Calibration Principle; Calibration Traceability To The Wrr - Kipp & Zonen SMP series Instruction Manual

.

6.4.1 Calibration principle

At the Kipp & Zonen factory pyranometers are calibrated, or re-calibrated, in our laboratory according to ISO 9847:1992 'Solar
energy - Calibration of field pyranometers by comparison to a reference pyranometer', Annex A 'Calibration devices using
artificial sources'. The specific method is given in Annex A.3.1 and is described in the standard as the 'Kipp & Zonen (calibration)
device and procedure'.
This is based on a side-by-side comparison of the test pyranometer with a reference pyranometer of the same type under a stable
artificial sun. Kipp & Zonen uses a 150 W Metal-Halide high-pressure gas discharge lamp with precise voltage stabilisation. The
irradiance at the radiometers is approximately 500 W/m².
The reference pyranometers are regularly calibrated outdoors at the World Radiation Centre (WRC) in Davos, Switzerland. The
spectral content of the laboratory calibration lamp di ers from the outdoor solar spectrum at the World Radiation Centre. However,
this has no consequences for the transfer of calibration, because the reference and test radiometers have the same characteristics.
To minimise stray light from the walls and the operator, the light is restricted to a small cone around the two radiometers. The
test radiometer and the reference radiometer are placed side by side on a small rotating table. The lamp is centred on the axis
of this table. The table is used to interchange the positions of the pyranometers to allow for inhomogeneity of the light field.
The pyranometers are illuminated and after time for the output to stabilise the readings of both radiometers are integrated over
a measurement period. The lamp housing and beam restrictors heat up and emit long-wave infrared radiation which warms up
the pyranometer dome(s) slightly. This causes a small o set that is embodied in the pyranometer response under illumination.
To determine this o set, both radiometers are shaded, and after time to stabilise the signals of both radiometers are integrated
over a period.
The radiometer positions are interchanged by rotating the table and the whole procedure is repeated.
The sensitivity of the test pyranometer is calculated by comparison to the reference pyranometer readings and the calibration
certificate is produced. At Kipp & Zonen the complete process is automated under computer control, including programming the
SMP pyranometer with the correct calibration factors and default output range settings.
Kipp & Zonen produces the manually operated CFR Calibration Facility for Radiometers for customers to make their own
pyranometer calibrations to ISO 9847, Annex A.3.1. A special interface is available as an accessory to enable calibration of SMP
series pyranometers with the CFR.

6.4.2 Calibration traceability to the WRR

Our reference pyranometers are calibrated at the World Radiation Centre (WRC) in Davos, Switzerland by comparison to the World
Radiometric Reference (WRR). They are also fully characterized for linearity, temperature dependence and directional response
to enable transfer of the sensitivity under the measurement conditions in Davos to our calibration laboratory conditions.
Kipp & Zonen keeps at least two reference instruments for each pyranometer model. These reference instruments are sent
alternate years to the WRC for calibration, so that production and calibration in Delft can carry on without interruption.
Kipp & Zonen calibration certificates include an overview of the calibration method, details of the reference pyranometer used,
traceability to the WRR, and the uncertainty in the full calibration chain from the WRR to the pyranometer being calibrated.
32