A humidity sensor is a low-cost electronic device, used for measuring the humidity of the air. They are also known as hygrometers. Humidity sensors can measure the humidity as relative humidity, absolute humidity, and specific humidity. Relative humidity sensors contain a humidity sensing element with a thermistor for measuring temperature. In the case of capacitive humidity sensors, the sensing element is a capacitor. The different types of atmospheric humidity sensors and their working principle are discussed below:
Capacitive Humidity Sensors
Capacitive humidity sensors are the most basic humidity sensors. They are used in applications where cost and rigidity are important concerns. The electrical permittivity of capacitive humidity sensors changes with a change in humidity. The space between the capacitor plates in these sensors is filled by a dielectric material. The dielectric constant of this material changes based on variances in temperature. Capacitive humidity sensors can provide stable results over long usage and can detect a wide range of humidity changes. The output voltage is also linear. These humidity sensors are used in HVAC systems, weather stations, automobile, food processing, refrigerators, ovens, etc.
Resistive Humidity Sensors
These humidity sensors measure the resistance or electrical conductivity. They are generally made of materials with low resistivity. The resistivity changes drastically with humidity variations. Humidity and resistivity are inversely proportional. The low resistivity material is deposited on the top of the electrodes. The resistivity between the electrodes changes when the top layer absorbs water which can be measure with an electric circuit. These sensors are small and cheap too. Since there are no calibration standards, they are interchangeable too. They are used in both domestic and industrial applications.
Thermal Conductivity Humidity Sensors
These are also known as absolute humidity sensors as they are used for measuring absolute humidity. They can measure the thermal conductivity of both dry air and humid air. A thermal conductivity humidity sensor uses two small thermistors with a negative temperature coefficient for a bridge circuit. One thermistor is sealed in a chamber filled with dry nitrogen while the other thermistor is exposed to the open environment through small venting holes. On powering the circuit, the resistance of the two thermistors is calculated. The difference between the resistance is directly proportional to the absolute humidity.
These sensors are more durable and have high resolution. They are used in ovens, drying kilns, pharmaceutical plants, etc.