Aspiration psychrometer according to Assmann
The aspiration psychrometer according to Assmann is a measuring instrument of the group of psychrometers , with which the true temperature of the air , not falsified by solar radiation, and indirectly the relative humidity is measured. It is used in meteorology , metrology and air conditioning .
functionality
The way it works is based on the fact that energy in the form of heat is required to evaporate water . When air passes over a damp surface, the water evaporates. The energy required for this is withdrawn from the air flowing past, which leads to a cooling of the surface, similar to the effect of determining the wind direction with a moistened finger. If the air is humid, only a little water evaporates from the swept area. Their cooling is correspondingly low. The opposite applies to dry air.
The Assmann aspiration psychrometer consists of two thermometers . The dry temperature t can be read on one, the wet temperature f on the other . In the case of the wet thermometer, a hose is pulled over the capillary ball and moistened with distilled water. The wet temperature f is read from this. Both thermometers are protected from radiant heat by the metal construction so that the actual air temperature can be measured unaffected. The process is supported by the aspirator attached at the top (aspirate = inhale, suck in), which ventilates both thermometers with a constant air flow of 2.5-3.0 m / s and thus guarantees the same evaporation ratios. The air flowing past causes the water to evaporate, as a result of which the temperature on the humid thermometer drops. This happens until the heat content of the air saturated above the hose corresponds to the heat content of the air being examined. The relative humidity can be determined from the temperature difference of the dry and wet thermometer ( t - f ) using the Sprung formula ; mostly it is read from precalculated tables.
If used correctly, an Aßmann aspiration psychrometer achieves a very high level of accuracy. The measurement accuracy depends on the one hand on the accuracy of the thermometer used, and on the other hand on the temperature itself (the accuracy increases at higher temperatures).
development
The aspiration psychrometer was developed between 1887 and 1892 by the meteorologist Richard Assmann in collaboration with the aeronaut Hans Bartsch von Sigsfeld . The first apparatus was built by the Berlin precision mechanic Rudolf Fuess . In 1898 it was recommended as a standard instrument by the International Commission for the Study of the Free Atmosphere .
The instrument was an important new development. It served to verify several physical theories for the atmosphere. With this instrument Aßmann succeeded in discovering the stratosphere in 1902 .
Aßmann aspiration psychrometers are nowadays often used as reference devices because of their high accuracy.
Individual evidence
- ↑ R. Assmann: The working methods of the aerological observatories . In: Bröckelmann (Ed.): Wir Luftschiffer. Ullstein, Berlin / Vienna 1909, p. 66.