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CNR4 Net Radiometer

5. Using the CNR4 in the Four Separate Components

Mode

In the four separate components mode configuration (measuring two short-

wave radiation signals, two long-wave signals), all signals are measured

separately. Calculation of net-radiation and albedo can be done on-line by the

datalogger, or off-line 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.

The following paragraphs describe how one should treat the instrument, and

how different parameters like albedo, net short-wave radiation, net long-wave

radiation, soil temperature, sky temperature, and net (total) radiation can be

calculated.

5.1 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

of solar irradiance E, is the so-called calibration constant C (or

sensitivity).

For each pyranometer

E = V/C (5-1)

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, as much as possible. Conversion of the voltage to irradiance can

be done according to equation 5-1, and this is done inside the datalogger

program.

With the upward-facing pyranometer the so-called 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 5.5.

5.2 Measuring Long-wave Far Infrared Radiation with

Pyrgeometer

When using the pyrgeometer, you should realize that the signal that is

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

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Campbell CVF4 Spécifications Page 15

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