Hygrometers are devices for measuring the wet-bulb temperature of the air.
The temperature reading is affected by air movement over the instrument. There are two types of measuring instrument, sling and
screen hygrometer. The sling hygrometer is the more accurate one, therefore it is preferred by
air conditioning engineers.
Sling hygrometer measures the wet-bulb temperature in a moving air stream, preferably above 2 m/s. The
instrument has two thermometers. One of these thermometers is covered with muslin sleeve which is kept
moist with distilled and clean water. This thermometer will measure the wet-bulb temperature and the other
one dry-bulb temperature. A sling wet-bulb temperature may also be obtained by an Assman hygrometer. In
the Assman hygrometer, the wet-bulb thermometer is installed in a duct where the air is flowing at
reasonable velocity.
Screen hygrometer measures the wet-bulb temperature in still air. Thermometers are installed in a
Stevenson screen as used by meteorologists.
