frost deposition maintaining its high measurement accuracy Patents on the SR25 working principle are pending When heating SR25 the data availability and accuracy are higher than when ventilating traditional pyranometers In addition SR25 needs very low power it only consumes 1 5 W compared to the usual 10 W for ventilation The low thermal offsets make SR25 very suitable for measuring diffuse radiation SR25 is available with analogue millivolt output and as SR25 D1 with digital output Modbus protocol SR25 measures the solar radiation received by a plane surface in W m from a 180 field of view angle SR25 offers the best measurement accuracy the specification limits of two major sources of measurement uncertainty have been greatly improved over competing pyranometers zero offset a and temperature response SR25 pyranometer can be employed outdoors under the sun as well as indoors with lamp based solar simulators Its orientation depends on the application and may be horizontal tilted for plane of array radiation or inverted for reflected radiation In combination with the right software also sunshine duration may be measured Using SR25 is easy It can be connected directly to commonly used data logging systems The irradiance E in W m is calculated by dividing the SR25 output a small voltage U by the sensitivity S The sensitivity is provided with SR25 on its calibration certificate The cen
16. FERENCE 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 SR25 manual v1508 39 47 Hukseflux Thermal Sensors 8 7 Appendix on meteorological radiation quantities A pyranometer measures irradiance The time integrated total is called radiant exposure In solar energy radiant exposure is often given in W h m Table 8 7 1 Meteorological radiation quantities as recommen
17. Once the thumb screw has 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 SR25 s sun screen SR25 manual v1508 20 47 Hukseflux Thermal Sensors 5 3 Electrical connection In order to operate a pyranometer should be connected to a measurement system typically a so called datalogger SR25 is a passive sensor that does not need any power Cables generally act as a source of distortion by picking up capacitive noise We recommend keeping the distance between a datalogger or amplifier and the sensor as short as possible For cable extension see the appendix on this subject Table 5 3 1 The electrical connection of SR25 versions T1 and T2 The heater is not necessarily used The temperature sensor is not necessarily used PIN WIRE SR25 T1 SR25 T2 2 Pt100 10 kQ thermistor 3 Pt100 10 kQ thermistor 6 Pt100 10 kQ thermistor 8 Grey Pt100 10 kQ thermistor 1 heater heater 4 heater heater 9 ground ground 7 White signal signal 5 Green signal signal Note 1 Pt100 s of version T1 may be connected in a 3 wire or 4 wire configuration Note 2 10k thermistors of version T2 are usually connected in a 2 wire configuration Note 3 the heater is not necessarily connected In cas
18. Thermal Sensors Hukseflux USER MANUAL SR25 Secondary standard pyranometer with sapphire outer dome Copyright by Hukseflux manual v1508 www hukseflux com info hukseflux com Thermal Sensors f Hukseflux Warning statements A Putting more than 12 Volt across the sensor wiring can lead to permanent damage to the sensor Do not use open circuit detection when measuring N the sensor output SR25 manual v1508 2 47 Hukseflux Thermal Sensors Contents Warning statements Contents List of symbols Introduction 1 AUNE BWNH U PW N N H W Ne co 00 Go 00 00 GD 00 09 00 W 00 W ONNNYNNAADAANUNNUARAARR AWWWNE EE AUNE FPrRPrFOONOUBWNFE N HO Ordering and checking at delivery Ordering SR25 Included items Ouick instrument check Instrument principle and theory Specifications of SR25 Specifications of SR25 Dimensions of SR25 Standards and recommended practices for use Classification standard General use for solar radiation measurement General use for sunshine duration measurement Specific use for outdoor PV system performance testing Specific use in meteorology and climatology Installation of SR25 Site selection and installation Installation of the sun screen Electrical connection Reguirements for data acguisition amplification Making a dependable measurement The concept of dependability Reliability of the measurement Speed of repair and maintenance instrument lifetime Uncert
19. ainty evaluation Maintenance and trouble shooting Recommended maintenance and guality assurance Trouble shooting Calibration and checks in the field Data guality assurance Appendices Appendix on heating SR25 Appendix on cable extension replacement Appendix on tools for SR25 Appendix on spare parts for SR25 Appendix on standards for classification and calibration Appendix on calibration hierarchy Appendix on meteorological radiation guantities Appendix on ISO and WMO classification tables Appendix on definition of pyranometer specifications Appendix on terminology glossary Appendix on conditions of sale warranty and liability EC declaration of conformity SR25 manual v1508 3 47 Hukseflux Thermal Sensors List of symbols Quantities Voltage output Sensitivity Sensitivity at reference conditions Temperature Electrical resistance Solar irradiance Solar radiant exposure Time in hours Temperature coefficient Temperature coefficient Temperature coefficient see also appendix 8 7 on meteorological guantities Subscripts Not applicable SR25 manual v1508 Symbol o oO TIMP AVUNC ow Unit V V W m V W m C Q W m W h m h 1 C 1 C 4 47 Hukseflux Thermal Sensors Introduction SR25 secondary standard pyranometer takes solar radiation measurement to the next level Using a sapphire outer dome it has negligible zero offsets SR25 is heated in order to
