Specifications

Appendix B. CNF4 Heater/Ventilator
Whenever the heater is used, the heating may cause errors in the
measurement of the sensor temperature. 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 pyranometer and the pyrgeometer, thermal sensors are used, and
these sensors in principle measure a heat flow. For optimal performance, these
sensors should be at thermal equilibrium with the ambient air. Heating the
sensor disturbs this equilibrium. The heating causes the zero offset error on the
pyranometer (10 W/m
2
typical), and the temperature measurement error on the
sensor (2 degree typical). Therefore, the heater should be used only if
absolutely necessary. The pyrgeometer is less sensitive to this. Offset values
for the pyrgeometer cannot be determined, and, therefore, are not specified.
B.1 General Information
The one major reason for heating the sensor is to avoid the water deposition on
the pyrgeometer sensor window and on the pyranometer domes. The water
deposition on the pyrgeometer window will ultimately obstruct the far infrared
radiation completely, causing the signal close to be zero. During a rain event,
this will probably not lead to significant errors, because with an overcast sky,
the signal is close to zero anyway. However, the dew deposition is far more
significant. Dew deposition will probably take place under conditions with
large far infrared irradiation from the pyrgeometer to the clear sky, typically
-100 W/m
2.
The dew on the windows of pyrgeometer can cause the -100 W/m
2
signal to go to zero. In such a case, the heater should be used because the error
described above is much smaller than the gain obtained by heating the sensor
to avoid the dew deposition.
Please refer to the following diagram to determine whether or not the heater
should be used.
B-1