Kipp&Zonen CGR 4 Pyrgeometers - Manual (Page 21)

4.8. Window heating offset
Currently the major source of error concerning common pyrgeometer measurements is caused by the
so-called ‘window heating offset’. 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 difference between dome/window and thermopile will cause heat transfer by
radiation and convection to the sensor. This affects the net thermal radiation as measured by the thermopile
and is commonly referred to as the ‘window heating offset’. The result is measurement of a too high value
for downward long-wave radiation.
This offset is not easily reduced by ventilation, which only cools off 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 offset. A complex calculation must be performed to eliminate
the offset.
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 coefficient of the inner dome/window
surface. Its presence increases the window-heating offset.
- 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 difference 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 effectively 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 offset 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 offset 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 offset commonly called Zero Offset type B. It is quantified as the
response in W/m² to a 5 K/hr change in ambient temperature.
Page 21 CGR 4 Manual