What is the definition of a relative humidity sensor? Relative humidity sensors measure the relative humidity in the air, which is the amount of water vapor in the air relative to the maximum amount of water vapor the air can hold. Relative humidity is expressed as a percentage, and the maximum amount of water vapor that the air can hold varies with temperature. These sensors work by measuring the electrical properties of the air, such as the resistance or capacitance. The resistance of the air is affected by the amount of water present in the air, while the capacitance of the air is affected by the moisture content. A relative humidity sensor is usually combined with a temperature sensor to provide accurate readings of the humidity over a range of temperatures. Relative humidity sensors are commonly used in industrial and agricultural applications, such as greenhouses, food production and storage, and in HVAC systems. They are also used to monitor the conditions in homes, work places, and other enclosed spaces.