Types of Humidity Sensors

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.

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.




Types of Temperature Sensors

Temperature sensors play an important role in many applications. For instance, maintaining a certain temperature is vital for any equipment that is used to fabricate medical drugs and heat liquids. The responsiveness and accuracy of the detention circuit can be critical for quality control for applications like these. However, little do people know that temperature detection is actually part of preventative reliability. Say while an applicant might not be able to perform any activities in high temperature, the system itself might have a risk of overheating. This risk arises from specific external factors like very harsh operating environment and internal factors like electronics self-heating. The system can take preventative action by sensoring when overheating occurs. A robust temperature sensor circuit is not as pricey as you think. Here we will look at the different types of temperature sensor technologies in the current market and see what each has to offer.

There are four commonly used temperature sensor types:

  1. Negative Temperature Coefficient (NTC) thermistor

Thermistors are thermally sensitive resistors which can exhibit huge, precise and predictable change in resistance that is correlated to temperature variations. NTC thermistors provide extremely high resistance at low temperatures. The resistance drops rapidly as temperature increases. Small changes in temperature are reflected very fast and with high precision (.05 to 1.5 degree Celsius) and this is because NTC thermistors experience a very large change in resistance per degree Celsius. Besides that, most NTC thermistors’ outputs require linearization because of its exponential nature.

  1. Resistance Temperature Detector (RTD)

RTDs are commonly known as resistance thermometers. It measures temperature by correlating the resistance of the RTD element with temperature. What consists of an RTD is a film for greater accuracy and a ceramic core which is wrapped around with wire. RTDs made of platinum are more accurate and precise, however they cost more. Cheaper RTDs are made of nickel and copper but these two metals are not as stable as platinum. RTDs made of platinum offer a linear output that is very accurate which is .1 to 1.0 degree Celsius. RTD offer the greatest precision compared to any other temperature sensor, but it can also be the most expensive one.

  1. Thermocouple

Thermocouple is a temperature sensor that is made up of 2 wires of 2 distinct metals connected at 2 points to form 2 junctions. The potential difference between both the points will bring about proportional changes in temperature. Thermocouples require conversion when used to control temperature as well as compensation because it is non-linear. Thermocouple is not very precise or accurate, which is .5 to 5.0 degrees Celsius. But the upside about it is that thermocouples operate across the broadest temperature, which is from -200 to 1700 degrees Celsius.