Kipp&Zonen CFR Calibration Facility - Manual (Page 22)

CFR manual
21
4.7 Voltage Stabilization
The voltage stabilizer provides a highly stable voltage to the lamp. For calibration it is necessary to have a
reliable voltage because variations in lamp output can affect the result of the calibration slightly. In the final
calculation there is a check on read-outs and lamp stability. This is shown in chapter 3. For the 230 VAC /
50Hz version the voltage stabilizer is mounted underneath the table. The 115 VAC / 60Hz facility has the
stabilisation circuitry built into the lamp.
4.8 Voltmeter
The voltmeter shown in figure 10 serves to measure the
output of the pyranometers. The specific type of voltmeter has
been selected for its capability to measure small microvolt
signals with high stability and low offsets. In the final
calibration calculation, the absolute accuracy of the voltmeter
is eliminated as a parameter, as long as the readout is linear
and repeatable.
It is recommended to set the voltmeter to measure on the 100
mV scale with an accuracy of 3 digits. To ensure a steady
value of the output signal, the sampling rate should be set to
medium with a moving average filter over 100 samples.
fi gure 10 Voltmeter
4.9 Switch and clamps
The switch located on the turntable changes the input of the voltmeter from the reference pyranometer to
the test pyranometer. The spring clamps located on the table connect the signal wires of the two
pyranometers to the switch.
4.10 Reference pyranometer
The reference pyranometer used for calibration is normally delivered with the Calibration Facility, selected
from optional list to suit the test pyranometers to be recalibrated. It should normally be of the same type as
the test pyranometers or of a higher class but similar construction. The reason for this is that for similar
types, there are no errors introduced due to differing instrument characteristics. It is therefore suggested to
obtain a reference for each type that is in use.
It is possible to calibrate other types of pyranometers that are not the same as the reference. However, it
should be noted that the estimated calibration transfer accuracy could be lower. This has to do with the fact
that the surface areas of the detectors, spectral selectivity, non-linearity, time constants and other factors
may differ.
The reference pyranometer should be kept in a clean, dry and dark place in order to avoid unnecessary
aging and the desiccant checked regularly. Under field conditions, the change in sensitivity should be less
than ± 1% per year. Properly stored, the stability should be ± 0.25% per year.
Recalibration can be done either at Kipp & Zonen, at a Regional Radiation Centre, or (with the highest
accuracy) at the World Radiation Center (WRC) in Davos, Switzerland.