25. INSTRUMENTS AND METHODS FOR THE GROUND-LEVEL
REFERENCE MEASUREMENT OF SOLAR RADIATION, ALBEDO
AND NET RADIATION
247
3.
PYRANOMETERS
Two types of the pyranometers are in use: devices with black and white
receiving surface and single glass dome, and with black receiving surface
and double glass dome (Fröhlich, London, 1986). The first ones are more
stable under variable conditions and their sensitivity depends less on the
temperature, but they are more selective and the sensitivity may change due
the aging of the white paint. Instruments with the black receiving surface do
not have the shortcomings mentioned above, but the output of these may be
affected by the thermal radiation of the domes when the temperature differs
from the temperature of the sensor. It is why the moderate ventilation of the
instrument is recommended to stabilize the thermal regime. Another possible
reason of instrumental errors is less well-known. Usually the instruments are
designed so that active thermocouples are in thermal contact with the
receiving surface and reference ones with the instrument body, the thermal
inertia of which is far larger. For that reason essential deviations of the zero
point are possible under variable temperature regime of the instrument. To
avoid these errors, a thermally symmetrical pyranometer TP-3 was designed
by J. Reemann (Fig. 5). The thermocompensated hermetical pyranometer is a
precision thermoelectrical sensor designed to measure global, diffuse or
reflected solar radiation under field conditions. Fully symmetrical
construction guarantees high stability of sensitivity and zero point at variable
temperature regime. The instrument is hermetical to avoid inside
condensation. The thin double glass domes and painted by NEXTEL 2010
Velvet Coating Black sensitive surface cover measurements in the spectral
REFERENCE MEASUREMENT OF SOLAR RADIATION, ALBEDO
AND NET RADIATION
247
3.
PYRANOMETERS
Two types of the pyranometers are in use: devices with black and white
receiving surface and single glass dome, and with black receiving surface
and double glass dome (Fröhlich, London, 1986). The first ones are more
stable under variable conditions and their sensitivity depends less on the
temperature, but they are more selective and the sensitivity may change due
the aging of the white paint. Instruments with the black receiving surface do
not have the shortcomings mentioned above, but the output of these may be
affected by the thermal radiation of the domes when the temperature differs
from the temperature of the sensor. It is why the moderate ventilation of the
instrument is recommended to stabilize the thermal regime. Another possible
reason of instrumental errors is less well-known. Usually the instruments are
designed so that active thermocouples are in thermal contact with the
receiving surface and reference ones with the instrument body, the thermal
inertia of which is far larger. For that reason essential deviations of the zero
point are possible under variable temperature regime of the instrument. To
avoid these errors, a thermally symmetrical pyranometer TP-3 was designed
by J. Reemann (Fig. 5). The thermocompensated hermetical pyranometer is a
precision thermoelectrical sensor designed to measure global, diffuse or
reflected solar radiation under field conditions. Fully symmetrical
construction guarantees high stability of sensitivity and zero point at variable
temperature regime. The instrument is hermetical to avoid inside
condensation. The thin double glass domes and painted by NEXTEL 2010
Velvet Coating Black sensitive surface cover measurements in the spectral