20. ated 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 SR25 manual v1508 26 47 Hukseflux Thermal Sensors 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 linearity 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
21. ation 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 Reference Pyrheliometers SR25 manual v1508 38 47 Hukseflux Thermal Sensors 8 6 Appendix on calibration hierarchy The World Radiometric Reference WRR is the measurement standard representing the SI unit o
22. d instrument indoors according to ISO 9847 or outdoors according to SO9846 SR25 manual v1508 28 47 Hukseflux Thermal Sensors 7 2 Trouble shooting Table 7 2 1 Trouble shooting for SR25 The sensor does not give any signal Check the electrical resistance of the sensor between the green and white wire Use a multimeter at the 1000 Q range Measure the sensor resistance first with one polarity than reverse the polarity Take the average value The typical resistance of the wiring is 0 1 Q m Typical resistance should be the typical sensor resistance of 100 to 200 Q plus 1 5 Q for the total resistance of two wires back and forth of each 5 m Infinite resistance indicates a broken circuit zero or a low resistance indicates a short circuit Check if the sensor reacts to light put the multimeter at its most sensitive range of DC voltage measurement typically the 100 x 10 VDC range or lower Expose the sensor to strong light source for instance a 100 W light bulb at 1 x 107m distance The signal should read gt 2 x 10 V 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 V Check the data acquisition by applying a 1 x 10 V source to it in the 1 x 10 V range Check the condition of the connectors on chassis as well as the cable The sensor signal is unrealistically high or low Note
23. d agency of the United Nations It is the 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 SR25 manual v1508 18 47 Hukseflux Thermal Sensors 5 Installation of SR25 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 instr
24. ded 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 EI downward irradiance EJ E E W m HI downward radiant exposure H H Hi J m for a specified time interval Et upward irradiance Et Est E W m Ht upward radiant exposure Ht Hat HT 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 Egl h global irradiance Ey E cos On W m GHI Global hemispherical irradiance on Egl Horizontal a specified in this case Irradiance horizontal surface Egi global irradiance E E cos 6 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 TJ apparent surface 2C or K temperature Tf apparent sky 2C or K temperature SD sunshine duration h 8 is the apparent solar zenith angle Oh relative to horizontal 6 relative to a tilted surface g global long wave t tilted h horizontal distinction horizontal and
25. e it is connected the polarity of the connection is not important Note 4 signal wires are insulated from ground wire and from the sensor body Insulation resistance is tested during production and larger than 1 x 10 Q Note 5 ground is connected to the connector the sensor body and the shield of the wire Housing Thermopile Temperature sensor Heater tf Figure 5 3 1 Electrical diagram of the internal wiring of SR25 The shield is connected to the sensor body SR25 manual v1508 21 47 Hukseflux Thermal Sensors 5 4 Requirements for data acquisition amplification The selection and programming of dataloggers is the responsibility of the user Please contact the supplier of the data acquisition and amplification equipment to see if directions for use with the SR25 are available In case programming for similar instruments is available this can typically also be used SR25 can usually be treated in the same way as other thermopile pyranometers Pyranometers usually have the same programming as heat flux sensors Table 5 4 1 Requirements for data acquisition and amplification equipment for SR25 in the standard configuration Capability to measure small voltage signals preferably 5 x 10 V uncertainty minimum requirement 20 x 10 V uncertainty valid for the entire expected temperature range of the acquisition amplification equipment Capability for the data logger or the software to store data and to
26. ear 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 x 10 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 horizontal 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 sen
