Pyrgeometer

A pyrgeometers (by ancient Greek. Πῦρ / pyr / "Fire" and γῆ / geo / "Earth" ) is used to measure the incident from the half-space atmospheric back-radiation is thermal radiation in the range from about 4 microns.

Design and function

A pyrgeometers consists in principle of an edge filter, which keeps the portion of the solar radiation, and a black surface which absorbs the radiation transmitted by, and is warm.

The filter consists of a dome- shaped support (DOM) having an inner vapor-deposited interference filters. Since the radius of the dome is large compared to the measurement surface, are rays which strike the absorber surface, roughly perpendicular passing through the layers of the interference filter, which is important for its function. The contrary will filter effect, with a sensitivity of 0.3 to 3 microns, the device would be a pyranometer for measuring the solar global radiation.

The carrier material must be transparent at least to 50 microns and consists of a very thin silicon or polyethylene, which is held by pressure in the form. The silicon may be thicker, which makes the device more robust and allows to measure the temperature of the dome at one or more points - because the material is not completely transparent, wearing his temperature with the radiation flux.

The black absorber surface is one of two faces of a thermopile, the applied voltage is proportional to the heat flow through the column. The other surface of the column is in thermal contact with the chassis of the device. The absorber area, in order to avoid irradiation of the column from the side, is fitted into a black plate, which is likewise in thermal contact with the chassis. The temperature of the chassis is also measured and used to infer the temperature of the absorber in order to take into account the radiation towards the sky can.

An accurate calibration of the Pyrgeometers happens against a blackbody at various combinations of temperatures of the radiator, the absorber and the cathedral.

A sun shield reduces in clear weather, the contribution of solar radiation to the measured value (2% of the solar radiation are longer wavelength than 3 microns ). A slightly heated air flow from an annular nozzle prevents nocturnal dew on the dome.

Pyrgeometergleichung

For this purpose, an ideal pyrgeometers is considered. The ideal pyrgeometers consists of an ideal black, horizontal foil over a black base plate ideal. The temperatures of the film () and base plate ( ) are measured. To find the Pyrgeometergleichung, the Stefan- Boltzmann law is assumed. This assumption is necessary to calculate the counter-radiation from these temperatures. For the Pyrgeometergleichung a radiative equilibrium is assumed about the film. From below, the film absorbs the black body radiation of the bottom plate, the top back-radiation. The emission of the sheet is the black body radiation by the top and bottom. This will:

Is it by rearranging:

This results in the thermodynamic sense, the counter-radiation as a computation size, a very good auxiliary variable for understanding the radiation but - especially in the atmosphere - is.

In practice, disturbances modify the simple equation and the instruments are calibrated.

Although the counter-radiation is treated as a reality, it is thermodynamically calculated only as a computation size with the insinuations of the validity of the Stefan- Boltzmann law.