LPPYRHE16 -4-V2.3
2WORKING PRINCIPLE
The pyrheliometer LPPYRHE16 is based on a passive thermopile sensor. The sensitive
surface of the thermopile is coated with a matt black paint, which makes the pyrheli-
ometer not selective to the different wave lengths. The spectral range of the pyrheli-
ometer is determined by the transmission of the quartz window, whose function is to
protect the sensor from dust and water. A special quartz allows to perform a non-
selective measurement from 200 nm to 4000 nm.
The adopted sensor allows the response time to be lower than ISO 9060:2018 re-
quirements for the classification of class B pyrheliometers (the response time is un-
der 9 seconds while the standard requires a response time lower than 15 seconds).
Radiant energy is absorbed by the blackened surface of the thermopile, thus creat-
ing a difference in temperature between the hot junction and the body of the pyr-
heliometer, which acts in this case as a cold junction. Thanks to the Seebeck effect,
the difference in temperature between hot and cold junction is converted into a Dif-
ference of Potential.
In order to reduce the variations of sensitivity depending on temperature and to fall
within the specifications requested to a class B pyrheliometer, LPPYRHE16 is pro-
vided with a passive compensation circuit. Figure 2.1 shows the typical variation of
sensitivity at different temperatures. Deviations are calculated starting from sensi-
tivity measured at 20 °C.
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
-30 -20 -10 010 20 30 40 50 60
Variazione % della sensibilità
Fig. 2.1: % variation of sensitivity of the LPPYRHE16 pyrheliometer with regard
to sensitivity at 20 °C in the temperature range from -20 to 50 °C
LPPYRHE16 is a sealed instrument, for that reason a cartridge of silica-gel crystals is
provided to absorb humidity inside the instrument, in order to prevent condensation
from forming on the quartz window of the instrument, invalidating the performed
measurements.
In accordance with WMO regulations, the angular field of view is 5° and the slope an-
gle is 1° (figure 1).
