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4.8. Window heating o set
Currently the major source of error concerning common pyrgeometer measurements is caused by the
so-called 'window heating o set'. When a pyrgeometer is exposed to the sun, heating of the silicon
dome/window occurs due to absorption of solar radiation by the material. As a consequence the
dome/window of most types of pyrgeometer will heat up proportionally to the solar irradiance.
The resulting temperature di erence between dome/window and thermopile will cause heat transfer by
radiation and convection to the sensor. This a ects the net thermal radiation as measured by the thermopile
and is commonly referred to as the 'window heating o set'. The result is measurement of a too high value
for downward long-wave radiation.
This o set is not easily reduced by ventilation, which only cools o 50 W/m²/°C at maximum while
solar radiation can be absorbed at a rate of about 500 W/m² on a sunny day. Currently certain types of
pyrgeometers are equipped with one or more thermistors to measure the dome/window absolute
temperature that represents the apparent o set. A complex calculation must be performed to eliminate
the o set.
Arguments against a thermistor to measure window temperature are:
-
The thermistor contacts a part of the dome/window, it is a blackbody radiator and heat source itself
and its material and adhesive increases the mean emission coe cient of the inner dome/window
surface. Its presence increases the window-heating o set.
-
Under clear skies the direct solar irradiance impacts mainly from one direction, so for a
hemispherical dome it is recommended to have 3 equally-spaced thermistors to determine the
mean dome temperature.
-
The dome/window thermistor(s) should be carefully matched with the housing thermistor because
calculations must be done using the temperature di erence of the two thermistors.
-
The customer needs between one and three extra data logger channels for the dome/window
thermistor inputs.
Because of the possible problems caused by dome / window thermistors Kipp & Zonen developed the
revolutionary CGR 4 pyrgeometer. In the CGR 4, dome heating is strongly suppressed by a unique
construction that very e ectively conducts the absorbed heat away into the housing. CGR 4 temperature
variations between dome and detector are less than 0.3°C, compared with up to 3°C for other types of
pyrgeometers. In CGR 4 the window heating o set is less than 4 W/m². This allows accurate daytime
measurements, even in full sunlight, without the need for a tracking shading disk.
4.9. Zero o set B due to ambient temperature changes
Proportionally to the ambient temperature the instrument temperature varies and causes heat currents
inside the instrument. This will cause an o set commonly called Zero O set type B. It is quantified as the
response in W/m² to a 5 K/hr change in ambient temperature.
CGR 4 Manual
Page 21
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