CNR1 Net Radiometer
4. Using the CNR1 in the Four Separate Components
Mode (4SCM)
In the 4SCM configuration (measuring two Solar Radiation signals, two Far
Infrared signals and, for calculation purposes, one Pt-100 signal), all signals are
measured separately. Calculation of Net-Radiation and albedo can be done by
the datalogger, or later by the computer from the radiation and temperature
data.
The two CM3s will measure the solar radiation, both incoming and reflected.
The two CG3s will measure the Far Infrared radiation. For proper analysis of
the CG3 measurement results, they must be temperature corrected using the
temperature measurement performed by the Pt-100.
The following paragraphs describe how one should treat the instrument, and
how different parameters like net Solar radiation, net Far Infrared radiation,
soil temperature, sky temperature, and Net (total) radiation can be calculated.
4.1 Measuring Solar Radiation with the CM3
Measuring with the upward-facing CM3 the so-called global (solar) radiation is
measured. The downward-facing CM3 measures the reflected solar radiation.
When calculating the Net radiation, the Reflected radiation must be subtracted
from the global radiation. See Section 4.5.
The CM3 pyranometer generates a mV signal that is simply proportional to the
incoming Solar radiation. The conversion factor between voltage, V, and
Watts per square meter of solar irradiance E, is the so-called calibration
constant C (or sensitivity).
For the CM3 E = V/C (4.1)
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:
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