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GENERAL INFORMATION
1.2
PHYSICAL PRINCIPLES
The pyranometer CM 11 is provided with a thermal detector.
This type of detector responds to the total power absorbed and
theoretically it is non-selective as to the spectral distribution of the
radiation. This implies that the naked thermal detector is also sensitive
to long wave infrared radiation (thermal radiation λ > 3000 nm) from
the environment. (e.g. the inner dome)
The radiant energy is absorbed by a black painted disk. The heat
generated flows through a thermal resistance to the heat sink (the
pyranometer body). The temperature difference across the thermal
resistance of the disk is converted into a voltage.
The rise of temperature is easily affected by wind, rain and thermal
radiation losses to the environment ('cold' sky). Therefore the detector
is shielded by two glass domes. Glass domes allow equal transmitting
of the direct solar component for every position of the sun on the
celestial sphere. The spectral range of the pyranometer is limited by
the transmission of the glass. See figure 3. A desiccator in the body
prevents dew on the inner side of the domes, which can cool down
considerably, at clear windless nights.
Construction details (See figure 1)
The sensing element of the pyranometer CM 11 is a black painted
ceramic (Al
O
) disk. 100 thermocouples forming a thermopile are
2
3
imprinted on it using thick film techniques. Only the border of the disk
is in good thermal contact with the pyranometer body (heat sink), and
along this border the 100 cold junctions are located. The 100 hot
junctions are near the centre in a rotational symmetric arrangement.
This fact plus a proper levelling of the sensor related to the spirit level
results in a low azimuth error.
When the pyranometer is illuminated, the absorbed radiation results in
a radial heat flow to the border of the disk. The temperature in the
centre of the disk will rise due to its thermal resistance. The thermal
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