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GENERAL INFORMATION
resistance of the Al
O
substrate is relatively low. E.g. an irradiance of
2
3
2
1000 W/m
results in a rise of centre temperature of 3 °C only and a
voltage of 4-6 mV. Natural convection inside the inner dome due to
this temperature difference appeared to be small and when tilting a
pyranometer CM 11, no change of sensitivity is observed.
1.2.1 Zero offsets
The following definition of zero offsets is used:
When the sensor does not absorb radiation with wavelengths in the
spectral range of the instrument and there still is a signal, we call it
zero offset. Two types of zero offset are distinguished.
Zero offset type B
This zero offset arises when the body (heat sink) temperature
increases or decreases. This results in a temperature difference
between cold junctions (connected to the heat sink) and the hot
junctions due to the heat flow necessary to load or unload the sensor
heat capacity. In the CM 11 there is principally no zero offset B
because there is a second non-illuminated compensation element.
See figure 1.
Other heat flows in the sensing element e.g. due to rising or falling
body temperature cause spurious voltage, sometimes called zero
offsets. See Appendix II. To compensate for one of these offsets, 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 the inner dome has another
temperature than 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
2
100 W/m
long wave infrared radiation upwards. The outer glass
7
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