GHM Delta OHM LPPIRG01, Operating Manual, Page: 4 / 26

LPPIRG01
- 4 -
V2.1
2 WORKING PRINCIPLE
LPPIRG01 pyrgeometer is based on a thermopile sensor. The thermopile sensitive sur-
face is coated with a black matt paint, which allows the pyrgeometer not to be selective
at different wavelengths.
The sensor is covered by a silicon window that has two basic purposes:
1.
protect the thermopile from bad weather;
2. determine the instrument spectral range: silicon is transparent to wavelengths
longer than 1.1 µm, therefore on the inside of the window there is a filter to block
radiation up to 4.5 - 5 µm. The silicon external surface, which is exposed to weather
agents, has a scratch-resistant coating (DLC) to ensure strength and durability in
all weather conditions. The scratch-resistant coating offers the advantage of clean-
ing the surface without the risk of scratching the window. The transmission of the
silicon window as the wavelength varies is shown in figure 2.1.
1.
Fig. 2.1: Transmission of the silicon window
Radiant energy is absorbed/radiated by the thermopile black surface, thus creating a
difference of temperature between the center of the thermopile (hot junction) and the
pyrgeometer body (cold junction). Thanks to the Seebeck effect, the difference of tem-
perature between hot and cold junction is converted into a Difference of Potential.
If the pyrgeometer temperature is higher than the radiant temperature of the portion
of sky framed by the pyrgeometer, the thermopile will irradiate energy and the output
signal will be negative (typical situation of clear sky); vice versa, if the pyrgeometer
temperature is lower than that portion of sky framed, the signal will be positive (typical
situation of cloudy sky).
Therefore, for the calculation of infrared radiation at ground level (E
FIR
↓
), besides the
thermopile output signal, it is necessary to know the temperature of the pyrgeometer,
as shown in formula 1: