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1. IR20 manual v1403 17 41 Thermal Sensors Hukseflux 3 2 Dimensions of IR20 Figure 3 2 1 Dimensions of IR20 and IR2OWS in 10 m IR20 manual v1403 18 41 Hukseflux Thermal Sensors 4 Standards and recommended practices for use Pyrgeometers are not subject to standardisation The World Meteorological Organization WMO is a specialised 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 paragraph 7 4 covers measurement of total and long wave radiation For ultra high accuracy measurements the following manual may serve as a reference Baseline Surface Radiation Network BSRN Operations Manual Version 2 1 L J B McArthur April 2005 WCRP 121 WMO TD No 1274 This manual also includes chapters on installation and calibration IR20 manual v1403 19 41 Hukseflux Thermal Sensors 4 1 Site selection and installation Table 4 1 1 Recommendations for installation of pyrgeometers Location the horizon should be as free from obstacles as possible Ideally there should be sources of longwave irradiance between the course of the sun and the instrument only free sky Mechanical mounting thermal insulation preferably use connection by bolts to the bottom plate of the instrument A pyrgeometer is sensitive to the
2. Measurand longwave radiation Measurand in SI radiometry units longwave irradiance in W m Optional measurand sky temperature Optional measurand surface temperature IR20 VERSUS I R20WS SPECTRAL RANGE amp USE Spectral range IR20 4 5 to 40x10 m nominal see product certificate for individual value Spectral range IR2OWS 1 0 to50x10 m based on typical material properties only IR2OWS restrictions for use only in the absence of solar radiation Solar offset IR20 only not specified for IR2OWS lt 10 W m at 1000 W m global horizontal irradiance on the dome MAIN SPECI FI CATI ONS Field of view angle 180 Response time 95 3s Sensitivity nominal 17 x 10 V W m Sensitivity range 10 to 25 x 10 V W m Rated operating temperature range 40 to 80 C Temperature dependence lt 0 4 30 to 50 C using the measurement function Temperature sensor 10 kQ thermistor Required sensor power zero passive sensor Heater 12 VDC 1 5 W see below for details Standard cable length 5m IR20 manual v1403 15 41 Hukseflux Thermal Sensors Table 3 1 1 Specifications of IR20 and IR2OWS continued ADDI TI ONAL SPECI FI CATI ONS Zero offset b response to 5 K h change in ambient temperature lt 2 W m Non stability lt 1 change per year Non linearity
3. lt 0 5 100 to 300 W m relative to 200 W m sensor to source exchange Measurement range 1000 to 1000 W m sensor to source exchange U So a T _ b T Tilt dependence lt 0 5 0 to 90 at 300 W m Sensor resistance range 300 to 500 Q Expected voltage output Application for outdoor measurement of downward longwave irradiance 7 5 to 7 5 x 10 V Measurement function required programming E U So a T2 b T c o T 273 15 Measurement function optional programming for sky temperature Tey E o 273 15 Measurement function optional programming for surface temperature T surface E t o 4 273 15 Required readout 1 differential voltage channel or 1 single ended voltage channel input resistance gt 10 Q 1 temperature channel STANDARDS Standard governing use of the instrument WMO No 8 Guide to Meteorological Instruments and Methods of Observation seventh edition 2008 paragraph 7 4 measurement of total and long wave radiation MOUNTING CABLING TRANSPORT 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 diameter 3 to 6 x
4. IR20 manual v1403 7 41 Hukseflux Thermal Sensors 1 Ordering and checking at delivery 1 1 Ordering IR20 The standard configuration of IR20 is with 5 metres cable and a connector Common options are e Longer cable in multiples of 5 m Specify total cable length e Five silica gel bags in an air thight bag for IR20 desiccant holder Specify order number DCO1 e Optional calibration to blackbody ITS 90 e IR2OWS for the special wide spectrum model of IR20 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 pyrgeometer IR20 or IR2OWS e cable of the length as ordered with connector e sun screen e product certificate matching the instrument serial number e calibration certificate matching the instrument serial number e temperature dependence report e any other options as ordered Please store the certificates in a safe place IR20 manual v1403 8 41 Hukseflux Thermal Sensors 1 3 Ouick instrument check A guick test of the instrument can be done by using a simple hand held multimeter and a
5. such as temperature position of the sun presence of clouds horizon representativeness of the location Therefore 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 inverted 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 IR20 manual v1403 24 41 Hukseflux Thermal Sensors 5 2 Reliability of the measurement A measurement is reliable if it measures within reguired uncertainty limits for most of the time We distinguish between two causes of unreliability of the measurement e related to the reliability of the pyrgeometer 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
6. thermal source 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 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 300 to 500 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 heat put the multimeter at its most sensitive range of DC voltage measurement typically the 100 x 10 VDC range or lower Make sure that the sensor is at 20 C or lower Expose the sensor to a strong heat source at a short distance from the window of more than 50 C for instance a hot cup of coffee The signal should read positive and gt 1 x 10 V now In case of using your hand as a heat source the signal should be significantly lower 3 Remove the sun screen see chapter on installation of the sun screen Inspect the bubble level 4 Check the electrical resistance of the thermistor This should be in the 10 Q range 5 Check the electrical resistance of the heater This should be in the 100 Q range 6 Inspect the instrument for any damage 7 Inspect if the humidity indicator is blue Blue indicates dryness The colour pink indicates it is humid in the l
