Kipp&Zonen CNR 4 Net Radiometers - Manual (Page 31)

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5 TROUBLE SHOOTING
This chapter describes what to do if there appears to be a problem. The following chapters give individual
information for checking the pyranometer, pyrgeometer, the temperature sensors and the ventilation unit with
heater.
All connections to the CNR 4 are made with connectors and cables that can be separated from the main instrument.
Check at all times that these connectors are properly attached and screwed to the body of the CNR 4.
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 you 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 output in the normal position. Record the measured values when the signals have stabilised,
i.e. after about 3 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 output once more. Record the measured values when the radiometers have stabilised.
4.
The calculated radiation for the sensors in the rotated position should be equal in magnitude, only differing
in sign. In a rough test like this, deviations of +/- 10 % should be tolerated. If deviations greater than this are
encountered, the following tests might help.
5.1 Testing the pyranometer
As a first test we recommend that you check the sensor impedance. It should have a nominal value between 20 and
200 Ohm. Zero, or infinite resistance indicates a failure in hardware connection.
Before starting the second test measurement, let the pyranometer 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 pyranometer with your hand. Another cause might be the zero offset of
the data logger. When this is the case, the same offset will also be present when the data logger is short-circuited
with a 200 Ohm resistor. This is an amplifier error from the data logger. 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 metre, which is roughly equal to normal outdoor daylight conditions, and the sensitivity of
the pyranometer is 15 µV per Watt per square metre, the expected output range of the pyranometer is 1500 times
15 which is equal to 22500 µV, or 0.0225 Volts). You can calculate the radiation intensity by dividing the
pyranometer output (0.0225 volts) by the calibration factor (0.000015 volt per watt per square metre). Still no faults
found? Your pyranometer is probably doing fine.
5.2 Testing of the pyrgeometer
It is assumed that the data logger (amplifier) circuit is the same as the one used for pyranometer, and that its zero
offset is no more than a few watts per square metre, let us say 5 Watts per square metre just as an example, (see
test in 5.1).