SR05-D1A3-PV manual v1801 70/83
9.5 Appendix on calibration hierarchy
The World Radiometric Reference (WRR) is the measurement standard representing the
Sl unit of irradiance. Use of WRR is mandatory when working according to the standards
of both WMO and ISO. ISO9874 states under paragraph 1.3: the methods of calibration
specified are traceable to the WRR. The WMO manual states under paragraph 7.1.2.2:
the WRR is accepted as representing the physical units of total irradiance.
The worldwide homogeneity of the meteorological radiation measurements is guaranteed
by the World Radiation Center in Davos Switzerland, by maintaining the World Standard
Group (WSG) which materialises the World Radiometric Reference.
See www.pmodwrc.ch
The Hukseflux standard is traceable to an outdoor WRR calibration. Some small
corrections are made to transfer this calibration to the Hukseflux standard conditions:
sun at zenith and 1000 W/m
2
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
2
level.
Table 9.5.1 Calibration hierarchy for pyranometers
WORKING STANDARD CALIBRATION AT PMOD / WRC DAVOS
Calibration of working standard pyranometers:
Method: ISO 9846, type 1 outdoor. This working standard has an uncertainty “uncertainty of
standard”. The working standard has been calibrated under certain “test conditions of the
standard”. The working standard has traceability to WRR world radiometric reference.
CORRECTION OF (WORKING) STANDARD CALIBRATION TO STANDARDISED
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)
CALIBRATION 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.
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