7. 10 m special version Connector protection class IP 67 IP 69 K per EN 60 529 connected Cable diameter 5 3 x 107m Cable replacement replacement cables with connector can be ordered separately from Hukseflux Instrument 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 x 10 m 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 x 10 m IR20 manual v1403 16 41 Hukseflux Thermal Sensors Table 3 1 1 Specifications of IR20 and IR2OWS started on previous pages HEATI NG Heater operation the heater is not necessarily switched on recommended operation is to activate the heater when there is a risk of dew deposition Reguired heater power 1 5 W at 12 VDC Heater resistance 95 Q Steady state zero offset caused by 0 W m heating CALI BRATION Calibration traceability to WISG Optional traceability to blackbody ITS 90 Calibration hierarchy from WISG through Huksef
8. 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 Hukseflux specifies a calibration uncertainty of lt 6 k 2 IR20 manual v1403 34 41 Hukseflux Thermal Sensors 7 6 Appendix on meteorological radiation guantities A pyrgeometer measures longwave irradiance The time integrated total is called radiant exposure Table 7 6 1 Meteorological radiation guantities 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 E E E W m HL downward radiant exposure H H H J m for a specified time interval Et upward irradiance ET E t E t W m Ht upward radiant exposure Ht Hgt Hit Jim 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 Ep 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
9. 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 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 rad
10. E U S 14 k 0 T 273 15 kp o T 273 15 Formula 7 5 1 Pyrgeometers are calibrated by WRC in two ways by nighttime comparison to WISG to determine S A typical uncertainty of S is 4 2 by calibration under a blackbody to determine k and k2 There also is a third constant k3 correcting for the difference between dome temperature and thermopile temperature The Hukseflux measurement equation for IR20 is E U So a T2 b T c o T 273 15 Formula 0 1 In other words Hukseflux ignores k and assumes that k 1 It is Hukseflux opinion that by the design of the instrument the influence of these parameters is negligible IR20 manual v1403 33 41 Hukseflux Thermal Sensors Table 7 5 1 Calibration hierarchy for pyrgeometers WORKING STANDARD CALIBRATION AT PMOD WRC DAVOS Calibration of working standard pyrgeometers traceable to WISG A typical uncertainty of So is 4 2 k 2 CORRECTION OF WORKING STANDARD CALIBRATION TO STANDARDISED REFERENCE CONDITIONS Correction from test conditions of the standard to reference conditions No corrections are applied Reference conditions are horizontal mounting atmospheric longwave irradiance clear sky nights 20 C OUTDOOR PRODUCT CALIBRATION AT HUKSEFLUX Calibration of products i e pyrgeometers Outdoor side by side comparison to a reference pyrgeometer CALI BRATION UNCERTAINTY CALCULATION
11. The main factor in instrument non stability is the aging of the pyrgeometer solar blind filter For IR20WS this filter is not present For that reason we expect IR2OWS to have a better non stability e Errors due to the temperature measurement T For this a 10 kQ thermistor must be read out Required accuracy of the readout is 0 2 C which results in around 1 W m uncertainty of the irradiance measurement To this the uncertainty of the thermistor itself should be added In measurement of net radiation in case the upfacing and downfacing instruments are thermally coupled the temperature measurement and also its uncertainty cancel from the equation IR20 manual v1403 27 41 Hukseflux Thermal Sensors 6 Maintenance and trouble shooting 6 1 Recommended maintenance and quality assurance IR20 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 6 1 1 Recommended maintenance of IR20 If possible the data analysis and cleaning 1 and 2 should be done on a daily basis MINIMUM RECOMMENDED PYRGEOMETER MAI NTENANCE INTERVAL SUBJ ECT ACTION 1 1 week data analysis compare measured data to maximum
12. Thermal Sensors Hukseflux USER MANUALIR20 Research grade pyrgeometer Copyright by Hukseflux manual v1403 www hukseflux com info hukseflux com Thermal Sensors 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 AN the sensor output IR20 manual v1403 2 41 Hukseflux Thermal Sensors Contents Warning statements Contents List of symbols Introduction 1 5 WNb WN Ne NOUBWNE W Ne NNNNNNNNNN QDQQ OUUYUYNNADN NWWWNNNNN NN NEPPE AUNE OANADUBWNEH Ordering and checking at delivery Ordering IR20 Included items Quick instrument check Instrument principle and theory Pyrgeometer functionality Solar and longwave radiation IR20 pyrgeometer design Typical measurement results Optional heating Optional shading Use as a net radiation sensor Specifications of IR20 and IR2OWS Specifications of IR20 and IR2OWS Dimensions of IR20 Standards and recommended practices for use Site selection and installation Installation of the sun screen Electrical connection Requirements for data acquisition amplification Making a dependable measurement The concept of dependability Reliability of the measurement Speed of repair and maintenance instrument lifetime Uncertainty evaluation Maintenance and trouble shooting Recommended maintenance and quality assurance Tro
