Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 28)

CMP/CMA series manual
Page 27
7. Frequently asked questions
The most frequently asked questions are listed below. For an update refer to the Kipp & Zonen website
at www.kippzonen.com
‰ Negative output during night-time measurements?
This error is related to 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’s surface emits
roughly 100 W/m
2
of 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 but is 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 second, 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 minimising zero offsets. Kipp & Zonen advises
the CV 2 ventilation unit for optimal ventilation and suppression of zero offset type A. Using the CV 2
zero offset type A will decrease by ~ 50%.
‰ Maximum and minimum irradiation quantities?
Due to possible reflection from clouds the global irradiance at sea level can rise above the extra-
terrestrial direct irradiance of 1367 W/m
2
at the top of the atmosphere. Values up to 1500 W/m
2
have
been reported.
Because the clouds move, this irradiance value mostly appears as short events of a few minutes
duration.
‰ What is the primary entry point for humidity?
The drying cartridge seal and the silicon glue of the domes are not fully airtight.
‰ Is the pyranometer sensitivity affected by the length of the signal cable?
With longer cable lengths the impedance increases, however it does not affect the radiometer
sensitivity for the following reason. The impedance of the voltage measurement device is at least
10000 times more than the impedance of the pyranometer plus cable. Therefore the current that goes
through the readout cable is negligible and won’t generate an offset. However, the loading may affect
the temperature compensation circuit to some extent.