Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 22)

CMP/CMA series manual
Page 21
5.2.3. Calculation
The sensitivity of the unknown pyranometer is calculated with formula 2:
s S
b a B B
A A
⋅
+
+
=
'
'
Formula 2
S
b
= Sensitivity of the reference radiometer at +20
°C.
A = Output of test radiometer at position 1
A’ = Output of test radiometer at position 2
B = Output of reference radiometer at position 2
B’ = Output of reference radiometer at position 1
S
a
= Sensitivity of the test radiometer at +20
°C.
Output = mean value at 100% response minus zero offset signal
5.2.4. Zero offset
The lamp housing and beam restrictors heat up and emit long-wave infrared radiation, which heats up
the outer glass dome and, indirectly, the inner one. When the radiometers are shaded, there still
remains a small signal of up to +20 µV due to long-wave infrared radiation from the inner dome to the
sensor. This zero offset is decreasing with a time constant (1/e) of several minutes.
A zero offset is also embodied in the response due to illumination. To correct for this unwanted
response, the zero offset read after 60 seconds of shading is subtracted.
5.3. Traceability to World Radiometric Reference
Reference radiometers, which are calibrated annually by the World Radiation Centre in Davos, are
used for the calibration of radiometers manufactured by Kipp & Zonen. The reference radiometers are
fully characterized, i.e. linearity, temperature dependence and directional response are recorded.
Kipp & Zonen keeps two reference radiometers for each radiometer model. These reference
radiometers are sent alternate years to WRC for calibration, so production and calibration in Delft can
carry on without interruption.
5.4. Recalibration
Radiometer sensitivity changes with time and with exposure to radiation. Periodic calibration every two
years is advised.
Accurate calibrations can be done outdoors under clear conditions by comparison with a reference
pyrheliometer. Many national or regional weather services have calibration facilities. Their standard
pyrheliometer is compared with the World Radiometric Reference at Davos, Switzerland. This
embodies several absolute cavity (black body) pyrheliometers. Information about regional calibration
centres can be found in appendix V.