27. ent system SR25 has an onboard heater and a temperature sensor Heating the sensor measuring the body temperature and using the correction of the temperature response all contribute to the dependability and accuracy of the measurement However also when not using these features SR25 still complies with the secondary standard reguirements The instrument should be used in accordance with the recommended practices of ISO IEC WMO and ASTM Table 3 1 1 Specifications of SR25 continued on next pages SR25 MEASUREMENT SPECIFICATI ONS LIST OF CLASSIFICATION CRITERIA OF ISO 9060 ISO classification ISO 9060 1990 secondary standard pyranometer WMO performance level WMO No 8 seventh edition 2008 high guality pyranometer Response time 95 3s Zero offset a response to 200 W m net thermal radiation 1 W m unventilated 1 W m ventilated Zero offset b response to 5 K h change in ambient temperature lt 2 W m 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 instrument report included Spectral selectivity lt 3 0 35 to 1 5 x 10 m Temperature response lt 1 10 to 40 C lt 0 4 30 to 50 C with correction in data processing Temperature response of individual instrument report included Ti
28. f irradiance It was introduced in order to ensure world wide homogeneity of solar radiation measurements and is in use since 1980 The WRR was determined from the weighted mean of the measurements of a group of 15 absolute cavity radiometers which were fully characterised It has an estimated accuracy of 0 3 The WMO introduced its mandatory use in its status in 1979 The world wide 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 http 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 8 6 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 the standard The working standard has traceability to WRR world radiometric reference CORRECTION OF WORKING STANDARD CALIBRATION TO STANDARDISED RE
29. he assumption in this analysis is that the technology of these radiometers is essentially the same irrespective of the exact model The product expected lifetime is defined as the minimum number of years of employment with normal level of maintenance support until the instrument is no longer suitable for its intended use cannot be repaired For pyranometers the product expected lifetime depends heavily on the environmental conditions Examples of environments with reduced expected lifetime are areas with high levels of air pollution SR25 manual v1508 25 47 Hukseflux Thermal Sensors and areas with high levels of salt in the air Both cause enhanced corrosion It is not possible to give a generally applicable statement about expected lifetime In Hukseflux experience it is not realistic to expect a lifetime longer than 10 years except in very dry environments such as very dry tropical or polar climates 6 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 participating 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 Measu
30. id IP protection class IP 67 SR25 manual v1508 14 47 Hukseflux Thermal Sensors Table 3 1 1 Specifications of SR25 started on previous pages Gross weight including 5 m cable 2 05 kg Net weight including 5 m cable 0 85 kg Packaging HPRC casing of 255 x 225 x 165 mm HEATING Heater operation the heater is not necessarily switched on recommended operation is to continually power the heater see appendix 8 1 Required heater power 1 5 W at 12 VDC the heater is not necessarily active Heater resistance 95 Q Steady state zero offset caused by heating 0 to 1 5 W m 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 calibration 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 MEASUREMENT ACCURACY Uncertainty of the measurement statements about the overall measurement uncertainty can only be made o
31. ivery of goods is subject to Hukseflux General Conditions of Sale Hukseflux has the following warranty and liability policy Hukseflux guarantees the supplied goods to be new free from defects related to bad performance of materials and free from faults that are clearly related to production and manufacturing Warranty on products is valid until 24 months after transfer of ownership The warranty does not apply if the application involves significant wear and tear if it involves use outside the specified range of application or if it involves accidental damage or misuse The warranty expires when anyone other than Hukseflux makes modifications to or repairs the products Hukseflux is in no event liable for damages to its customers or anyone claiming through these customers associated to the goods or services it supplies SR25 manual v1508 44 47 Hukseflux Thermal Sensors 8 12 EC declaration of conformity We Hukseflux Thermal Sensors B V Delftechpark 31 2628 XJ Delft The Netherlands in accordance with the reguirements of the following directive 2004 108 EC The Electromagnetic Compatibility Directive hereby declare under our sole responsibility that Product model SR25 Type Pyranometer has been designed to comply and is in conformity with the relevant sections and applicable reguirements of the following standards Emission EN 61326 1 2006 Immunity EN 61326 1 2006 Emission EN 61000 3 2 2006 Emission
32. lt response lt 0 2 0 to 90 at 1000 W m For the exact definition of pyranometer ISO 9060 specifications see the appendix SR25 manual v1508 13 47 Hukseflux Thermal Sensors Table 3 1 1 Specifications of SR25 continued SR25 ADDITIONAL SPECIFICATIONS Measurand hemispherical solar radiation Measurand in SI radiometry units irradiance in W m Optional measurand sunshine duration Field of view angle 180 Measurement range 0 to 4000 W m Sensitivity range 7 to 25 x 105 V W m Sensitivity nominal 12 x 10 V W m Expected voltage output application under natural solar radiation 0 1 to 50 X 10 W Measurement function reguired programming E U S Optional measurement function reguired programming for correction of sensitivity as a function of instrument body temperature E U So a T 2 b T c Measurement function optional programming for sunshine duration programming according to WMO guide paragraph 8 2 2 Reguired readout 1 differential voltage channel or 1 single ended voltage channel input resistance gt 10 Q Internal temperature sensor measuring the body temperature version code T1 for Pt100 DIN class A version code T2 for thermistor 10 kQ at 25 C Optional readout 1 temperature channel in case the temperature sensor is used Rated operating temperature range 40 to 80
