Relative humidity is the ratio of the actual
water vapor in the air compared to the water vapor capacity of
the air. If the relative humidity is 33%, the air contains about one-third as much water
vapor as totally saturated air at the same temperature.
Relative humidity is essential information for
weather watchers as well as musicians, wine connoisseurs, woodworkers, health care
professionals, race car mechanics and gardeners. Almost as varied as the users of humidity
instruments are the instruments themselves.
Three of the categories are:
Digital instruments involve a
sensor that changes its electrical properties based on the humidity. This change is
converted electronically to a digital display.
Analog or dial instruments
use a de-oiled human hair or a synthetic hygroscopic material that expands and contracts
based on humidity. Through a mechanical linkage, this expansion and contraction turns the
indicating needle.
Sling psychrometers use a wet
and dry bulb thermometer system. These instruments consist of two identical thermometers
mounted on a frame with wet wicking covering the bulb of one thermometer. Using the
temperatures of the two thermometers and a table or slide rule, the current relative
humidity is determined.
