Campbell Scientific CNR1 Net Radiometer - Manual (Page 9)

CNR1 Net Radiometer
3.1 Connecting and Using the Heater
Only use the sensor’s heater when there is risk of dew forming on the sensors,
especially for low power installations. Furthermore, the heater should be
turned on and off infrequently because it may take some time for the sensor to
come to thermal equilibrium. No damage will result if the heater is powered
continuously, but as with all thermopile sensors, it is best if the sensor operates
at ambient temperatures and is not subject to rapid temperature changes.
The sensor power can be controlled using one of the 12V power switches built
into Campbell dataloggers or using an external solid state switch such as a
PSW12/SW12. The heater’s current drain is approximately 500 mA when
using a 12V battery. Connect the ground return from the heater either directly
to the battery or to a G terminal close the power input to the logger (i.e., not to
an analog ground near the measurement inputs).
The heater power can be controlled by adding instructions to the datalogger
program that turns on the heater only when the light level falls below 20 W m
-2
or, if a measurement of air humidity is available, when the dew point of the air
falls to within 1ºC of the sensor body temperature. More details about using
the heater are provided in Appendix B. Appendix C provides an example
CR3000/CR5000 program that controls the CNR1 heater.
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)
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