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Version 1003
State of the art of indoor calibration of pyranometers and pyrheliometers
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2Indoor calibration
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Main points
• Most field pyranometers are calibrated indoors
• Many procedures for indoor calibration
• Not all optimally connected to ISO 98-3 GUM
• Industry requires straightforward approach
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Industry
• Meteorology - Solar renewable energy • Site assessment• Installation performance• Professionalisation / IEC
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Future
• A few high accuracy outdoor calibrations
• A lot of indoor facilities• Accredited labs
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Conclusion
• Points for discussion• Normal Incidence NI calibration is
preferred (Diffuse Sphere Source DSS not)
• Uncertainty & accuracy of reference can be optimised
• Indoor calibration complies with GUM• Pyrheliometer indoor calibration must
be allowed by ISO
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Myself
• Kees VAN DEN BOS• Director / owner Hukseflux Thermal
Sensors• Last 20 years sensor design
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Hukseflux DR01 pyrheliometer
• Founded 1993
• Thermal sensors
• 15 employees
• 5 radiometry
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9Hukseflux 2010
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10Reolith thermal properties on moon rover
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11Snow thermal conductivity
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My interest
• Hukseflux company cannot work with outdoor calibration
• Our customers want a understandable accuracy statement
• Feedback• More questions than answers
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Background
• Most pyranometers and pyrheliometers have indoor calibration
• Exception: highest accuracy (BSRN, outdoor)
• Exceptions on national level: Japan, China, … (outdoor)
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Background
• Cost, time, weather; outdoor calibration is unacceptable to industry
•DISADVANTAGE: Indoor methods only work with reference type = field type
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Present status (excerpt)
• Eppley, US Weather Bureau: indoor integrating diffuse source
• Kipp, Hukseflux: indoor normal incidence
• EKO: outdoor tracker with collimation tube
• KNMI: indoor (network) and outdoor (BSRN)
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16ISO 9060
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17ISO 9060
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Background
• Measurement uncertianty is a function of:
• Characterisation / class • Calibration (+characterisation and
class)• Measurement & maintenance
conditions• Environmental conditions
(+characterisation and class)
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Background
• Indoor calibration covered by ISO 9847
• Present ASME: “Indoor Transfer of Calibration from Reference to Field Pyranometers”
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20ISO 9846
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21ISO 9847 also indoor
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22ISO 98-3 GUM
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Hierarchy of Traceability
• A: Reference calibration (uncertainty)• B: Correction of reference to indoor
conditions (uncertainty)• C: Indoor calibration of field
instrument (uncertainty)• Indoor calibration uncertainty
estimate (A+B+C)• Field measurement uncertainty
estimate
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Hierarchy of Traceability
• A: Reference calibration (uncertainty)• B: Correction of reference to indoor
conditions (uncertainty)• C: Indoor calibration of field
instrument (uncertainty)• Indoor calibration uncertainty
estimate (A+B+C)
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25ISO 98-3 GUM
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26Hierarchy of traceability
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28Indoor calibration Normal Incidence NI
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29ISO 98-3 GUM
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Hierarchy of Traceability
• KNMI TR 235 "uncertainty in pyranometer and pyrheliometer measurements at KNMI in De Bilt".
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Hierarchy of Traceability
• A: Reference calibration (uncertainty)• B: Correction of reference to indoor
conditions (uncertainty)• C: Indoor calibration of field
instrument (uncertainty)• Indoor calibration uncertainty
estimate (A+B+C)• Field measurement uncertainty
estimate
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34ISO 98-3 GUM
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NI Hierarchy of Traceability
• A: Reference calibration (uncertainty) (conditions and class)
• B: Correction of reference to indoor conditions (uncertainty)
• C: Indoor calibration of field instrument (uncertainty)
• Indoor calibration uncertainty estimate (A+B+C)
• Field measurement uncertainty estimate (conditions & class)
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Strange…
• Errors in reference calibration re-appear in measurement errors
• Counted double• At least systematic errors (Zero offset
A and directional errors) can be avoided.
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One step back
• Calibration with restricted conditions results in lower uncertainty
• See yesterday’s presentation by Ibrahim Reda
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One step back
• Present reference works well if calibrated pyranometers are used:
• Outdoor / unventilated• At same latitude
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One step back
• Present approach does NOT work well calibrated if instruments are used:
• As indoor reference• At different latitudes• Ventilated
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Typical secondary standard calibration• Irradiance 800 W/m2
• 40 to 60 degrees angle of incidence, + / - 30 degrees azimuth• Zero offset A: -9 +/- 3 W/m2 (larger
than ISO9060)• Directional: +/- 10 W/m2 @ 1000
W/m2 , now estimated +/- 5 W/m2
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Typical calibration
• PMOD specified uncertainty +/- 1.3%• Systematic error -1%? Type B.
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NI reference improved
• Restricted conditions• Zero offset A: -9 +/- 3 W/m2 (larger
than ISO9060)• Directional: +/- 10 W/m2
• Solution 1: ventilation• Solution 2: single angle of incidence
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For consideration
• Japanese collimated tube with tilt correction and ventilation
• Tilted sun-shade method
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Diffuse Sphere Source DSS
• Uniformity of sphere top-edge (experimental -13%)
• Weighing for non uniform source requires weighing of reference with source
• Diffuse sphere: weighing requires weiging of field instrument with source. Complicated!
• Normal incidence: weighing of field instrument is not necessary
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DSS Hierarchy of Traceability
• A: Reference calibration (uncertainty) (conditions and class)
• B: Correction of reference to indoor conditions (uncertainty)
• C: Indoor calibration of field instrument (uncertainty) (conditions and class)
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DSS Hierarchy of Traceability
• Indoor calibration uncertainty estimate (A+B+C)
• Field measurement uncertainty estimate (conditions & class)
• Additional uncertainty under C compared to NI calibration
• Bottom line: DSS has less restricted conditions than NI
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Conclusion
• Indoor calibration offers only acceptable solution for manufacturers and “general users” in solar industry
• Indoor calibration fits within ISO 98-3 GUM
• detailed statements about field measurement still need to be agreed upon
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Conclusion
• Indoor calibration: Normal Incidence calibration is preferred (Diffuse Sphere Source is not)
• Accuracy and precision of reference can be optimised to serve as indoor calibration reference (restricted: single angle, ventilated)
• Pyrheliometer indoor calibration must be added /allowed by ISO
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