Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 12)

CMP/CMA series manual
Page 11
Figure 5 Non-linearity sensitivity variation of a CMP 22
2.4. Operation
After completing the installation the radiometer will be ready for operation.
The irradiance value (E
↓Solar ) can be simply calculated by dividing the output signal (U
emf
) of the
pyranometer by its sensitivity (S
ensitivity
) as shown in Formula 1.
For calculation of the solar irradiance (global or reflected) the following formula must be applied:
ensitivity
emf
Solar S
U
E =
↓
Formula 1
E ↓Solar = Solar radiation [W/m
2
]
U
emf
= Output of radiometer [
µV]
S
ensitivity
= Sensitivity of radiometer [
µV/W/m 2
]
To be certain that the quality of the data is of a high standard, care must be taken with daily maintenance
of the radiometer. Once a voltage measurement is taken, nothing can be done to retrospectively improve
the quality of that measurement.
2.5. Measurement uncertainty
When a pyranometer is in operation, the performance of it is correlated to a number of parameters,
such as temperature, level of irradiance, angle of incidence, etc. Normally, the supplied sensitivity
figure is used to calculate the irradiances. If the conditions differ significantly from calibration
conditions, uncertainty in the calculated irradiances must be expected.
For a secondary standard instrument (the highest quality) the WMO expects maximum errors in the
hourly radiation totals of 3%. In the daily total an error of 2% is expected, because some response
variations cancel each other out if the integration period is long. Kipp & Zonen expects maximum
uncertainty of 2% for hourly totals and 1% for daily totals for the CMP 22 pyranometer. Many years of
experience has shown that pyranometer performance can be improved concerning zero offset type A by
using a well designed ventilation system. The Kipp & Zonen CV 2 ventilation unit is recommended to
minimise this small remaining error.
For the CMP 22 the effect of each parameter on the sensitivity can be shown separately.
The non-linearity error, the sensitivity variation with irradiance, is the same for any CMP 22 and is
shown in Figure 5 for a range from 0 to 1000 W/m
2
referred to the calibration at 500 W/m
2
.