13. ance In case the two instruments are thermally coupled the body temperatures of the instruments are identical In that case the body temperature cancels from the equation for the net radiation However for calculation of sky temperature and surface temperature the instrument temperature still needs to be measured IR20 manual v1403 14 41 Hukseflux Thermal Sensors 3 Specifications of IR20 and IR2ZO0WS 3 1 Specifications of IR20 and IR2OWS IR20 research grade pyrgeometer measures the longwave irradiance received by a plane surface in W m from a 180 field of view angle In meteorological terms IR20 measures downward and upward longwave irradiance Working completely passive using a thermopile sensor IR20 generates a small output voltage proportional to the radiation balance between the instrument and the source it faces It can only be used in combination with a suitable measurement system The instrument is not subject to classification It should be used in accordance with the recommended practices of WMO IR20 measures during both day and night In the absence of solar radiation IR20WS offers a higher accuracy because of its wider spectral range For ultra high accuracy measurements the user should consider to use the incorporated heater and should consider shading the instrument during daytime Table 3 1 1 Specifications of IR20 and IR2ZO0WS continued on next pages IR20 amp IR2OWS SPECIFICATIONS MEASURANDS
14. atter case replace the desiccant see chapter on maintenance IR20 manual v1403 9 41 Hukseflux Thermal Sensors 2 Instrument principle and theory 2 1 Pyrgeometer functionality IR20 s scientific name is pyrgeometer IR20 measures the longwave or far infra red FIR radiation received by a plane surface in W m from a 180 field of view angle In meteorological terms pyrgeometers are used to measure downward and upward longwave irradiance WMO definition As secondary measurands the sky temperature T xy and the equivalent surface ground temperature T surface Can be measured Both are so called equivalent blackbody radiative temperatures i e temperatures calculated from the pyrgeometer measurement assuming these are uniform temperature blackbodies with an emission coefficient of 1 2 2 Solar and longwave radiation Longwave radiation is the part of the radiation budget that is not emitted by the sun The spectral range of the longwave radiation is not standardised A practical cut on is in the range of 4 to 5 x 10 m see figure 2 2 1 In meteorology solar and longwave radiation are typically measured as separate parameters The instrument to measure solar radiation is called pyranometer _ 1 000 E E z mmm dJownwelling 5 0 100 longwave solar el x o u c S ae 0 010 A T 5 u o un 0 001 7 i 1 10 100 wavelength x 10 6 m Figure 2 2 1 Atmospheric radiat
15. cally quiet environment the IR20 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 7 1 1 Preferred specifications for IR20 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 10 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 se
16. cquisition by applying a 1 x 10 V source to it in the 1x10 V range The sensor signal is unrealistically high or low Check if the measurement function including the constant a b and c has been implemented properly Please note that each sensor has its own individual calibration factor and constants as documented in its production certificate Check if the pyrgeometer has a clean dome Check the location of the pyrgeometer are there any obstructions sources that could explain the measurement result Check the condition of the wiring at the logger Check the cable condition looking for cable breaks Check the range of the data logger signal is usually 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 to 1000 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 etc Check the condition of the shielding Check the condition of the sensor cable Check if the cable is not moving during the measurement The instrument shows internal condensation Replace the desiccant and wait a few days to see if the sit
17. ction of solar irradiance The solar offset can serve as a quality indicator of the pyrgeometer filter condition 6 4 Data quality assurance Quality assurance can be done by e analysing trends in longwave irradiance signal e plotting the measured irradiance against mathematically generated expected values e comparing irradiance measurements between sites e analysis of daytime signals against solar irradiance 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 IR20 manual v1403 30 41 Hukseflux Thermal Sensors 1 Appendices 7 1 Appendix on cable extension replacement The sensor cable of IR20 is eguipped 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 IR20 is eguipped 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 electri
18. ed IR20 can usually be treated in the same way as other thermopile pyrgeometers Table 4 4 1 Reguirements for data acguisition and amplification eguipment for IR20 in the standard configuration Capability to measure small voltage signals preferably better than 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 perform division by the sensitivity to calculate the longwave irradiance E U So a T2 b T c o T 273 15 Formula 0 1 see also optional measurands 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 Capability to measure temperature a 10 kQ thermistor must be read out Required accuracy of the readout is 0 2 C which results in around 1 W m uncertainty of the irradiance measurement Capability to power the heater OPTIONAL IR20 has a 12 VDC 1 5 W heater on board which may optionally be activated to keep the inst
19. ees VAN DEN BOS Director Delft March 20 2013 IR20 manual v1403 39 41 2014 Hukseflux Thermal Sensors B V www hukseflux com Hukseflux Thermal Sensors B V reserves the right to change specifications without notice
