Humidity Sensors

Developments in semiconductor technology have made possible humidity sensors that are incredibly accurate, durable, and inexpensive.  The three most commons types of humidity sensors are capacitive, resistive, and thermal conductivity.

Resistive humidity sensors measure the change in impedance of a medium which has an inverse exponential relationship to relative humidity.  Mediums used include conductive polymers, salt, or treated substrate.  These sensors typically use ceramic as a coating for protection from condensation.  The voltage output provided from the sensor become directly proportional to the relative humidity when signal conditioning is applied.

Thermal conductivity humidity sensors calculate the difference between thermal conductivities of dry air and air containing water vapor to measure absolute humidity.  Because of their ability to measure absolute humidity, these sensors are also referred to as absolute humidity sensors.  This sensor consists of two negative temperature coefficient thermistor elements within a DC bridge circuit.  One element gets sealed in dry nitrogen and the other remains exposed to the environment.  Absolute humidity is directly proportional to the difference between the resistances of each element.

Capacitive sensors are the only type of full-range sensor that can accurately measure to 0% relative humidity.  This type of relative humidity sensor is most commonly used in industrial, commercial, and weather applications and is used over wide ranges of temperature due to their low temperature effect.  Capacitive sensors measure the change in dielectric constant which is proportional to the environment’s relative humidity.  A 0.2-0.5 pF change in capacitance is related to a 1% change in relative humidity. This type of sensor would most likely be used in HVAC applications.

Regardless of the type of relative humidity sensor, all outputs are affected by both temperature and perfect of relative humidity.  When higher accuracy or wide operating temperature ranges are considered, temperature compensation is included in the application. Relative humidity integrated circuits, or RHIC, have linear voltage outputs that are a function of the supply voltage, percent of relative humidity, and temperature.  This allows for the sensor to translate supply and output voltages to determine the true relative humidity of an area.