Campbell Scientific CNR1 Net Radiometer - Manual (Page 10)

CNR1 Net Radiometer
4.2 Measuring Far Infrared Radiation with the CG3
The downward-facing CG3 measures the Far Infrared radiation that is emitted
by the ground. The upward-facing CG3 measures the Far Infrared radiation
from the sky. As the sky is typically colder than the instrument, one can expect
negative voltage signals from the upward-facing CG3. For this measurement,
the Pt-100 output is required. The Equation 4.2 is used to calculate the Far
Infrared irradiance of the sky and of the ground.
When using the CG3 pyrgeometer, one should realize that the signal that is
generated by the CG3 represents the exchange of Far Infrared (thermal)
radiation between the CG3 and the object that it is facing. This implies that the
CG3 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. This means that for estimating the
Far Infrared radiation that is generated by the object that is faced by the
pyrgeometer, usually the sky or the soil, one will have to take the pyrgeometer
temperature, T, into account. This is why a Pt-100 is incorporated in the
CNR1's body. (This body is in very good thermal contact with the CG3 and
has the same temperature as the CG3 sensor surface.) The calculation of the
Far Infrared irradiance, E, is done according to the following equation:
For the CG3 only E = V/C + 5.67 ⋅ 10 -8 ∗ T 4 (4.2)
In this equation C is the sensitivity of the sensor. Please bear in mind that T is
in Kelvin, and not in Celsius or Fahrenheit.
4.3 Measuring the CNR1’s Temperature with the Pt-100
The Pt-100 temperature sensor is located in the CNR1 body. It will not
measure the exact temperature of the CG3 unless the whole instrument is in
thermal equilibrium. Errors, however, are minimized in the design by making
solid metal connections between the sensors and the temperature sensor. When
the sun is shining, the largest expected deviation between real sensor
temperature and Pt-100 reading is 2 degrees. This results in a worst-case error
for the CG3 of 10 Watts per square meter.
The Pt-100 will not give a good indication of ambient air temperature; at 1000
Watts per square meter Solar radiation, and no wind, the instrument
temperature will rise approximately 12 degrees above ambient temperature.
The offsets of both pyranometers and pyrgeometers might be larger than 10
Watts per square meter if large temperature gradients are forced on the
instrument (larger than 5 K/hr). This happens, for example, when rain hits the
instrument. The occurrence of this can be detected using the Pt-100 readout. It
can be used as a tool for quality assurance of your data.
The 4WPB100 module is used to interface the Pt-100 to the datalogger, and is
included with the CNR1 sensor purchased from CSI. The 4WPB100
configures the Pt-100 as a 4-wire half bridge circuit that requires one excitation
and two differential channels on the datalogger (Section 5).
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