20. el 9 connector By definition a pyrgeometer should not measure solar radiation and in the longwave have 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 radiation hits the sensor perpendicularly normal to the surface 0 angle of incidence zero response when the source is at the horizon 90 angle of incidence 90 zenith angle and 50 of full response at 60 angle of incidence A pyrgeometer should have a so called directional response older documents mention cosine response that is as close as possible to the ideal cosine characteristic IR20 manual v1403 12 41 Hukseflux Thermal Sensors In order to attain the proper directional and spectral characteristics a pyrgeometer s main components are e a thermal sensor with black coating It has a flat spectrum covering the 0 3 to 50 x 10 m range and has a near perfect directional response The coating absorbs all longwave 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 irradiance exchange between sensor and source The sensor not only absorbs but also irradiates heat as a blackbody e a silicon dome This dome limits the spectral
21. erm research grade is used to indicate that this instrument has the highest attainable specifications IR20 manual v1403 32 41 Hukseflux Thermal Sensors 7 5 Appendix on calibration hierarchy Hukseflux pyrgeometers are traceable to the World Infrared Standard Group WISG WISG is composed of a group of pyrgeometers The calibration hierarchy of Hukseflux IR20 is from WISG through Hukseflux internal calibration procedure involving outdoor comparison to a reference pyrgeometer The WISG group of instruments is maintained by World Radiation Center WRC in Davos Switzerland An absolute sky scanning radiometer provides the absolute longwave irradiance reference Comparisons between the reference and the WISG are performed on a regular basis to maintain the WISG and supervise its long term stability It is essential that these intercomparisons take place under various sky conditions but the predominant condition is a clear sky which means that the validity of WISG calibration is a clear sky spectrum Typical exchange between pyrgeometer and sky is in the 70 to 120 W m At Hukseflux in an independent lab experiment the detector properties are determined as a function of temperature resulting in temperature coefficients a b and c WRC works with a measurement equation involving additional constants k which corrects for thermal resistance of the thermopile sensor k2 which corrects for emissivity that does not equal 1
22. fers even better accuracy because of its wider spectral range IR20 measures the longwave or far infra red radiation received by a plane surface in W m from a 180 field of view angle In meteorological terms pyrgeometers are used to measure downward and upward longwave irradiance WMO definition Longwave radiation is the part of radiation that is not emitted by the sun The spectral range of longwave radiation is not standardised A practical cut on is in the range of 4to5x10 m IR20 has a dome with a solar blind filter with a cut on at 4 5 x 10 m making it suitable for day and night observations Model IR2OWS has a wide spectral range with a cut on at 1 0 x 10 m It offers a superior accuracy during night time when solar radiation is absent The main purpose of a pyrgeometer is to measure longwave radiation As secondary measurands the sky temperature T xy and the equivalent surface temperature T surface can be measured Both are so called equivalent blackbody temperatures i e temperatures calculated from pyrgeometer data assuming the source behaves as a blackbody with an emission coefficient of 1 Using IR20 is easy It can be connected directly to commonly used data logging systems The irradiance in W m is calculated by dividing the R20 output a small voltage by the sensitivity and taking in account the irradiated heat by the sensor itself Planck s law The sensitivity is provided with IR20 on its calibration cer
23. flux 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 IR20 manual v1403 38 41 Hukseflux Thermal Sensors 7 9 EC declaration of conformity We Hukseflux Thermal Sensors B V Elektronicaweg 25 2628 XG 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 IR20 and IR2OWS Type Pyrgeometer 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 EN 61000 3 3 1995 Al 2001 A2 2005 Ae Oo O D N K
24. g platforms such as aircraft and buoys e on board heater Heating prevents condensation of water on the pyrgeometer dome which when occurring leads to very large measurement errors e instrument cut on wavelength 5 and the two 50 transmission points are displayed on the product certificate for individual sensors E IR20 fisen 4006 t 16 V W m pes Sen 3 z 8 7110 b 2 26x10 c 0 9582 cf ia gt Figure 0 1 IR20 research grade pyrgeometer with its sun screen removed More about the instrument principle theory and specifications can be found in the following chapters IR20 manual v1403 6 41 Hukseflux Thermal Sensors Pyrgeometers are not subject to a classification standard Calibration of pyrgeometers is usually traceable to the World Infrared Standard Group WISG This calibration takes into account the spectral properties of typical downward longwave radiation As an option calibration can be made traceable to a blackbody and the International Temperature Scale of 1990 ITS 90 This alternative calibration is appropriate for measurements of upward longwave radiation IR20 facing down See the specific paragraph in this manual about calibration and uncertainty assessment for more information This manual is intended for users of both IR20 and IR20WS The specifications of IR2OWS are identical to IR20 s except for its spectral range Figure 0 1 IR2OWS research grade pyrgeometer
25. iation from the universe is involved passing through the atmospheric window In case of upwelling E t composed of longwave 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 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 radiati
