B-1
Appendix B. CNF4 Heater/Ventilator
Whenever the heater is used, the hea ting may cause errors in th e
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.
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.
10 Watt power available?
Not available
DO NOT HEAT
Available
Consider options below
Clock and relay available?
Not Available
DO NOT HEAT
(CSI recommendation)
Available
Heat from 1 hour before the sunset
until 1 hour after the sunrise.
NOTE
