Kipp & Zonen CM 11, Instruction Manual

Page: 44 / 64

FREQUNTLY
ASKED QUESTIONS
43
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 nighttime 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 (<0
°
C) the earth
surface emits roughly 100 W/m
2
long wave 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 state of the art is to use a thin aluminium foil cup
covering only the outer dome. With this configuration only the fluxes (short
wave and long wave) to the outer dome are blocked. Other heat fluxes from
wind etc. remain undisturbed.
The response to solar radiation will decay
with a time constant (1/e) of 4 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.