Campbell CM 21, Инструкция по эксплуатации

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Figure 1 Construction details of a CM 21 pyranometer
1.2.1 Zero offsets
Zero offset type B
Heat flows in the sensing element e.g. due to rising or falling body
temperature normally cause a spurious voltage., called zero offset. To
compensate for this offset, a second non-illuminated element is installed, in
which the same heat flow will arise. By anti-series arrangement of both
elements the spurious voltage is cancelled out greatly. The white plastic
screen reduces the body temperature variations due to solar radiation and
(cold) rain showers.
Zero offset type A
This zero offset is present when there is a temperature difference between
the inner dome and the cold junctions of the sensor.
In practice a zero offset will be present when there is a clear sky. Because of
the low effective sky temperature (<0ºC) the earth surface emits roughly 100
W/m
2
longwave infrared radiation upwards. The outer glass dome of a
pyranometer also has this emission and is cooling down several degrees
below air temperature (the emissivity of glass for the particular wavelength
1. GENERAL INFORMATION