C-1
Appendix C. 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 gained by heating will be larger than the errors
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.
C.1 General Information
The primary 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. During a rain event, this will probably not cause
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
significantly 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.
NOTE
