Assmann psychrometer

For measurement of atmospheric humidities the sling psychrom-eter is widely used. This is composed of a wet- and dry-bulb thermometer mounted in a sling which is whirled manually to give the desired gas velocity across the bulb. In the Assmann psychrometer the air is drawn past the bulbs by a motor-driven fan. 

Assmann psychrometer An apparatus which uses a clockwork or electrical fan located above wet and dry bulb thermometers, positioned in a cylinder to provide a set airflow rate over the thermometer bulbs for accurate temperature readings of both bulbs. 

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... 

For exposure to ozone, a special chamber, 4 feet wide, 7 feet high, and 6 feet long, constructed by Armour Research Foundation, was used. Usually one, but sometimes two, subjects were placed simultaneously in this chamber. An exhaust fan provided a complete air change in the chamber about once every minute, thus avoiding any accumulation of carbon dioxide and water vapor. Control with a thermometer and an Assmann psychrometer showed that average room temperature and humidity were never exceeded. 

The simplest way to calibrate a hygrometer is to use ambient air. The humidity of ambient air is measured using both the calibrated meter and the reference meter. A simple psychrometer or an Assmann hygrometer can be used as a reference. Provided the air conditions are stable, this method is used to check hygrometers in field measurements and other not too demanding purposes. 

Figure 23.4 Psychrometers. (a) Assmann. (b) Sling, (c) Screen housing (Courtesy of Casella London Ltd)...

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). 

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...

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