CNR4 Net Radiometer
10
For the pyrgeometer only
E = V/C + 5.67
*
10
-8
*
T
4
(5-2)
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.
The downward-facing pyrgeometer measures the far infrared radiation that is
emitted by the ground. The upward-facing pyrgeometer 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 pyrgeometer. The
Equation 5-2 is used to calculate the far infrared irradiance of the sky and of the
ground.
5.3 Measuring CNR4 Temperature with Thermistor
The CNR4 has two temperature sensors built inside: thermistor and Pt-100. They
both have the identical accuracy. We recommend using the thermistor with
Campbell Scientific, Inc. dataloggers. The thermistor has a larger resistance (10
k
Ω
@ 25°C) than Pt-100 sensor (100
Ω
@ 0°C), and the change in resistance with
respect to temperature, in absolute terms, is greater. Therefore, the cable
resistance can be neglected, and the thermistor can easily be measured using half-
bridge measurement instruction on Campbell Scientific, Inc. dataloggers. This
makes it simpler to program, uses less number of measurement channels, and offer
better compatibility across the family of our dataloggers.
Table 5-1 shows the thermistor resistance values as a function of temperature.
Relatively small errors occur when the CNR4 is not in thermal equilibrium. This
happens for example when the heater is on, or when the sun is shining. When the
heater and ventilator are on, the largest expected deviation between the real sensor
temperature and the thermistor reading is 1 degree. This results in a worst case
error for the pyrgeometer of 5 W/m
2
. When the sun is shining, the largest
expected deviation between the real sensor temperature and the thermistor reading
is again 1 degree. This results in a worst case error for the pyrgeometer of 5
W/m
2
.
The thermistor will not give a good indication of ambient air temperature; at 1000
W/m
2
solar radiation, and no wind, the instrument temperature will rise
approximately 5 degrees above the ambient temperature.
The offsets of both the pyranometers and the pyrgeometers might be larger than
5W/m
2
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 thermistor readout, and can be used for data
filtering.
The thermistor measurement can be done by the datalogger, using the half bridge
measurement method that requires one voltage excitation and one single-ended
analogue channel.
A 1K precision resistor is incorporated into the free end of the
temperature cable to complete the thermistor bridge. Do not shorten
or extend the cable without replacing/moving this resistor at the end
of the cable.
NOTE
