The aspirator psychrometer by Assmann serves for measuring the air temperature and the humidity. Two parallel mounted, equal thermometers with coloured petroleum are used. The bulb of one thermometer (wet bulb thermometer) is covered by a wick, which must be moistened for a measurement.
The bulb of the other thermometer remains without wick. Both bulbs are encased by two radiation tubes. In order to achieve an effective radiation shield the surface of the instrument is polished. Depending on the amount of water vapour in the ambient air the water evaporates more or less from the wick of the wet bulb thermometer. The evaporative cooling causes the column of the wet bulb thermometer to drop. The dry bulb thermometer is indicating the true air temperature. From the difference in temperature of both thermometers results the psychrometric depression. From this the relative humidity, the dew point temperature and the vapour pressure of the air may be computed or determined from tables. Both thermometer bulbs are ventilated during measurements by fan. The ventilating speed at the bulbs averages 3 m/s.
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