Humidity Sensor Working and Its Applications

Humidity is the measure of the amount of water vapor present in the air. Humidity is calculated as Relative humidity and Absolute humidity. For industrial and medical environments relative humidity becomes an important factor. A rise in the values of humidity, beyond threshold levels, can lead to malfunctioning of control systems, errors in weather prediction systems. So, as a security and safety factor, measurement of humidity values is very important. Humidity sensors are used to measure the humidity values. Relative sensors also measure air temperature. But this type of sensor is not useful for temperatures above 100 degree Celsius.

What is a Humidity Sensor?

Humidity Sensors are the low cost-sensitive electronic devices used to measure the humidity of the air. These are also known as Hygrometers. Humidity can be measured as Relative humidity, Absolute humidity, and Specific humidity. Based on the type of humidity measured by sensor, these are classified as Relative Humidity sensor and Absolute Humidity sensor.

Based on the parameters used to measure humidity, these sensors are also classified as Capacitive Humidity Sensor, Resistive Humidity Sensor, and Thermal Conductivity Humidity Sensor.

Some of the parameters to consider while choosing these sensors are the Accuracy, Linearity, Reliability, Repeatability and Response time.

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 measured to calculate the relative humidity values.

Low resistivity materials are used for the construction of a Resistive sensor. This resistive material is placed on top of two electrodes. Change in the resistivity value of this material is used to measure the change in humidity.

Salt, solid electrolytes and conductive polymers are the examples of resistive material used in Resistive sensor. Thermal conductive sensors measure Absolute humidity values.

Applications

The capacitive sensor is used for various applications for measuring humidity in HVAC systems, Printers, Fax machines, Weather stations, automobiles, food processing, refrigerators, etc…

Due to there low cost and small size, resistive sensors are used in residential, industrial and domestic applications. Thermal Conductive sensors are commonly used in pharmaceutical plants, food dehydration, drying machines, etc…

Examples

Some of the examples of these sensors available in the market are DHT11, DHT22, AM2302, SHT71, etc…For accurate and reliable measurements digital sensors are used. SHT3X is the most versatile sensor. SHTW2 is the smallest humidity sensor available in the market. SHT85 provides easy replaceability.

Humidity measurement doesn’t measure humidity directly. They depend on the measurement of quantities such as temperature, pressure, mass, resistivity to calculate humidity. Digital sensors measure the change in resistivity and conductivity values to calculate the humidity.

These sensors output the digital values which makes them easy to interface and use with microcontrollers such as Arduino, Raspberry Pi boards. These sensors proved to be a very useful tool for the safety of sensitive electronic equipment which is less tolerable to the environmental changes. Which parameter was a deciding factor for you to choose the sensor for your application?