Pyrgeometer

A pyrgeometer for measuring the atmospheric counter-radiation

A pyrgeometer (from ancient Greek πῦρ / pyr / "fire" and γῆ / geo / "earth") is used to measure the atmospheric counter-radiation arriving from the half-space ; this is thermal radiation in the range from about 4 µm.

Layout and function

A pyrgeometer basically consists of an edge filter , which keeps out the solar part of the radiation, and a black surface, which absorbs the transmitted radiation and thus becomes warm.

The filter consists of a dome-shaped support (dome) with an interference filter evaporated on the inside . Since the radius of the dome is large compared to the measuring surface, rays that hit the absorber surface have passed through the layers of the interference filter approximately perpendicularly, which is important for its function. With the opposite filter effect, with a sensitivity of 0.3 to 3 µm, the device would be a pyranometer for measuring global solar radiation.

The carrier material must be at least 50 µm transparent and consists of silicon or very thin polyethylene , which is held in shape by overpressure. The silicon can be thicker, which makes the device more robust and allows the temperature of the dome to be measured at one or more points - since the material is not completely transparent, its temperature contributes to the radiation flow.

The black absorber surface is one of two surfaces of a thermopile , the terminal voltage of which 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. In order to avoid irradiation of the column from the side, the absorber surface is fitted into a black sheet metal that is also in thermal contact with the chassis. The temperature of the chassis is also measured and the temperature of the absorber is deduced from this in order to be able to consider its radiation towards the sky.

The pyrgeometer is precisely calibrated against a black body with various combinations of the temperatures of the body, the absorber and the dome.

When the weather is clear, a sun shield reduces the contribution of solar radiation to the measured value (2% of solar radiation is longer than 3 µm). A slightly warmed air stream from a ring nozzle prevents nighttime dew formation on the cathedral.

Pyrgeometer equation

For this purpose, an ideal pyrgeometer is considered. The ideal pyrgeometer consists of an ideally black, horizontal film over an ideally black base plate. The temperatures of the film ( ) and base plate ( ) are measured. To find the pyrgeometer equation, the Stefan-Boltzmann law is used. This is necessary in order to calculate the counter radiation from these temperatures. The pyrgeometer equation is based on the radiation equilibrium of the film. From below the film absorbs the black body radiation of the base plate, from above the counter radiation. The emission of the foil is the black body radiation upwards and downwards. This will: ${\ displaystyle T_ {F}}$ ${\ displaystyle T_ {B}}$ ${\ displaystyle G}$

${\ displaystyle \ sigma T_ {B} ^ {4} + G = 2 * \ sigma * T_ {F} ^ {4}}$

The conversion results in:

${\ displaystyle G = \ sigma (2 * T_ {F} ^ {4} -T_ {B} ^ {4})}$

The atmospheric counter-radiation is thus the difference between the radiation emitted by the measuring film and the radiation from the base plate.

In practice, disturbance variables modify the simple equation and the measuring devices must be calibrated.

literature

Kirill I͡Akovlevich Kondratʹev: Radiation in the Atmosphere , Vol. 12, Academic Press, 1969, publ. by Elsevier under ISBN 978-0-12-419050-4 , limited preview in the Google book search (equipment details, historical variants).
Claus Fröhlich: Investigation of the surface radiation budget in the Alps and in comparison to Switzerland. Mittelland , vdf Hochschulverlag ETH, 1998, ISBN 3-7281-2504-0 , limited preview in the Google book search (state of the art).