CNR4 Net Radiometer
17
7.4.1 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
(p. D-1)
, for more details.
The sensor sensitivity is in
µ
V/(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
µ
V/(W/m
2
), the multiplier is 1000/7.3 =
136.99 (W/m
2
)/mV.
8. Operation
In the four separate components mode configuration (measuring two short-
wave radiation signals and two long-wave signals), all signals are measured
separately. Calculation of net-radiation and albedo can be done online by the
datalogger, or offline by the user during post-processing, using the stored raw
data.
The two pyranometers will measure the short-wave radiation, both incoming
and reflected. The two pyrgeometers will measure the long-wave radiation. For
proper analysis of the pyrgeometer measurement results, they must be
temperature corrected using the temperature measurement performed by the
onboard thermistor or Pt-100 sensor.
8.1 Measuring Short-wave Solar Radiation with Pyranometer
The pyranometer generates an mV signal that is simply proportional to the
incoming short-wave radiation. The conversion factor between voltage, V, and
W/m
2
of solar irradiance E, is the calibration constant C or sensitivity
For each pyranometer,
E = V/C
(8-1)
Measuring with a pyranometer can be done by connecting two pyranometer
wires to a datalogger. Incidental light results in a positive signal. The
pyranometer mounting plate and ambient air should be at the same
temperature. Conversion of the voltage to irradiance can be done according to
Equation
, and is computed by the datalogger program.
With the upward-facing pyranometer, the global (solar) downwelling radiation
is measured. The downward-facing pyranometer measures the reflected
upwelling solar radiation. When calculating the net radiation, the upwelling
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