CNR1 Net Radiometer
zero. Bear in mind that the response takes about one minute. Small deviations
from zero are possible; this is caused by thermal effects like touching the
pyranometer with your hand. The latter effect can be demonstrated by
deliberately heating the CM3 with your hand. Another cause might be the zero
offset of the amplifier. When this is the case, the same offset will also be
present when the amplifier is short-circuited with a 200 Ohm resistor. This is
an amplifier error. This amplifier error should not be larger than 5 Watts per
square meter. If the amplifier error is within specifications, proceed with the
third test.
In the third test the sensor should be exposed to light. The signal should be a
positive reading. Set the voltmeter range in such a way that the expected full-
scale output of the pyranometer is within the full-scale input range of the
voltmeter. The range can be estimated on theoretical considerations. (When
the maximum expected radiation is 1500 Watts per square meter, which is
roughly equal to normal outdoor daylight conditions, and the sensitivity of the
pyranometer is 15 mV per Watt per square meter, the expected output range of
the pyranometer is 1500 times 15 which is equal to 22500 mV, or 0.0225
Volts). One can calculate the radiation intensity by dividing the pyranometer
output (0.0225 volts) by the calibration factor (0.000015 volt per watt per
square meter). Still no faults found? Your pyranometer is probably doing fine.
8.2 Testing of the CG3
It is assumed that the amplifier circuit is the same as the one used for the CM3,
and that its zero offset is no more than a few watts per square meter, let us say
5 Watts per square meter just as an example (see second test in 7.1).
The pyrgeometer, the mounting plate, and ambient air should be at the same
temperature as much as possible. Let the CG3 rest for at least five minutes to
regain its thermal equilibrium. Set the voltmeter to its most sensitive range.
To test if the CG3 is working properly, we suggest putting your hand in front
of the CG3. The thermal radiation will cause CG3 to generate a positive
voltage when the hand's surface temperature is higher than the pyrgeometer
temperature. The CG3 will generate a negative voltage if the hand is colder.
The signal is proportional to the temperature difference (see the rule of thumb
of Section 4.5). The radiation that is emitted by the hand can be calculated by
dividing the pyrgeometer output by the calibration factor, and subsequently
correcting for the temperature, according to Equation 4.2. Still no faults
found? Your pyrgeometer is probably doing fine.
8.3 Testing the Pt-100
Using a meter which measures resistance, check the operation of the Pt-100. If
connected properly, the resistance of two opposite wires of the Pt-100 should
measure about 100 ohms (this includes the cable resistance for the standard 10-
meter cable). The cable resistance should measure about 0.1 ohms per meter
cable.
23
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