Kipp&Zonen CGR 4 Pyrgeometers - Manual (Page 24)

5. Calibration
5.1. Calibration principle
An ideal radiometer gives voltage output that is proportional to the absolute irradiance level. This
relationship can be expressed as a constant ratio called ‘sensitivity’ (Sensitivity).
The sensitivity figure of a particular pyrgeometer is unique. In the case of the CGR 4 it is determined
outdoors by comparison against a reference CGR 4 pyrgeometer. The reference pyrgeometer is
calibrated outdoors regularly at the World Radiation Centre (WRC) at Davos, Switzerland.
5.2. Calibration procedure at Kipp & Zonen
CGR 4 pyrgeometers are calibrated outdoors at Kipp & Zonen under a mainly clear sky during nighttime.
The test instruments are installed next to the reference CGR 4. The pyrgeometer detector outputs (U
emf
)
and housing temperatures (T
b
) are measured each second and compressed to one minute averages.
Afterwards the downward radiation (L
d
) on the reference pyrgeometer is calculated using Formula 2.
Formula 2
For the test CGR 4’s a one minute average sensitivity is calculated using the Formula 3.
Formula 3
A final S
t
is determined using only one minute S
t
’s determined under clear sky conditions when;
Net IR-exchange > 40 W/m².
The sum of all periods must be at least 6 hours for a successful calibration.
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 sky conditions by comparison
to a reference pyrgeometer of equal or higher standard.
Page 24 CGR 4 Manual
L
d
=
b
U
emf
S
+ 5.67 • 10
-8
• T
4
(L
d
U
t S
t
=
b
- ) 5.67 • 10
-8
• T
4
(L
d b
- ) 5.67 • 10
-8
• T
4