CNR4 Net Radiometer
17
to irradiance can be done according to Equation
, and is computed by the
data logger program.
With the upward-facing pyranometer, the 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
Calculation of Net Short-wave Radiation
(p. 20)
.
8.2 Long-wave Far Infrared Radiation Measurements
The signal 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 a negative voltage signal when it faces an object that is colder. Therefore,
when estimating the far infrared radiation that is generated by the object facing
the pyrgeometer, usually the sky or the soil, you will have to take the
pyrgeometer temperature, T, into account. This is why the temperature sensors
are incorporated in the CNR4 body near the pyrgeometer sensing element, and
has, therefore, the same temperature as the pyrgeometer sensor surface. The
calculation of the long-wave far infrared irradiance, E, is done according to
Equation
For the pyrgeometer only
E = V/C + 5.67 × 10
-8
× T
4
(8-2)
In this equation, C is the sensitivity of the sensor.
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. Equation
is used to calculate the far infrared irradiance of the
sky and of the ground.
8.3 Internal Temperature Sensors Measurements
The CNR4 has two temperature sensors built inside: thermistor and Pt-100;
both have identical accuracy. The thermistor is recommended when using
Campbell Scientific data loggers. The thermistor has a greater resistance
(10 k
Ω
at 25 °C) than Pt-100 sensor (100
Ω
at 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
data loggers.
shows the thermistor resistance values as a function of
temperature.
NOTE
