A humidity sensor, also called a hygrometer, measures and regularly reports the relative humidity in the air. They may be used in homes for people with illnesses affected by humidity; as part of home heating, ventilating, and air conditioning (HVAC) systems; and in humidors or wine cellars. Humidity sensors can also be used in cars, office and industrial HVAC systems, and in meteorology stations to report and predict weather.
A humidity sensor senses relative humidity. This means that it measures both air temperature and moisture. Relative humidity, expressed as a percent, is the ratio of actual moisture in the air to the highest amount of moisture air at that temperature can hold. The warmer the air is, the more moisture it can hold, so relative humidity changes with fluctuations in temperature.
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The most common type of humidity sensor uses what is called “capacitive measurement.” This system relies on electrical capacitance, or the ability of two nearby electrical conductors to create an electrical field between them. The sensor itself is composed of two metal plates with a non-conductive polymer film between them. The film collects moisture from the air, and the moisture causes minute changes in the voltage between the two plates. The changes in voltage are converted into digital readings showing the amount of moisture in the air.
A person with a respiratory illness or certain allergies might use a home humidity sensor because low humidity can exacerbate breathing problems and cause joint pain, while high humidity encourages bacteria, mold, and fungus growth. Home humidors and wine cellars often have a humidity sensor that helps to maintain a consistent relative humidity optimal to safe long-term storage. Humidity sensors can also be used in homes or museums where valuable antiques or artwork are kept, because these items can be damaged or degraded from constant exposure to too much moisture.
Commercial and office buildings often have humidity sensors in their HVAC systems, which help to ensure safe air quality. Many automobiles use a humidity sensor as part their defrosting and defogging systems to automatically adjust the temperature and source of air used for heating and air conditioning. Humidity sensors also have industrial applications for production of materials that are sensitive to moisture. Humidity sensors give regular, ongoing readings of relative humidity, so they are used for data collection in oceanography and weather stations where humidity must be measured over time to analyze patterns and predict weather.