About Relative Humidity Sensor

What is a relative humidity sensor?

A relative humidity sensor is an instrument which is used for measuring the amount of humidity and water vapors in the atmosphere, in the soil, or in confined areas. Humidity measurement instruments usually rely on the measurements of some other quantity like temperature, pressure, mass, a mechanical or electrical change in a substance as moisture is absorbed. These measured quantities by calibration and calculation can lead to a measurement of humidity. Modern electronic equipment use temperature of condensation which is called the dew point, or changes in electrical capacitance or resistance for measuring humidity differences. The first relative humidity sensor was invented by the Italian Renaissance polymath Leonardo da Vinci in 1480. The maximum amount of water vapors which can be held in a given volume of air varies greatly by temperature. Most temperature probes respond to relative humidity (RH), which is the amount of water relative to the maximum at a particular temperature expressed as a percent.

Applications of a Relative Humidity Sensor

Apart from greenhouses and industrial areas, relative humidity sensors are also applicable in some incubators, saunas, humidors, and museums. They are also very useful in the care of wooden musical instruments like pianos, guitars, violins, and harps which can be damaged by improper humidity conditions. In residential settings, these devices are used to assist in humidity control as too low humidity can damage human skin and body, while very high humidity favors the growth of mildew and dust mite. Temperature probes are also used in the coating industry as the application of paint and other coatings may be very sensitive to humidity and dew point.

Types of Relative Humidity Sensor

Capacitive: This device is suitable for applications where cost, space, or fragility are relevant. In capacitive devices, the effect of humidity on the dielectric constant of a polymer or metal oxide material is measured. With calibration, these devices have an accuracy of ±2% RH in the range 5–95% RH. Without calibration, the accuracy can be 2 to 3 times worse.

Resistive: In resistive temperature probes, the change in electrical resistance of a material due to humidity is measured. These devices are less sensitive than capacitive sensors as the change in material properties is less, so they need more complex circuitry.

Thermal: In thermal sensors, the change in thermal conductivity of air because of the humidity is measured. These devices measure absolute humidity rather than relative humidity.

Gravimetric: This device measures the mass of an air sample compared to an equal volume of dry air. It is considered the most accurate primary method for determining the moisture content of the air.