What is a Humidity Sensor?

A humidity sensor is an electronic device that measures the humidity in its environment and converts its findings into a corresponding electrical signal. Humidity sensors vary widely in size and functionality; some humidity sensors can be found in handheld devices (such as smartphones), while others are integrated into larger embedded systems (such as air quality monitoring systems). Humidity sensors are commonly used in the meteorology, medical, automobile, HVAC and manufacturing industries. 

Humidity sensors can be divided into two groups, as each category uses a different method to calculate humidity: relative humidity (RH) sensors and absolute humidity (AH) sensors. Relative humidity is calculated by comparing the live humidity reading at a given temperature to the maximum amount of humidity for air at the same temperature. RH sensors must therefore measure temperature in order to determine relative humidity. In contrast, absolute humidity is measured without reference to temperature.

The two most common RH sensors are the capacitive and resistive humidity sensors. Capacitive sensors use two electrodes to monitor the capacitance (i.e. the ability to store an electric charge) of a thin metal strip placed between them. The metal’s capacitance increases or decreases at a rate that is directly proportional to the change of humidity in the sensor’s environment. The difference in charge (voltage) generated by an increase in humidity is then amplified and sent to the embedded computer for processing. Resistive humidity sensors operate on a different principle. These sensors utilize a small polymer comb that increases and decreases in size as the humidity changes, which directly affects the system’s ability to store charge.

Schematic of a resistive humidity sensor.
Schematic of a resistive humidity sensor. (ResearchGate)

Thermal humidity sensors are used to measure absolute humidity. Unlike RH sensors, thermal humidity sensors utilize two probes, one to measure dry nitrogen and one to measure the air of its surrounding environment. When humidity is collected on the exposed probe, the difference in thermal conductivity is perceived by the sensor, and AH is calculated.