Assmann’s psychrometer

Fig. 10.3-1 Assmann s psychrometer for the measurement of the steady state temperature air flows over a small moist surface, at steady state = ss...

In reverse, by measuring the wet bulb temperature the moisture content XA of the humid air can be calculated using the two equations. The wet bulb hygrometer, sometimes called Assmann s Aspiration Psychrometer works by this method. It consists of two thermometers, one of which is covered by a porous textile, the lower part of which is dipped into a container full of water, so that the bulb of the thermometer is always wet. Air is blown over the thermometer and after some time it reaches the wet bulb temperature Meanwhile the second, so called dry thermometer, indicates the temperature of the air flowing around it. With these measured values the moisture content XA5 of the humid air can be found using equation (1.210) or (1.211). 

In order that consistent conditions can be obtained, the air speed over the thermometers should be not less than 1 m/s. This can be done with a mechanical aspiration fan (the Assmann psychrometer) or by rotating the thermometers manually on a radius arm (the sling psychrometer). If the thermometers cannot be in a moving airstream, they are shielded from draughts by a perforated screen and rely only on natural convection. In this case the wet bulb... 

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