Campbell Scientific CNR1 Net Radiometer - Manual (Page 27)

CNR1 Net Radiometer
8. Troubleshooting
If there is no clue as to what may be the problem, start performing the
following "upside-down test", which is a rough test for a first diagnosis. It can
be performed both outdoors and indoors. Indoors, a lamp can be used as a
source for both Solar and Far Infrared radiation. Outdoors one should
preferably work with a solar elevation of more than 45 degrees (45 degrees
above horizon) and of course under stable conditions (no large changes in solar
irradiance, preferably cloudless).
1. Measure the radiation outputs in the normal position. Record the
measured values when the signals have stabilized, i.e. after about three
minutes.
2. Rotate the instrument 180 degrees, so that the upper and the lower sensors
are now in the reverse orientation as to the previous position.
3. Measure the radiation outputs once more. Record the measured values
when the radiometers have stabilized.
4. Since of the all sensors are trimmed, the values in the rotated position
should be equal in magnitude, only differing in sign. In a rough test like
this, deviations of +/- 10 % can be tolerated. If deviations greater than
this are encountered, the following tests might help.
8.1 Testing the CM3
As a first test we recommend that one check the sensor impedance. It should
have a nominal value as indicated in the specifications. Zero, or infinite
resistance, indicates a failure in hardware connection.
Before starting the second test measurement, let the CM3 rest for at least five
minutes to let it regain its thermal equilibrium. For testing, set a voltmeter to
its most sensitive range setting. Darken the sensor. The signal should read
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.
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