Campbell Scientific CNR1 Net Radiometer - Manual (Page 31)

Appendix B. Details about Using the
Heater
Whenever the heater is used, the heating may cause errors in the
measurement of the sensor temperature, see chapter 1.1.2.3. in
the Kipp and Zonen CNR1 manual
(
wo degrees typical, and
zero offsets in the CM3 (10 Watts per square meter typical).
Under most conditions the accuracy that is gained by heating will
be larger than the errors that are introduced by heating.
NOTE
In both the CM3 and CG3, thermal sensors are used. These sensors in principle
measure a heat flow. For optimal performance, these sensors should be at
equilibrium with the ambient air. Heating the sensor disturbs this equilibrium.
This will give rise to zero offsets particularly in the CM3. A typical value
under conditions with no wind is 10 Watts per square meter. Therefore heating
should only be done if absolutely necessary. CG3 is less sensitive to this.
Offset values for CG3 cannot be determined, and therefore are not specified.
B.1 General Information
There is one major reason for heating: avoiding water deposition on the CG3
window and on the CM3 dome. In case of dew deposition on the CG3
window, the dew will ultimately obstruct the Far Infrared radiation completely,
causing a signal close to zero. In the case of rain, this will probably not lead to
significant errors, because with an overcast sky the signal is close to zero
anyway. The case of dew deposition is far more significant. Dew deposition
will probably take place under conditions with large Far Infrared irradiation
from the pyrgeometer detector to the clear sky, typically minus (-) 100 Watts
per meter square. Under conditions where clouds or wind are present, dew is
less likely to occur. A situation where dew depositing on the window of the
CG3, causing a signal of -100 Watts per square meter to drop to zero, is
certainly significant. If heating can avoid this, it certainly should be done
because all errors that are described above are much smaller than the gain in
accuracy of 100 Watts per meter square.
B-1