Campbell NR01, Инструкция по эксплуатации

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NR01 Four-Component Net Radiation Sensor
17
However, the sensor itself also irradiates LW radiation. This is according to
Plank’s law, so that the pyrgeometer thermopile signal is composed of the
incoming radiation minus the outgoing radiation. In order to estimate the
outgoing component, the pyrgeometer temperature is measured independently,
using a PT100 or a user-supplied temperature sensor. Equation 7-12 calculates
the incoming LW radiation assuming T
pyrgeo
is in Kelvin:
LW
in
= (U
pyrgeo, up
/ E
pyrgeo, up
) + 5.67.10 -8 (T
pyrgeo
) 4 7-12
For LW
out
a similar formula is valid. The equations are the same for up and
down facing instruments.
It is possible to calculate temperatures of the objects within the field of view of
the instrument, assuming these are uniform- temperature blackbodies (emission
coefficient of 1). For example, equation 7-13 calculates, in Kelvin, the sky
temperature:
T
sky
= (LW
in
/5.67.10
-8
)
1/4
7-13
The NR01’s pyrgeometers are type IR01. Pyrgeometers are not classified by
the ISO or WMO.
The atmospheric LW
in
radiation essentially consists of two components:
1 Low temperature radiation from the universe, filtered by the atmosphere.
The atmosphere is transparent for this radiation in the so-called
atmospheric window (around 10 to 15 micrometer wavelength).
2 Higher temperature radiation emitted by atmospheric gasses.
Down facing instruments are presumably looking directly at the surface, which
behaves like a normal blackbody.
As a first approximation, the sky can, be seen as a cold temperature source with
its lowest temperatures at zenith and getting warmer at the horizon. The
uniformity of this LW source is much better than that in the solar (SW) range,
where the sun is a dominant and non-uniform contributor. This explains why a
pyrgeometer with 150 degrees field of view can perform a good measurement.
TABLE 7-2 summarizes the main measurement errors for the IR01. The error
in the directional response is caused by non-perfect field of view. The
window-heating offset occurs when solar radiation heats up the instrument
window, producing a positive sensor offset.
TABLE 7-2. Main Measurement Errors in the LW Signal
Source Maximum Error
Directional response 8 W/m 2 on LW
in
at -100 W/m 2 LW
net
Window heating offset +15 W/m 2 on LW
in
at 1000 W/m 2 SW
in
Temperature dependence ±5% for the entire range