26. ion as a function of wavelength plotted along two logarithmic axes to highlight the longwave radiation Longwave radiation is mainly present in the 4 to 50 x 10 m range whereas solar radiation is mainly present in the 0 3 to 3 x 10 m range In practice the two are measured separately using pyrgeometers and pyranometers IR20 manual v1403 10 41 Hukseflux Thermal Sensors In the longwave spectrum the sky can be seen as a temperature source colder than ground level ambient air temperature with its lowest temperatures at zenith getting warmer closer to ambient air temperature at the horizon The uniformity of this longwave source is much better than that in the range of the solar spectrum where the sun is a dominant contributor The eguivalent blackbody temperature as a function of zenith angle roughly follows the same pattern independent of the exact sky condition cloudy or clear This explains why for pyrgeometers the directional response is not very critical The downwelling longwave radiation essentially consists of several components 1 low temperature radiation from the universe filtered by the atmosphere The atmosphere is transparent for this radiation in the so called atmospheric window roughly the 10 to 15 x 10 m wavelength range 2 higher temperature radiation emitted by atmospheric gasses and aerosols 3 in presence of clouds or mist the low temperature radiation from the universe is almost completely bloc
27. ked by the water droplets The pyrgeometer then receives the radiation emitted by the water droplets The spectral distribution of longwave irradiance varies significantly as a function of the source composition A pyrgeometer is relatively insensitive to these variations but all the same blackbody calibration tends to differ from WISG calibration by up to 5 In addition there may be effects that are uncompensated for in the calibration for instance related to atmospheric water vapour content in the 5 to 10 W m range These effects are still under investigation by the international scientific community Comparison between IR20 and IR20WS may serve to investigate this effect Upwelling longwave irradiance is measured with downfacing instruments These are presumably looking directly at the surface absorption and emission of the atmosphere is low over a short distance of around 2 m which behaves like a normal blackbody Hukseflux suggests calibrating downfacing instruments against a blackbody rather than having WISG as a reference IR20 manual v1403 11 41 Hukseflux Thermal Sensors 2 3 IR20 pyrgeometer design Figure 2 3 1 Overview of IR20 pyrgeometer 1 cable standard length 5 metres optional longer cable 2 fixation of sun screen 3 dome with solar blind filter 4 sun screen 5 humidity indicator 6 desiccant holder 7 levelling feet 8 bubble lev
28. logger or amplifier and the sensor as short as possible For cable extension see the appendix on this subject Table 4 3 1 The electrical connection of IR20 The heater is not necessarily used The temperature sensor must be used PIN WIRE IR20 10 kQ thermistor 10 kQ thermistor 10 kQ thermistor Grey 10 kQ thermistor J e OD Ww N heater heater ground White signal u N O Note 1 10 kQ thermistors are usually connected in a 2 wire configuration Note 2 the heater is not necessarily connected In case it is connected the polarity of the connection is not important Note 3 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 4 ground is connected to the connector the sensor body and the shield of the wire Housing Thermopile Temperature sensor Heater A Figure 4 3 1 Electrical diagram of the internal wiring of IR20 The shield is connected to the sensor body IR20 manual v1403 22 41 Hukseflux Thermal Sensors 4 4 Reguirements for data acguisition amplification The selection and programming of dataloggers is the responsibility of the user Please contact the supplier of the data acguisition and amplification eguipment to see if directions for use with the IR20 are available In case programming for similar instruments is available this can typically also be us
29. lt in a lower sensitivity S than WISG traceable calibraton e Errors due to water deposition at clear nights these completely block the longwave irradiance exchange between pyrgeometer and may cause the signal U So9 a T b T c to change from a large negative value 100 W m to around 0 W m Water deposition at clear nights may largely be avoided by using the on board heater of IR20 e Errors due to solar offset which is of the order of 10 W m at 1000 W m global horizontal irradiance This offset can be reduced by around 60 by shading of the instrument typically by using a shading disk on a solar tracker This error is partially caused by heating of the dome partially by transmission of solar radiation by the dome filter combination This uncertainty is not taken into account in the WISG calibration of the reference instrument With IR20WS only measuring at night this uncertainty does not play a significant role e Errors due to the choice of the cut on wavelength of the pyrgeometer Depending on the atmospheric water content the pyrgeometer will block a variable percentage of the downward longwave irradiance This causes an uncertainty of the sensitivity So With IR20 this uncertainty is already taken into account in the WISG calibration of the reference instrument With IR2OWS this uncertainty does not play a significant role e Errors due to instrument non stability This is now estimated at lt 1 change per year
30. lux internal calibration procedure involving outdoor comparision to a reference pyrgeometer Calibration method outdoor comparison to a reference pyrgeometer Calibration uncertainty lt 6 k 2 Recommended recalibration interval 2 years Reference conditions horizontal mounting atmospheric longwave irradiance clear sky nights 20 C 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 Hukseflux recommends ITS 90 traceable calibration for upward longwave irradiance measurement Characterisation of the dependence of sensitivity to temperature temperature coefficients a b and c of the measurement equation are determined in an independent experiment and reported on the product certificate MEASUREMENT ACCURACY Uncertainty of the measurement statements about the overall measurement uncertainty can only be made on an individual basis See the chapter on uncertainty evaluation Achievable uncertainty 95 confidence level daily totals 8 Hukseflux own estimate VERSIONS OPTIONS Longer cable in multiples of 5 m option code total cable length Calibration optional to blackbody ITS 90 ACCESSORIES Bags of silica gel for desiccant set of 5 bags in an air tight bag option code DCO1
