Campbell CNR4, Инструкция по эксплуатации

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CNR4 Net Radiometer
18
radiation must be subtracted from the downwelling radiation. See Section 8.5 ,
Calculation of Net Short-wave Radiation
(p. 21)
.
8.2 Measuring Long-wave Far Infrared Radiation with
Pyrgeometer
When using the pyrgeometer, you should realize 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 that it will give 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’s 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 8-2 .
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 8-2 is used to calculate the far infrared irradiance of the
sky and of the ground.
8.3 Measuring CNR4 Temperature with Thermistor
The CNR4 has two temperature sensors built inside: thermistor and Pt-100;
both have identical accuracy. Using the thermistor is recommended when using
Campbell Scientific dataloggers. The thermistor has a greater 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 dataloggers.
TABLE 8-1 shows the thermistor resistance values as a function of
temperature.
NOTE