The pyrophometer LP PYRHE 16 is based on a new passive sensor to thermopile. The sensitive surface of the thermopile is coated with a black varnish opaque which allows the instrument not to be selective towards the different wavelengths. The spectral range of the pyrheliometer is determined by the transmission of the quartz window which has the function of protecting the sensor from dust and water. A special quartz makes it possible to carry out a measurement selective of 250nm-4000nm.
The adopted sensor makes it possible to obtain a response time inferior to specifications of the ISO9060 standard for the classification of pyrheliometers first class (the response time is less than 9 seconds while the
standard provides a response time of less than 20 seconds) The radiant energy is absorbed by the blackened surface of the thermopile, causing therefore a temperature difference between the hot junction and the
body of the pyrheliometer which, in this case, will act as a cold junction. The temperature difference between hot junction and cold junction is going to be transformed into Difference of Potential thanks to the Seebeck effect.
In order to minimize variations in sensitivity according to the temperature and to comply with the specifications laid down for a pyrheliometer of the second class, the LP PYRHE 16 is provided with a circuit of
passive compensation. Graph 1 shows the typical variation of sensitivity to different temperatures