The presence of water in air is called humidity. The amount of water vapour in air can significantly affect comfort of as well as many manufacturing process in various industries. The presence of water vapour influences physical, chemical and biological processes in the world. Humidity measurement is very critical in industries because it can affect the cost of business product as well as the safety and health of the personnel. With that being said, humidity sensing is very important especially in the control systems for human comfort and industrial processes.
Monitoring and controlling humidity is very vital in many domestic and industrial applications. In semiconductor industry, moisture and humidity levels have to be carefully monitored during water processing. In medical applications, humidity control is necessary for respiratory equipments, incubators, sterilizers and pharmaceutical processing. Humidity control is also required in chemical gas purification, film desiccation and textile production. Plantation protection in agriculture also depends a lot on humidity control. Humidity control in domestic applications is needed for living environment in microwave ovens and cooking control. Mostly, humidity sensors are used to provide an indication of the environment’s moisture levels.
Classification & Principles of Humidity Sensing
Humidity sensors are divided into 2 types according to the measurement units, namely relative sensors (RH) as well as absolute humidity sensors. Majority of humidity sensors are relative humidity sensors and they use very distinct sensing principles.
Humidity measurement can be done using wet or dry bulb hygrometers, electronic hygrometers or dew point hygrometers. Electronic hygrometers are the most popular variety of all and it is the most demanded one in the markets today. Electronic hygrometers are also known as humidity sensors. Electronic hygrometers can be further divided into 2 categories: one uses resistive effects and the other uses capacitive sensing principle.
Resistive Effect Humidity Sensors
Resistive type humidity sensors tend to respond to the change in humidity by picking up changes in the resistance value of the sensor element. The electrode is formed in the shape of the comb by printing and calcinating previous metals like gold into a very thick film conductor. After that, a polymeric film is applied on the electrode. This film acts as a humidity sensing film thanks to the presence of movable ions. The change in the number of movable ions brings about change in impedance.
Capacitive Effect Humidity Sensors
Humidity sensors will rely on this principle typically consists on a hygroscopic dielectric material which forms a small capacitor between 2 electrodes. Plastic or polymers are usually used as the dielectric material in most capacitive effect humidity sensors these days. A typical dielectric constant ranges from 2 to 15. When moisture is absent, the dielectric constant and the geometry of the sensor will determine the capacitance value accurately. At normal room temperature, the dielectric constant is 80, which is a lot bigger than the constant of the sensor material. Therefore, the sensor absorbs this water vapour and this result in an increase in the capacitance of the sensor.