31. n 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 Optional shading for ultra high accuracy measurements the solar offset can be reduced by around 60 by shading which means preventing the direct radiation to reach the instrument Shading is typically done by using a shading disk on a solar tracker IR20 manual v1403 20 41 Hukseflux Thermal Sensors 4 2 Installation of the sun screen IR20 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 turned the sun screen loose the screen can be lifted off manually After removal the user may inspect the bubble level item 8 of the drawing and remove the cable connector item 9 Figure 4 2 1 Installation and removal of IR20 s sun screen IR20 manual v1403 21 41 Hukseflux Thermal Sensors 4 3 Electrical connection In order to operate a pyrgeometer should be connected to a measurement system typically a so called datalogger IR20 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 data
32. nsor 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 IR20 manual v1403 31 41 Hukseflux Thermal Sensors 7 2 Appendix on tools for IR20 Table 7 2 1 Specifications of tools for IR20 tooling reguired for sun screen fixation and removal by hand tooling reguired for bottom plate fixation and removal hex key 2 5 x 10 m tooling reguired for desiccant holder fixation and removal spanner size 20 x 10 m tooling reguired for wire fixation and removal internal wiring inside IR20 body screwdriver blade width 2 x 10 m 7 3 Appendix on spare parts for IR20 e Desiccant holder with glass window and rubber ring e Desiccant set of 5 bags in an air tight bag e Humidity indicator e Levelling feet set of 2 e Static foot e Sunscreen with metal ring and thumb screw e R20 cable with connector specify length in multiples of 5 m e O ring IR20 NOTE The dome of IR20 and IR2OWS cannot be supplied as spare part 7 4 Appendix on standards for classification and calibration Unlike pyranometers pyrgeometers are not subject to a system of classification At Hukseflux we distinguish between normal pyrgeometers like our model IRO2 and research grade pyrgeometers like IR20 and IR2OWS The t
33. o 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 pyrgeometer 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 Hukseflux flat window pyrgeometers for example model IR20 manual v1403 25 41 Hukseflux Thermal Sensors IRO2 this may slowly develop into a defect For model IR20 extra desiccant in a set of 5 bags in an air tight bag is available e One of the larger errors in the daytime measurement of downwelling longwave irradiance is the offset caused by solar radiation the solar offset Errors due to solar offset are of the order of 10 W m at 1000 W m global horizontal irradiance For ultra high accuracy measurements this offset can be reduced by around 60 by shading which means preventing the direct radiation to reach the instrument Shading is typically done by using a shading disk on a solar tracker Another way to improve measurement reliability is to introduce redundant sensors e The use of redundant instruments allo
34. on projected in the plane of the sensor surface Varies from 0 to 360 0 is by definition the cable exit direction also called north west is 90 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 IR20 manual v1403 36 41 Hukseflux Thermal Sensors WISG World Infra Red Standard Group Group of pyrgeometers maintained by PMOD Davos Switzerland that forms the reference for calibration of pyrgeometers WISG is traceable to international standards through an absolute sky scanning radiometer WISG has been formally recognised by the World Meteorological Organisation WMO as interim WMO Pyrgeometer Infrared Reference Sky equivalent blackbody radiative temperature of the sky i e the temperature temperature calculated from pyrgeometer data measuring downwelling longwave radiation assuming the sky behaves as a blackbody with an emission coefficient of 1 Surface equivalent blackbody radiative temperature of the surface i e the temperature temperature calculated from pyrgeometer data measuring upwelling longwave radiation assuming the ground behaves as a blackbody with an emission coefficient of 1 IR20 manual v1403 37 41 Hukseflux Thermal Sensors 7 8 Appendix on conditions of sale warranty and liability Delivery of goods is subject to Hukseflux General Conditions of Sale Hukse
35. possible maximum expected irradiance and to other measurements nearby redundant instruments Also historical seasonal records can be used as a source for expected values 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 cable glands inspect mounting position inspect cable clean instrument clean cable inspect levelling change instrument tilt in case this is out of specification inspect mounting connection 4 desiccant desiccant replacement if applicable Change in case the replacement humidity indicator shows more than 50 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 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 cable gland desiccant holder sun screen NOTE use Hukseflux approved parts only 8 internal if applicable open in