33. 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 SR25 manual v1508 23 47 Hukseflux Thermal Sensors 6 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 unreliability of the measurement e related to the reliability of the pyranometer and its design manufacturing calibration hardware reliability e related to the reliability of
34. ment Check the condition of the connectors on chassis as well as the cable The outer dome shows internal condensation In case there is a minor layer of moisture that is hardly visible replace the desiccant and wait a few days to see if the situation improves In case of condensation of droplets disassemble the instrument and dry out the parts The inner dome shows internal condensation Arrange to send the sensor back to Hukseflux for diagnosis SR25 manual v1508 29 47 Hukseflux Thermal Sensors 7 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 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 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
35. ments to correct deviations of more than 3 Lower deviations should be interpreted as acceptable and should not lead to a revised sensitivity SR25 manual v1508 30 47 Hukseflux Thermal Sensors 7 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 http www dqms com SR25 manual v1508 31 47 Hukseflux Thermal Sensors SR25 manual v1508 32 47 Hukseflux Thermal Sensors 8 Appendices 8 1 Appendix on heating SR25 SR25 was tested extensively before its release Hukseflux has tested both the nighttime offsets and the data availability of various pyranometer configurations The test results and recommendations regarding use of the internal heater are given in appendix 8 1 8 1 1 Recommendations on heater use in SR25 SR25 is equipped with an internal heater which is not necessarily connected The combined use of sapphire and internal heating keeps the outer dome dew and frost free This highly increases data availability while maintaining high measurement accuracy Therefore the general recommendation is t
36. n an individual basis See the chapter on uncertainty evaluation VERSIONS OPTIONS Digital output Modbus protocol option code D1 for specifications see the SR25 D1 user manual Longer cable in multiples of 5 m option code total cable length ACCESSORIES Ventilation unit VUO01 Separate amplifiers AC100 and AC420 Hand held read out unit L119 Bags of silica gel for desiccant set of 5 bags in an air tight bag option code DC01 SR25 manual v1508 15 47 Thermal Sensors f Hukseflux 3 2 Dimensions of SR25 M5 2x H M6 85 Figure 3 2 1 Dimensions of SR25 in x 10 m SR25 manual v1508 16 47 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 EOUIVALENT WMO ASTM STANDARD ISO 9060 1990 Not available WMO No 8 Guide to Solar energy specification and Meteorological Instruments classification of instruments for and Methods of Observation mea
37. ncertainty 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 SR25 secondary standard pyranometer extra desiccant in a set of 5 bags in an air tight bag is available SR25 manual v1508 24 47 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 addition 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
38. nighttime offset than the ventilated secondary standard pyranometers both when heated and when unheated SR25 manual v1508 34 47 Hukseflux Thermal Sensors 8 1 3 Heating and data availability SR25 heated N A E 500 pe z PA 4 E 2 Unheated secondary standard yb PA e pyranometer 4 oO i Pa clear sky value n 6 00 9 00 12 00 time hh mm Figure 8 1 3 1 Morning solar radiation measurement data comparing clear sky value with SR25 heated suppressing dew deposition successfully and one unheated secondary standard pyranometer SR25 heated follows the ideal clear sky solar radiation while the unheated secondary pyranometer is covered with dew and deviates from ideal During a period of 26 days in spring 2015 the reduction in data availability due to dew deposition on domes was monitored for various sensor configurations The Hukseflux outdoor test facility was used for this experiment SR25 with heating had no reduction in data availability due to dew SR20 ventilated with VUO1 had 1 one dew event The unheated SR25 performs better than SR20 in this dew experiment This can be explained by the better thermal coupling between body and dome in SR25 During clear nights the temperature of the outer dome of SR25 is higher than the temperature of the outer dome of SR20 making it less susceptible to dew a 80 2 e 70 s o 60 5 f 50 8B Ue 40 Lo 2 30 E v 20 O o 10 gt
