Campbell Scientific CNR4 Net Radiometer - Manual (Page 21)

CNR4 Net Radiometer
7. Operation
7.1 Using the CNR4 in the Four Separate Components Mode
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.
7.1.1 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
(Equation 7-1).
For each pyranometer,
E = V/C (7-1)
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 7-1, 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
radiation must be subtracted from the downwelling radiation. See Section
7.1.5, Calculation of Net Short-wave Radiation.
7.1.2 Measuring Long-wave Far Infrared Radiation with Pyrgeometer
When using the pyrgeometer, you should realize the signal generated by the
pyrgeometer represents the exchange of long-wave far infrared (thermal)
radiation between the pyrgeometer and the object that it is facing. This implies
that the pyrgeometer will generate a positive voltage output, V, when it faces
an object that is hotter than its own sensor housing, and that it will give a
negative voltage signal when it faces an object that is colder. Therefore, when
estimating the far infrared radiation that is generated by the object facing the
pyrgeometer, usually the sky or the soil, you will have to take the pyrgeometer
temperature, T, into account. This is why the temperature sensors are
incorporated in the CNR4’s body near the pyrgeometer sensing element, and
has, therefore, the same temperature as the pyrgeometer sensor surface. The
calculation of the long-wave far infrared irradiance, E, is done according to
Equation 7-2.
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