Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 18)

CMP/CMA series manual
Page 17
4.5. Non-linearity
The non-linearity of a radiometer is the percentage deviation in the sensitivity over an irradiance range
from 0 to 1000 W
.
m
-2
compared to the sensitivity calibration irradiance of 500 W
.
m
-2
. The non-linearity
effect is due to convective and radiative heat losses at the black absorber surface which make the
conditional thermal equilibrium of the radiometer non-linear.
4.6. Temperature dependence
The sensitivity change of the radiometer with ambient temperature change is related to the thermo-
dynamics of the radiometer construction. The temperature dependence is given as percent deviation
with respect to the calibrated sensitivity at +20
°C. Some of the CMP/CMA series radiometer models
have passive electrical compensation circuits to minimise this effect. Each CMP 21 and CMP 22
pyranometer is supplied with an individual test certificate stating the temperature dependency in the
range from -20
°C to +50°C, at 10°C intervals. The CMP 21 and CMP 22 are fitted as standard with an
internal temperature sensor to allow sensitivity corrections to be applied if desired.
4.7. Tilt error
This is the deviation from the sensitivity at 0
° tilt (exactly horizontal) over the range from 0° to 90° tilt
under 1000 W
.
m
-2
normal incidence irradiance. The tilt response is proportional to the incident
radiation. The error could be corrected for, in applications where it is necessary to install the
pyranometer on an inclined surface, but is usually insignificant.
4.8. Zero offset A
By physical laws any object having a certain temperature will exchange radiation with its surroundings.
The domes of upward facing radiometers will exchange radiation primarily with the relatively cold
atmosphere. In general, the atmosphere will be cooler than the ambient temperature at the Earth’s
surface. For example, a clear sky can have an effective temperature up to 50
°C cooler, whereas an
overcast sky will have roughly the same temperature as the Earth’s surface. Due to this the
pyranometer domes will ‘lose’ energy to the colder atmosphere by means of radiative transfer. This
causes the dome to become cooler than the rest of the instrument. This temperature difference
between the detector and the instrument housing will generate a small negative output signal which is
commonly called Zero Offset type A. This effect is minimized by using an inner dome. This inner dome
acts as a ‘radiation buffer’. The above is illustrated in Figure 10.
This effect can be minimized by applying appropriate
ventilation of the instrument. The CV 2 ventilation unit is
specially designed for the CMP series, except CMP 3.
Figure 10 Zero Offset type A