39. o continually apply 12 V to the heater of SR25 with a heating power of 1 5 W When the lowest possible zero offset a is required for example for diffuse measurements SR25 can be used without heating a Figure 8 1 1 1 Heating of SR25 improves data availability it keeps the outer dome free of dew and frost deposition This figure shows a heated SR25 left and an unheated pyranometer without sapphire dome right on a frosty morning SR25 manual v1508 33 47 Hukseflux Thermal Sensors 8 1 2 Heating and accuracy During a 26 day period the nighttime offset of various pyranometer configurations was monitored Looking at nighttime offsets the performance of both the heated and the unheated SR25 is better than that of all other measured pyranometer configurations SR25 unheated e SR25 heated offset W m2 m SR20 unheated opremium brand A unheated secondary net longwave radiation W m2 standard pyranometer Figure 8 1 2 1 nighttime offsets of unventilated pyranometers versus net longwave radiation SR25 has lower nighttime offset than the other secondary standard pyranometers both when heated and when unheated 100 A o P60 4 O 08 e SR25 unheated e SR25 heated m SR20 ventilated offset W m2 opremium brand A ventilated secondary standard pyranometer net longwave radiation W m Figure 8 1 2 2 nighttime offsets of ventilated pyranomers and SR25 versus net longwave radiation SR25 has lower
40. process Table 6 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 equator 2 6 1 9 1 7 pole 7 9 5 6 4 5 winter mid latitude 3 4 2 5 2 1 first class summer mid latitude 4 7 3 3 3 4 eguator 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 eguator 7 8 5 5 5 3 pole 29 5 21 6 18 0 winter mid latitude 11 4 8 1 9 9 6 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
41. r desiccant holder levelling feet bubble level connector SR25 manual v1508 10 47 Hukseflux Thermal Sensors SR25 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 m 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 coating 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 ab
42. r 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 exceeds 120 W m ref WMO SR25 manual v1508 43 47 Hukseflux Thermal Sensors 8 11 Appendix on conditions of sale warranty and liability Del
43. rement or GUM 6 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 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 calcul
44. reverse the polarity Take the average value The typical resistance of the wiring is 0 1 m Typical resistance should be the typical sensor resistance of 100 to 200 Q plus 1 5 Q for the total resistance of two wires back and forth of each 5 m Infinite resistance indicates a broken circuit zero or a low resistance indicates a short circuit 2 Check if the sensor reacts to light put the multimeter at its most sensitive range of DC voltage measurement typically the 100 x 10 VDC range or lower 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 2 x 10 V 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 O V 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 SR25 manual v1508 9 47 Thermal Sensors 4 Hukseflux 2 Instrument principle and theory Figure 2 1 Overview of SR25 cable standard length 5 metres optional longer cable fixation of sun screen thumb screw glass inner dome thermal sensor with black coating sapphire outer dome sun screen humidity indicato
45. shine 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 SR25 is very well applicable in outdoor PV system performance testing See also Hukseflux model SR25 D1 Digital secondary standard pyranometer with sapphire outer dome and SR20 D1 Digital secondary standard pyranometer Modbus protocol 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 specialise
46. sitivity 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 SR25 manual v1508 42 47 Hukseflux Thermal Sensors 8 10 Appendix on terminology glossary Table 8 10 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 diffuse component of the solar radiation ref WMO Hukseflux Hemispherical solar radiation solar radiation received by a plane surface from a 180
47. sorption 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 asapphire outer dome The high thermal conductivity of the sapphire outer dome ensures excellent thermal coupling between body and outer dome even when the pyranometer is heated As a result both zero offset a and heating offset are very low e asecond inner dome made of glass 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 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 eguilibrium between the sensor and inner dome compared to using a single dome The effect of having a second dome is a further 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 classification 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
48. suring 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 HEMI SPHERI CAL SOLAR RADIATI ON ISO STANDARD EOUIVALENT 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 SR25 manual v1508 17 47 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 recommendations for instrument use in sun
