Campbell CVF4 Spécifications télécharger pdf (Page 43)

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

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

The dew on the windows of pyrgeometer can cause the -100 W/m

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

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Campbell CVF4 Spécifications Page 43