A humidity sensor or hygrometer can be used to sense, measure, and report air humidity and air temperature. Air humidity is an important factor to consider when designing electronic equipment. Excessive ambient humidity causes condensation and corrosion, which may cause performance exceptions or even faults. To prevent this, engineers use humidity sensors. These devices are used for systems deployed in humid environments, such as industrial control, instrumentation, climatology, and agricultural applications.

Different Types of Humidity Sensors

There are three main types of humidity sensors which are defined around what approach is used to sense humidity and deliver an electrical signal that can be used to establish the value. These types of humidity sensors include:

Capacitive humidity sensors

Resistive humidity sensors

Thermal conductivity humidity sensors

1, Capacitive Humidity Sensors and their Working Principle

Capacitive humidity sensors as implied by their name make use of a capacitor, which consists of two electrode layers between which is a dielectric material. In the case of capacitive humidity sensors, the dielectric material is one that is hygroscopic, meaning that it is capable of absorbing moisture from the surrounding air. A commonly used dielectric for capacitive humidity sensors is a polymer film, whose dielectric constants are somewhere around 2-15.

In the absence of moisture, the capacitance (the ability to store electric charge) is determined by the geometry of the capacitor and the permittivity (dielectric constant) of the dielectric material. The dielectric constant of water vapor at normal room temperature is around 80, much larger than that of the dielectric material. As the dielectric material absorbs water vapor from the surrounding air, the dielectric constant increases, which increases the capacitance of the sensor.

There is a direct relationship between the relative humidity in the air, the amount of moisture contained in the dielectric material, and the capacitance of the sensor. The change in the dielectric constant is directly proportional to the value of the relative humidity. By measuring the change in capacitance (dielectric constant), the relative humidity level can be established. The sensor is one element in a chain that also includes a probe, cable, and an electronics unit (signaling circuit) that takes the signal from the sensor and produces an output signal conditioned for the desired use and application.
Capacitive humidity sensors provide stable readings over time and are capable of detecting a wide range in relative humidity. They also provide near linearity with signal amplitude over the range of humidity. They are limited by the distance between the sensor and the signaling circuit.

2, Resistive Humidity Sensors and their Specifications

A resistive humidity sensor sometimes referred to as a hygristor or an electrical conductivity sensor, is one that makes use of the change in the resistivity measured between two electrodes to establish a value of relative humidity. The device contains a hygroscopic conductive layer in the form of a polymer humidity sensing film that is mounted on a substrate.

The conductive film contains a set of comb-like electrodes, usually deposited from a noble metal like gold, silver, or platinum that are laid out in an interdigitated pattern to increase the amount of contact area between the electrodes and the conductive material. The resistivity of the conductive material will vary inversely with the amount of moisture that is absorbed. As more water vapor is absorbed, the non-metallic conductive material increases in conductivity hence decreases in resistivity.

Resistive humidity sensors are low-cost devices with a small footprint and are readily interchangeable. Unlike capacitive humidity sensors, resistive humidity sensors can function in remote monitoring applications where the distance between the sensor element and the signaling circuit is large.

3, Thermal conductivity humidity sensors and Their Usages

Thermal conductivity humidity sensors are used to measure absolute humidity. They do so by calculating the difference in the thermal conductivity of dry air versus humid air.

Two NTC thermistors are suspended by thin wires with the sensor. One of the thermistors sits in a chamber that is exposed to the air through a series of ventilation holes. The second thermistor is placed in a different chamber within the sensor that is hermetically sealed in dry nitrogen. An electrical bridge circuit passes current to the thermistors which begin to self-heat. Since one of the thermistors is exposed to humidity from the air, it will have different conductivity. A measurement of the difference in resistance of the two thermistors can be made which will be directly proportional to the absolute humidity.