Solar radiation calibration scale change
WRR solar irradiance scale is expected to go down
WRR (World Radiometric Reference) contains a systematic error relative to SI (International System of Units). To correct this, WRR scale will likely shift downward by 0.34 %. The change is highly relevant to climatology, solar resource assessment, and solar system performance assessment.
Introduction
Since 1980, the WRR scale has been used as the international reference for representing the physical units of solar irradiance.
PMOD: designated institute under METAS
The WRR scale is maintained by PMOD (Physikalisch-Meteorologisches Observatorium Davos), which holds the formal status in international metrology as “designated institute.” Therefore, PMOD is responsible for maintaining the highest standard for solar irradiance measurement with pyranometers and pyrheliometers. For more details, visit the Swiss Institute of Metrology (METAS) website.
What will happen to WRR?
It has become clear that WRR contains a systematic error relative to SI. To correct this, the WRR scale will likely shift by 0.34 %.
Since the WRR scale was too high, all irradiance measured with traceability to WRR must be corrected to lower values in [W/m2]. Correspondingly, with the correction from WRR to the new scale (name yet to be determined), the sensitivities of instruments like pyranometers in [V/(W/m2)] will increase by 0.34 %.
WMO (World Meterological Organization) does not recommend retroactively correcting past data. It does, however, advise documenting which scale was used when obtaining data from pyranometers and pyrheliometers. The decision about the introduction of the new solar irradiance scale will be made at the WMO Congress in 2027.
Literature reference for scale change
A reference for the proposed scale change is: Fehlmann A., Kopp G., Schmutz W. et al. Fourth World Radiometric Reference to SI radiometric scale comparison and implications for on-orbit measurements of the total solar irradiance. Metrologica 49 (2012) p34-38
| PMOD pyranometer calibration* | |
|---|---|
| calibrated quantity | responsivity to solar irradiance |
| calibrated instrument | pyranometer |
| measurand | V/(W/m2) |
| uncertainty | 0.66 % |
| calibration method |
|
| calibration reference | World Radiometric Reference (WRR) |
| type of source | sun and clear sky |
| * Reference: BIPM website | |
What we do at Hukx to prepare
Hukx’s latest industrial pyranometers allow users to add metadata, to indicate if calibration is traceable to WRR or the new scale, and on-board calibration records and calibration history. All records are conveniently stored in the instrument.
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