36. range from 1 0 to 40 x 10 m cutting off the part below 1 0 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 a solar blind interference coating deposited on the dome not for model IR2OWS this coating limits the spectral range It now becomes 4 5 to 40 x 10 m cutting off the part below 4 5 x 10 m Pyrgeometers can be manufactured to different specifications and with different levels of verification and characterisation during production Hukseflux also manufactures lower accuracy pyrgeometers see our pyrgeometer model IRO2 Model IR20 has a dome with a solar blind filter with a cut on at 4 5 x 10 m making it suitable for day and night observations Model IR2OWS has a wide spectral range with a cut on at 1 0 x 10 m It offers a superior accuracy under night time conditions when solar radiation is absent See also the appendix on uncertainty evaluation IR20 manual v1403 13 41 Hukseflux Thermal Sensors 2 4 Typical measurement results Please note that the signal generated by an upfacing pyrgeometer usually has a negative sign The most important factors determining downward longwave irradiance are e ambient air temperature e sky condition cloud cover e atmospheric moisture content Table 2 4 1 Expected pyrgeometer output U S at different ambient air temperatures Tambient and at diffe
37. rent cloud conditions Under clear sky conditions the U S is around 100 W m while under cloudy conditions it will be close to 0 W m Also calculated the sky temperature T xy and the longwave downward irradiance E EXPECTED PYRGEOMETER OUTPUT CONDITIONS Tambient Sky condition U So la T b T c Tsky E C cloudy clear IW m C W m 20 cloudy 0 20 232 20 clear sky 100 53 132 0 cloudy 0 0 314 0 clear sky 100 24 214 30 cloudy 0 30 477 30 clear sky 100 12 377 2 5 Optional heating A low power heater is located in the body of the pyrgeometer The heater is not necessarily switched on recommended operation is to activate the heater when there is a risk of dew deposition 2 6 Optional shading One of the larger errors in the daytime measurement of downwelling longwave irradiance is the offset caused by solar radiation the solar offset Errors due to solar offset are of the order of 10 W m at 1000 W m global horizontal irradiance For ultra high accuracy measurements this offset can be reduced by around 60 by shading which means preventing the direct radiation to reach the instrument Shading is typically done by using a shading disk on a solar tracker 2 7 Use as a net radiation sensor Two pyrgeometers mounted back to back may be used to measure net longwave radiation Net longwave radiation is defined as downwelling minus upwelling longwave irradi
38. 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 field of view e the environmental conditions for instance when assessing net radiation the downfacing pyrgeometer measurement may not be representative of irradiance received in that particular area The measurement reliability can be improved by maintenance support Important aspects are e dome fouling by deposition of dust dew rain or snow With pyrgeometers the most important source of unreliability is deposition of water on the dome Water completely blocks the longwave radiation flux between sensor and sky In particular at clear nights this causes very large errors Water deposition under clear sky nighttime conditions can largely be prevented by using the instrument heater Fouling results in undefined measurement uncertainty sensitivity and directional error are n
39. rmal 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 under the bottom plate of the instrument Performing a representative measurement the pyrgeometer measures the solar radiation in the plane of the sensor This may require installation in a tilted or inverted position The sensor surface sensor bottom plate should be mounted parallel to the plane of interest In case a pyrgeometer 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 Installatio
40. rument above dew point Some users prefer to have the heater on full time others prefer to switch it on during nighttime only IR20 manual v1403 23 41 Hukseflux Thermal Sensors 5 Making a dependable measurement 5 1 The concept of dependability A measurement with a pyrgeometer 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 pyrgeometer 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 In case of pyrgeometers the measurement uncertainty as obtained during outdoor measurements is a function of e the instrument properties e the calibration procedure uncertainty e the presence of natural sunlight involving the instrument specification of solar offset e the measurement conditions such as tilting ventilation shading heating instrument temperature e maintenance mainly fouling and deposition of water e the environmental conditions
41. s heavily on the environmental conditions Examples of environments with reduced expected lifetime are areas with high levels of air pollution 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 IR20 manual v1403 26 41 Hukseflux Thermal Sensors 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 5 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 pyrgeometer 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 Measurement or GUM The main ingredients of the uncertainty evaluation for pyrgeometers are e Calibration uncertainty which is in the order of 6 k 2 for upfacing instruments measuring downward longwave irradiance e Calibration uncertainty which is larger for other than upfacing instruments for downfacing instruments a blackbody calibration seems preferable Blackbody calibration will resu
42. strument 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 at the manufacturer or a reference institute Also recalibrate the temperature sensor IR20 manual v1403 28 41 Hukseflux Thermal Sensors 6 2 Trouble shooting Table 6 2 1 Trouble shooting for IR20 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 heat put the multimeter at its most sensitive range of DC voltage measurement typically the 100 x 10 VDC range or lower Make sure that the sensor is at 20 C or lower Expose the sensor to a strong heat source at a short distance from the window of more than 50 C for instance a hot cup of coffee The signal should read positive and gt 1 x 10 V now In case of using your hand as a heat source the signal should be significantly lower Check the data a
