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will decay with a time constant 1/e of 4s, but the dome temperature
will go to equilibrium with a time constant of several minutes. So after
one minute the remaining signal is the main part of the last zero offset
A.
Good ventilation of domes and body is the solution to decrease zero
offset A. Kipp & Zonen advises CV 2 for optimal ventilation and
suppression of zero offset type A. Using the CV 2 zero offset type A
2
will be less than 7 W/m
SUMMARIZED: Zero offset is the result of lack of thermal equilibrium
in the instrument.
Upper values of zero offset in CM 11
2
Zero offset A:
7 W/m
(ventilated domes)
12 W/m
ventilation)
Zero offset B:
< 3 W/m
temperature
1.2.2 Spectral range and spectral selectivity
Spectral selectivity is the product of spectral absorptance of the black
coating (see figure 2) and spectral transmittance of the glass domes
(see figure 3).
Shifts in the solar spectrum, due to changes from clear to overcast
sky, are mainly in the mid of the spectral range. No significant spectral
selectivity errors have to be expected. E.g. at a sun's elevation of 30°
(airmass 2) only 1% of the solar radiation has wavelengths below 335
nm and only 1% of the solar radiation has wavelengths above 2200
mm.
GENERAL INFORMATION
.
2
response to 200 W/m
thermal radiation
2
2
response to 200 W/m
2
response to 5 °C/hr change of body
thermal radiation (no
9
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