Humidity Measurement (Hygrometer)

Relative humidity is measured by a hygrometer. A hygrometer consists of two identical thermometers. The bulb of one is uncovered and dry, (dry bulb). The bulb of the other thermometer is covered with a wick and it is kept moist, (wet bulb). The two thermometers will not read alike. The bulb of the wet bulb thermometer will be cooled slightly by the evaporation of the moisture from the wick surrounding the bulb. Consequently, this thermometer will read lower than the dry bulb thermometer.

The lower the humidity the more rapidly the moisture on the wet bulb will evaporate and this rapid evaporation will cool the wet bulb to a much lower temperature than the dry bulb. If the humidity is high, approaching 100%, the difference between the dry bulb temperature and the wet bulb temperature will be very slight, one degree or less.

To determine the relative humidity, the operator records the difference between the dry bulb and wet bulb temperatures and refers to established tables which indicate the relative humidity for the air being tested. An in­strument for measuring relative humidity using a wet and a dry bulb the thermometer is called a psychrometer. Hygrometers using a moisture absorbing wood, hair and other substances are being used, however, the wet and dry bulb ther­mometer (psychrometer) is simple, re­liable and easy to operate.