CNR4 Net Radiometer
16
8. Operation
Where,
Rf = thermistor resistance in ohms (for example, for a 10 kohm thermistor, Rf
is 10000)
Vx = value returned by the
BrHalf()
instruction
The Steinhart-Hart equation is used to convert resistance to temperature. The
Steinhart-Hart equation for converting resistance to degree Celsius is as
follows:
Temperature = 1(1.0295e-3+2.391e-4 x LN(Rs)+1.568e-7 x (LN(Rs))^3)-
273.15
In CRBasic, the Steinhart-Hart equation is entered as a mathematical
expression. A downloadable example program that uses a thermistor is
/downloads/cnr4-thermistor-example-
The Pt-100 is measured with the
Resistance()
CRBasic instruction. The value
provided by the
Resistance()
instruction needs to be divided by 100 then
converted to temperature using the
PRTCalc()
instruction. A downloadable
program that uses the Pt-100 is available at:
/downloads/cnr4-pt100-example-program
Details about the internal sensors are provided in Section
Temperature Sensors Measurements
.
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
data logger, 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.
8.1 Short-wave Solar Radiation Measurements
The pyranometer generates a millivolt 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
For each pyranometer,
E = V/C
(8-1)
Incidental light results in a positive signal. The pyranometer mounting plate
and ambient air should be at the same temperature. Conversion of the voltage