43. surface Es Lt global irradiance E E cos 6 W m POA Plane of hemispherical irradiance on Eg t Erte Array a specified in this case tilted surface Es downward diffuse solar W m DHI Diffuse radiation Horizontal Irradiance Eit Ei upward downward W m longwave irradiance E t reflected solar irradiance W m E net irradiance E E Ef W m T surtace equivalent blackbody eC radiative temperature of the surface T sky eguivalent blackbody eC radiative temperature of the sky SD sunshine duration H 8 is the apparent solar zenith angle 6 relative to horizontal 6 relative to a tilted surface g global longwave t tilted h horizontal distinction horizontal and tilted from Hukseflux kE T symbols introduced by Hukseflux contributions of Ey andE t are Eq and E t both corrected for the tilt angle of the surface IR20 manual v1403 35 41 Hukseflux Thermal Sensors 7 7 Appendix on terminology glossary Table 7 7 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
44. tificate Please note that the IR20 sensitivity is corrected for temperature dependence in the measurement equation by using 3 additional constants These coefficients are provided as well The central measurement equation governing IR20 is E U So a T2 b T c o T 273 15 Formula 0 1 lea So aT bT c Formula 0 2 The instrument should be used in accordance with the recommended practices of WMO Suggested use for IR20 and IR20WS e climatological networks e extreme climates polar tropical e moving platforms aircraft buoys e uncertainty assessment IR20 IR2OWS e calibration reference IR20WS IR20 manual v1403 5 41 Hukseflux Thermal Sensors Distinguishing features and benefits of IR20 are e correction of temperature dependence by use of the measurement function This is far more accurate than temperature compensation in the instrument especially at very low and high temperatures Every pyrgeometer is supplied with temperature coefficients to enter into the eguation e high sensitivity With sufficient input signal a typical datalogger no longer significantly contributes to the uncertainty of the measurement e low thermal resistance of the sensor Competing designs need a significant correction for the difference in temperature between pyrgeometer body and sensor surface For IR20 this is not needed e fast response time 3 s A low response time is a benefit for measurements on movin
45. uation improves In case of condensation of droplets disassemble the instrument and dry out the parts The instrument shows persistent internal condensation Arrange to send the sensor back to Hukseflux for diagnosis IR20 manual v1403 29 41 Hukseflux Thermal Sensors 6 3 Calibration and checks in the field Recalibration of field pyrgeometers is typically done by comparison in the field to a reference pyrgeometer There is no standard for this procedure Hukseflux recommendation for re calibration if possible perform calibration indoor by comparison to an identical or a higher class reference instrument under nighttime as well as daytime conditions Use nighttime data only to determine So Do not change the constants a b and c Hukseflux main recommendations for field intercomparisons are 1 perform field calibration during several days 2 to 3 days and if possible under cloudless conditions 2 to take a reference of the same brand and type as the field pyrgeometer or a pyrgeometer 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 to analyse downward irradiance values at nighttime only to determine So 6 to analyse the daytime data independently and look at the residuals between the calibration reference and calibrated instrument as a fun
46. uble shooting Calibration and checks in the field Data quality assurance Appendices Appendix on cable extension replacement Appendix on tools for IR20 Appendix on spare parts for IR20 Appendix on standards for classification and calibration Appendix on calibration hierarchy Appendix on meteorological radiation quantities Appendix on terminology glossary Appendix on conditions of sale warranty and liability EC declaration of conformity IR20 manual v1403 3 41 Hukseflux Thermal Sensors List of symbols Ouantities Symbol Unit Voltage output U V Sensitivity S V W m Sensitivity at reference conditions So V W m Temperature T C Equivalent blackbody radiative temperature T C Electrical resistance Re Q Longwave irradiance E W m Stefan Boltzmann constant 5 67 x 105 tj W m K temperature coefficient a 1 C temperature coefficient b 1 C temperature coefficient c see also appendix 7 6 on meteorological quantities Subscripts sky relating to the atmosphere surface relating to the ground surface ambient relating to ambient air body relating to the instrument body sensor relating to the sensor IR20 manual v1403 4 41 Hukseflux Thermal Sensors Introduction IR20 is a research grade pyrgeometer suitable for high accuracy longwave irradiance measurement in meteorological applications IR20 is capable of measuring during both day and night In absence of solar radiation model IR2OWS of
47. ws remote checks of one instrument using the other as a reference which leads to a higher measurement reliability 5 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 pyrgeometers 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 Hukseflux pyrgeometers are designed to be suitable for the intended use for at least 5 years under normal meteorological conditions Factory warranty granting free of charge repair for defects that are clearly traceable to errors in production is 2 years 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 pyrgeometers the product expected lifetime depend

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