| A hygrometer is an instrument used to measure the moisture content of a gas, such as determining the relative humidity of air. Most hygrometers fall into one of four categories, as listed below. Humidity sensors for weather stations and stand-alone sensors usually use a capacitance or resistance hygrometer to measure the humidity. Remote sensors then relay the information to the base display.
While there are other types of hygrometers, they are not generally encountered at the consumer electronics level. RadioShack sells several hygrometers (and hygrometer systems) as stand-alone weather sensors.
The most common type of hygrometer is the dry- and wet-bulb psychrometer. It consists of two identical mercury or electrical thermometers, one of which has a wet cotton or linen wick around its bulb. Evaporating water from the wick absorbs heat from the thermometer bulb, causing the thermometer reading to drop. The readings are then cross-referenced.
A mechanical hygrometer uses a material that changes physical properties as the humidity changes. Older types of mechanical hygrometers used human hair, which stretches as it absorbs moisture, with a linkage connecting the center of a bundle of hairs under tension to a pointer. Another uses goldbeater's skin, a membrane from the intestines of oxen. Other materials are used to determine relative changes in humidity because they change color as they absorb moisture.
A thin film capacitor hygrometer measures humidity by detecting the change in capacitance of a thin polymer film; this sensor has a relatively fast response compared to other types of hygrometers. Another type of hygrometer works on the principle that electrical resistance varies in a material that absorbs moisture, with the resistance to a current passing between wires measured by special sensors.
The dew point hygrometer uses a cooled mirror as a sensor; in this case, the temperature of the mirror is monitored to determine the temperature at which dew (or frost) first appears. A light sensor detects when frost or dew forms, and the temperature of the surface is typically measured by an electronic temperature sensor, such as a thermistor. |
