A temperature and humidity sensor is an electronic device that measures the humidity in its environment and converts its findings into a corresponding electrical signal. Humidity sensors are commonly used in the meteorology, medical, automobile, HVAC and manufacturing industries. It is the measure of the amount of water vapor present in the air. Humidity is calculated as Relative humidity and Absolute humidity. Relative sensors also measure air temperature. But this type of sensor is not useful for temperatures above 100 degree Celsius.
Working Principle of Humidity Sensor
Relative humidity sensors usually contain a humidity sensing element along with a thermistor to measure temperature. For a capacitive sensor, the sensing element is a capacitor. Here the change in electrical permittivity of the dielectric material is measure to calculate the relative humidity values.
Low resistivity materials are use for the construction of a Resistive sensor. This resistive material is place on top of two electrodes. Change in the resistivity value of this material is use to measure the change in humidity.
Salt, solid electrolytes and conductive polymers are the examples of resistive material use in Resistive sensor. Thermal conductive sensors measure Absolute humidity values.
Types of humidity sensors
There are three primary types of humidity sensors used 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
The first two of these are design to sense relative humidity (RH) – the last one is use to detect absolute humidity (AH). Relative humidity sensors usually also contain a thermistor to establish the temperature reading.
Absolute Humidity: Absolute Humidity (AH) is the ratio of mass of the water vapour to the volume of the air. If m is the mass of the water vapour and V is the total volume i.e. volume of air and water vapour mixture, then Absolute Humidity AH is given by
AH = m/V
Absolute Humidity doesn’t take temperature in to account but it changes with temperature and pressure.
Relative Humidity: Whenever we talk about measuring Humidity, it usually Relative Humidity that we are talking about (unless otherwise specified).
Relative Humidity or RH is the ratio of the actual water vapour pressure present in the air at a temperature to the maximum water vapour pressure present in the air at the same temperature.
In weather reports and forecasts, the probability of precipitation or dew or fog is indicated using Relative Humidity and hence, it is considered an important metric.
Relative Humidity takes both temperature and pressure in to consideration. Hence, the Humidity Sensors which measure Relative Humidity, measure both the moisture content as well as the air temperature.
Specific Humidity: Specific Humidity (SH) is the ratio of mass of the water vapor to the total mass of the air.
Mixing Ratio or Humidity Ratio: Mixing Ratio is the ratio of mass of the water vapor to mass of the dry air.
AM1001 (Analog Output)
DHT11 (Digital Output)
SHT20 Temperature and Humidity Sensor (I2C interface)
SHT20 I2C Temperature & Humidity Sensor (Waterproof Probe)
Applications of Capacitive Humidity Sensors
Capacitive Humidity Sensors are use in a wide range of applications including but not limited to:
- HVAC Systems
- Printers and Fax Machines
- Weather Stations
- Automobiles
- Food Processing
- Refrigerators, Ovens and Dryers
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