2.3.4 Analogue current output
The SGR-A (current output versions) have been factory set such that an output of 4 mA represents 0 W/m² and the full-scale
output of 20 mA represents 1000 W/m².
The current output range in W/m² can be changed by the user with the supplied PC software. The maximum recommended
irradiance for the SGR pyrgeometers is 1000 W/m².
Negative inputs will make the output go below 4 mA and no zero offset is needed. For the default setting of 4 to 20 mA representing
0 to 1000 W/m². The irradiance value (
L
d↓
) for the default setting can be simply calculated as shown below.
L
d↓
= Downward atmospheric long-wave radiation
[W/m²]
mA = Output of radiometer
[mA]
2.3.5 Recommended cable types
Where cables need to be extended, or the customer prefers to provide their own cables, they should be suitable for outdoor used
and UV resistant.
Recommended types
RS-485
Ethernet CAT 5 shielded twisted pair (STP)
0 to 1 V
Shielded 2-core signal cable
4 to 20 mA
Shielded twisted pair control cable
2.4 Operation
After completing the installation the pyrgeometer will be ready for operation. The downward atmospheric long-wave radiation
is calculated automatically inside the pyrgeometers with formula L
d
by measuring the detector output voltage U
emf
[µV], the
housing temperature T
b
[K], and taking the sensitivity calibration factor S [µV/W/m²] into account.
Note
The net radiation term (U
emf
/S) is mostly negative, so the calculated downward atmospheric long-wave radiation
is smaller than the detector’s upward irradiance (5.67 • 10
-8
• T
4
).
In the Baseline Surface Radiation Network (BSRN) manual (WMO/TD-No.897) an extended formula is described. This formula
corrects for window heating and so called ‘solar radiation leakage’. Due to the very low window heating offset and optimal
spectral cut-on wavelength, these corrections are not necessary for the pyrgeometer.
Ld
↓
= (mA-4) x (1000/16)
18
formula L
d
= Downward atmospheric long-wave radiation
[W/m²]
Net radiation (difference between the downward [W/m²]
= longwave radiation emitted from the atmosphere
and the upward irradiance of the SGR detector)
= Upward irradiance of the SGR detector
[W/m²]
L
d
= + 5.67 • 10
-8
• T
4
U
emf
S
b
L
d
5.67 • 10
-8
• T
4
b
U
emf
S
b