49. that night time signals may be negative down to 5 W m on clear windless nights due to zero offset a Check if the pyranometer has clean domes Check the location of the pyranometer are there any obstructions that could explain the measurement result Check the orientation levelling of the pyranometer Check if the right calibration factor is entered into the algorithm Please note that each sensor has its own individual calibration factor as documented in its calibration certificate Check if the voltage reading is divided by the calibration factor in review of the algorithm Check the condition of the wiring at the logger Check the cable condition looking for cable breaks Check the condition of the connectors on chassis as well as the cable Check the range of the data logger signal can be negative this could be out of range or the amplitude could be out of range Check the data acquisition by applying a 1 x 10 V source to it in the 1 x 10 V range Look at the output Check if the output is as expected Check the data acquisition by short circuiting the data acquisition input with a 100 Q resistor Look at the output Check if the output is close to 0 W m The sensor signal shows unexpected variations Check the presence of strong sources of electromagnetic radiation radar radio Check the condition of the shielding Check the condition of the sensor cable Check if the cable is not moving during the measure
50. 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 reliability 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 u
51. tilted from Hukseflux T symbols introduced by Hukseflux contributions of Eg and E are Eg and E f both corrected for the tilt angle of the surface SR25 manual v1508 40 47 Hukseflux Thermal Sensors 8 8 Appendix on I SO and WMO classification tables Table 8 8 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 15 s 30 s 60 s 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 350 to 1500 x 10 m 3 5 10 WMO 300 to 3000 x 10 m Temperature response interval of 50 K 2 4 8 Tilt response 0 5 2
52. to secondary standard the achievable accuracy improves by a factor 2 SR25 manual v1508 11 47 Hukseflux Thermal Sensors 1 2 0 wt ee ce 1 solar radiation Ys C S ja 0 8 s 5 pyranometer geg 0 6 response v n ow 0 Y a 0 4 Te 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 J m North se 2 an 4 gt I 2 o r gt 2 2 East Bi T oO ea ee DE omaa ia ee ee aa 0 E 3 South 0 Z K 80 GE 9 Se eee ss 5 HTa West u S k S 2 t N gt ISO secondary a N standard L directional N response limit 4 zenith angle Figure 2 3 Directional response of a SR25 pyranometer of 4 azimuth angles compared to secondary standard limits SR25 manual v1508 12 47 Hukseflux Thermal Sensors 3 Specifications of SR25 3 1 Specifications of SR25 SR25 is a pyranometer of the highest category in the ISO 9060 classification system secondary standard It measures the solar radiation received by a plane surface from a 180 field of view angle This quantity expressed in W m is called hemispherical solar radiation Working completely passive using a thermopile sensor SR25 generates a small output voltage proportional to this flux It can only be used in combination with a suitable measurem
53. tral equation governing SR25 is E U S Formula 0 1 Figure 0 1 SR25 secondary standard pyranometer with sapphire outer dome SR25 manual v1508 5 47 Hukseflux Thermal Sensors SR25 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 This information can be used for further reduction of temperature dependence during post processing In case the sensitivity is corrected for the instrument body temperature the optional measurement eguation becomes E U So a T b T c Formula 0 2 The temperature coefficients a b and c can be found on the calibration certificate of each instrument SR25 has the following distinguishing features and benefits e sapphire outer dome negligible zero offsets e internal heater because of dew and frost suppression by heating better data availability and accuracy than ventilated instruments e 1 5 W very low power consumption e test certificates for temperature response and directional response included all sensors tested individually for ISO 9060 compliance The instrument should be used in accordance with the recommended practices of ISO WMO and ASTM The ASTM E2848 Standard Test Method for Reporting Photovoltaic Non Concentrator System Performance issued end 2011 confirms that a pyranometer is the preferred instr
54. ument connection from below under 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 SR25 manual v1508 19 47 Hukseflux Thermal Sensors 5 2 Installation of the sun screen SR25 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
55. ument for PV system performance monitoring SR25 pyranometer complies with the requirements of this standard For more information see our pyranometer 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 SR25 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 Suggested use for SR25 e all situations where ventilated pyranometers are employed e PV system performance monitoring e indoor PV testing with solar simulators e airborne measurements e diffuse measurements e environments with dew e environments with frost SR25 manual v1508 6 47 Hukseflux Thermal Sensors Model SR25 D1 outputs irradiance digitally SR25 D1 uses a high end A D converter and the industry standard Modbus RTU over 2 wire RS 485 communication protocol This user manual covers SR25 use Specifications of model SR25 D1 the secondary standard pyranometer with digital output differ from those of SR25 For SR25 D1 use please consult the SR25 D1 user manual SR25 manual v1508 7 47 Hukseflux Thermal Sensors 1 Ordering and checking at delivery 1 1 Ordering SR25 The standard configuration of SR25 is with 5 metres cable Common options are

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