Campbell Scientific NR-LITE2 Net Radiometer - Manual (Page 14)

NR-LITE2 Net Radiometer
5. Overview
The NR-LITE2 is used for measuring solar and far infrared radiation balance.
This balance is known as the net (total) radiation. Its upwards facing sensor
measures the solar energy and far infrared energy that is received from the
entire hemisphere (180° field of view). Its downwards facing sensor measures
the energy received from the surface of the soil. The two readings are
automatically subtracted and the result converted to a single output signal.
This output represents the net radiation (which can be interpreted as meaning
the radiative energy that is seen at the surface) and is expressed in Watts per
square meter (W m
-2 ).
The NR-LITE2 is designed for continuous outside use. The sensor surfaces are
coated with PTFE. This ensures sensor stability, long life, and easy
maintenance compared to the more usual radiometers fitted with plastic domes.
However, it does have some disadvantages, particularly a higher sensitivity to
wind speed with a subsequent lessening of accuracy. It is, though, possible to
correct for the wind speed sensitivity if the sensor is installed in a system
where wind speed is also being measured.
Although net radiometers are usually used in meteorology to measure radiation
balance, the NR-LITE2 can also be used to measure indoor climate radiative
stress.
The NR-LITE2 is manufactured by Kipp & Zonen, but cabled for use with
Campbell Scientific dataloggers. Its cable can terminate in:
• Pigtails that connect directly to a Campbell Scientific datalogger
(cable termination option –PT).
• Connector that attaches to a prewired enclosure (cable termination
option ─PW).
5.1 Electrical Properties
The thermopile consists of a number of thermocouples connected in series,
essentially providing a highly sensitive differential temperature sensor. The
thermopile generates a voltage output — the sensor itself is passive, and so no
power supply is required.
The upwards and downwards facing sensor surfaces are connected to the upper
and lower thermopile junctions respectively, allowing the sensor to measure
the differential temperature (FIGURE 5-1). This temperature differential can
be measured to a high accuracy (in the order of 0.001 degrees), and is
proportional to the net radiation.
The thermopile determines the electrical characteristics of the instrument. Both
upper and lower facing sensors have a field of view of 180 degrees, and their
angular characteristics conform closely to the cosine response (see following
sections).
The electrical sensitivity for the thermopile changes with temperature, and no
nominal sensitivity value is available.
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