Humidity Sensor working principle and applications

 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.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. 

Introduction 

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.

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.

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.

Types of humidity sensors

• Capacitive
• Resistive
• Thermal

Capacitive

A capacitive humidity sensor measures relative humidity by placing a thin strip of metal oxide between two electrodes. The metal oxide’s electrical capacity changes with the atmosphere’s relative humidity. Weather, commercial and industries are the major application areas.

The capacitive type sensors are linear and can measure relative humidity from 0% to 100%. The catch here is a complex circuit and regular calibration. However, for designers this a lesser hassle over precise measurement and hence these dominate atmospheric and process measurements. These are the only types of full-range relative humidity measuring devices down to 0% relative humidity. This low-temperature effect often leads to them being used over wide temperature ranges without active temperature compensation.

Resistive

Resistive humidity sensors utilize ions in salts to measure the electrical impedance of atoms. As humidity changes, so do the resistance of the electrodes on either side of the salt medium.

Thermal

Two thermal sensors conduct electricity based upon the humidity of the surrounding air. One sensor is encased in dry nitrogen while the other measures ambient air. The difference between the two measures the humidity.

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

• Industrial process control systems. Storage and warehouses
• Climate control for green houses
• Meteorological applications
• Office automation, room comfort control
• Automotive cabin air control
• Home appliances, Air conditioners
• Food processing
• Medical applications
• Chicken Coops, Pig Barns