FAQs Solar Resource Assessment

Solar Resource Assessment

Not directly. The output signal from this pyranometer is in milivolts and the input channels on the EOL Zenith ranges from 0-5 volts. You therefore need an amplifier for this pyranometer. The amplifier we supply is called AMPVAR.

This error is related to the zero offset type A. Normally this zero offset is present when the inner dome has a different temperature from the cold junctions of the sensor. Practically this is always the case when there is a clear sky. Because of the low effective sky temperature (<0 °C) the earth surface emits roughly 100 W/m2 longwave infrared radiation upwards. The outer glass dome of a pyranometer also has this emission and is cooling down several degrees below air temperature (the emissivity of glass for the particular wavelength region is nearly. The emitted heat is attracted from the body (by conduction in the dome), from the air (by wind) and from the inner dome (through infrared radiation). The inner dome is cooling down too and will attract heat from the body by conduction and from the sensor by the net infrared radiation. The latter heat flow is opposite to the heat flow from absorbed solar radiation and causes the well-known zero depression at night. This negative zero offset is also present on a clear day, however, hidden in the solar radiation signal.

Zero offset type A can be checked by placing a light and IR reflecting cap over the pyranometer. The response to solar radiation will decay with a time constant (1/e) of 1 s, but the dome temperature will go to equilibrium with a time constant of several minutes. So after half a minute the remaining signal represents mainly zero offset type A.

Good ventilation of domes and body is the solution to reducing zero offsets even further.

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