Kipp&Zonen CFR Calibration Facility - Manual (Page 15)

CFR manual
14
The possibility of making errors due to inhomogeneous light is still present. Therefore, the positions of the
pyranometers are interchanged, to position 2, by rotating the turntable, and the procedure is repeated. The
resulting values of this second measurement are called R2 and T2.
The sensitivity ST of the test pyranometer can now be calculated based on the known sensitivity of the
reference pyranometer SR:
) 2 1 (
) 2 1 (
R R
T T
SR ST
+
+
⋅ = (Formula 1)
Finally, a check on errors and lamp stability can be done, using the information of the output of the
pyranometers. If the stability is not sufficient, the calibration should be rejected. The rule is that the
calibration has not succeeded if the following expression is outside its boundaries:
0.98 <
) 2 2 (
) 1 1 (
T R
T R
⋅
⋅
< 1.02 (Formula 2)
This expression is sensitive to:
• read-out errors by the operator;
• accidental shading of one pyranometer;
• a non-perfect interchange of position;
•
light field instability.
Lamp drift influences the result of the expression, although the effect on the calculated sensitivity ST is
negligible due to the near simultaneous read-outs.