Hygrometer, << hy GROM uh tuhr, >> is an instrument used to measure the amount of water vapor in the air. Scientists use hygrometers to determine the relative humidity. Relative humidity is the amount of water vapor in the air compared with the amount required for saturation of the air at the same temperature. The most common types of hygrometers are the psychrometer and the hair hygrometer.
The psychrometer
consists of two thermometers mounted on the same frame. One type of psychrometer, called the sling psychrometer or the whirled psychrometer, has a frame that can be whirled in the air by hand. The bulb of one thermometer is covered with a tight-fitting muslin sack and wetted with water. This thermometer is known as the wet-bulb thermometer. The other is the dry-bulb thermometer. The psychrometer is whirled to provide ventilation for the bulbs. The dry bulb indicates the temperature of the air. The wet bulb helps determine the relative humidity.
When the sling psychrometer whirls through the air, water from the muslin evaporates. The evaporating water cools the wet bulb. The amount of cooling that occurs depends on the relative humidity. The lower the humidity, the faster the water in the muslin will evaporate, and the more the bulb will cool. High humidity will cause less evaporation, slowing the cooling process.
In air that has less than 100 per cent relative humidity, the wet bulb will record a lower temperature than the dry bulb. This difference in temperature is known as wet-bulb depression. A special chart is used to convert the wet-bulb depression to relative humidity.
The Assman psychrometer, an extremely accurate type of psychrometer, has a built-in fan. The fan draws the proper amount of air through two stationary metal tubes. These tubes hold wet-bulb and dry-bulb thermometers.
The hair hygrometer
uses a bundle of human hairs to detect relative humidity. The length of the hair increases as it absorbs moisture from the air. One end of the bundle of hair is anchored. The other end of the bundle is attached to a lever that moves a pointer on a scale. As the humidity lengthens or shortens the hair, the lever moves the pointer on the scale.
Other hygrometers
measure relative humidity by different methods. An absorption hygrometer uses a chemical that absorbs water vapor from the air. The chemical gets heavier as it absorbs the moisture, and the change in weight is measured to determine relative humidity. An electrical hygrometer uses carbon or some other substance whose electrical resistance responds to changes in humidity. A dew-point hygrometer measures dew point, the temperature at which the air has 100 per cent relative humidity (see Dew point). Its polished surface is chilled until dew appears. A device on the surface measures the temperature at which the dew forms.
