Campbell Scientific NR01 Net Radiometer - Manual (Page 10)

NR01 Four-Component Net Radiation Sensor
The black coating on the thermopile sensor absorbs the solar radiation. This
radiation is converted to heat. The heat flows through the sensor to the
pyranometer housing. The thermopile sensor generates a voltage output signal
that is proportional to the solar radiation.
SW
in
= U
pyrano, up
/ E
pyrano, up
2.3-1
In case of the NR01, the pyranometer is type SR01. This is a second-class
pyranometer according to the WMO and ISO classification system (ISO 9060).
The atmospheric solar radiation consists of two components—direct radiation
(in a beam from the sun) and diffuse radiation from the sky.
For down facing instruments, the earth surface reflects part of the solar
radiation, depending on the local surface properties. If there is direct radiation,
this often is the dominant source of energy. Because the solar position is
changing, this implies that for a pyranometer the directional response is quite
important.
Table 2.3-1 summarizes the main sources of measurement errors for the SR01.
The error in the directional response is caused by non-perfect optical properties
of the dome and coating. The infrared offset is produced when the low
temperature “sky” cools off the instrument dome. Because the LW radiation
balance between dome and sky is negative, a negative sensor offset occurs as
the dome cools.
TABLE 2.3-1. Main Measurement Errors in the SW Signal
Source Maximum Error
Directional response +/- 30 W/m 2 on SW
in
in practice this level is
+/- 15 W/m 2 on SW
in
at 1000 W/m 2 SW
in
Infrared offset - 15 W/m 2 on SW
in
at -200 W/m 2 LW
net
Temperature dependence +/- 5 % for the entire range
2.4 Pyrgeometers
A pyrgeometer should measure the far infrared or LW radiation flux from a
field of view of 180 degrees. The atmospheric LW radiation spectrum extends
roughly from 4500 to 50000 nm. The pyrgeometer should cover that spectrum
with a spectral sensitivity that is as “flat” as possible.
For a flux measurement, by definition, the response to “beam” radiation varies
with the cosine of the angle of incidence. For example, full response occurs
when the radiation hits the sensor perpendicularly (normal to the surface,
source at zenith, 0 degrees angle of incidence); zero response occurs when the
radiation comes from the horizon (90 degrees angle of incidence, 90 degrees
zenith angle), and half a response occurs at 60 degrees angle of incidence. It
follows from the definition that a pyrgeometer should have a so-called
“directional response” or “cosine response” that is close to the ideal cosine
characteristic.
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