Instruction manual

CNR4 Net Radiometer
The CNR4 outputs four voltages that typically range from 0 to 15 mV for the
pyranometers, and ± 5 mV for the pyrgeometers. A differential voltage
measurement is recommended because it has better noise rejection than a
single-ended measurement. If differential channels are not available, single-
ended measurements can be used. The acceptability of a single-ended
measurement can be determined by simply comparing the results of single-
ended and differential measurements made under the same conditions.
Additionally, one voltage excitation channel and one single-ended analog
channel are required to make the temperature measurement of the sensor body,
using the thermistor.
7.3.1 Sensor Sensitivity
The CNR4 comes with four different sensor sensitivity values for four separate
probes. The CNR4 sensor comes with two copies of its ‘Certificate of
Calibration’ by the manufacturer. They show the sensor serial number and
sensitivity values for four individual probes: one copy for pyranometers, and
another copy for pyrgeometers. The serial number and sensitivity values are
also shown on a label affixed to the bottom of the sensor. If you choose to
attach the CNF4 heater/ventilator unit to the CNR4, the label showing the
serial number and sensitivity values will be covered. After attaching the CNF4
heater/ventilator, affix the extra label to the bottom of the CNF4 in a visible
location. The extra label containing the serial number and sensitivity values is
supplied with the purchase of the CNR4. Please refer to Appendix D, CNF4
Heater/Ventilator, for more details.
The sensor sensitivity is in µV/(W/m
2
). This needs to be converted into
(W/m
2
)/mV to be used as a multiplier parameter inside the datalogger program.
To convert the units, divide the sensor sensitivity value into 1000. For
example, if the sensitivity is 7.30 µV/(W/m
2
), the multiplier is 1000/7.3 =
136.99 (W/m
2
)/mV.
8. Troubleshooting
If there is no indication 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 short-wave and long-wave radiation. Outdoors, one should
preferably work with a solar elevation of more than 45 degrees (45 degrees
above horizon) and under stable conditions (no large changes in solar
irradiance, and preferably no clouds).
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. The computed net radiation values in rotated position should be equal in
magnitude but only differing in sign. In a rough test like this, deviations
of ± 10 % can be tolerated. If deviations greater than this are encountered,
additional testing is warranted.
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