Campbell CM 21, Инструкция по эксплуатации

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7 FREQUENTLY ASKED QUESTIONS (FAQ’s)
The most frequently asked questions are listed below. For an update refer to
the Kipp & Zonen web page: http://www.kippzonen.com
1. Negative output during night time measurements?
This error is related to the zero offset type A. Normally this zero offset is
present when the inner dome has a different temperature from the cold
junctions of the sensor. Practically this is always the case 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 region is nearly 1). The emitted heat is attracted from
the body (by conduction in the dome), from the air (by wind) and from the
inner dome (through infrared radiation). The inner dome is cooling down too
and will attract heat from the body by conduction and from the sensor by the
net infrared radiation. The latter heat flow is opposite to the heat flow from
absorbed solar radiation and causes the well known zero depression at
night. This negative zero offset is also present on a clear day, however,
hidden in the solar radiation signal.
Zero offset type A can be checked by placing a light and IR reflecting cap
over the pyranometer. The response to solar radiation will decay
with a time constant (1/e) of 1 s, but the dome temperature will go to
equilibrium with a time constant of several minutes. So after half a
minute the remaining signal represents mainly zero offset type A.
Good ventilation of domes and body is the solution to reducing zero offsets
even further. Kipp & Zonen advises the CV 2 for optimal ventilation and
suppression of zero offset type A. Using the CV 2 zero offset type A will be
less than 5 W/m
2
.
7. FREQUENTLY ASKED QUESTIONS