Humidity Monitoring: Humidity Sensors, Controllers and Monitors

An imbalance in the Earth's environment has occurred due to global warming, which is caused due to the increased greenhouse effect. Greenhouse effect releases gases in the atmosphere, and warming due to the increase in greenhouse gases leads to higher humidity in the atmosphere. Water vapor, being a greenhouse gas, causes additional warming. This process goes on in a never-ending loop, causing the temperature and humidity to rise continuously. Different humidity monitoring devices like humidity sensors, humidity monitors and humidity controllers have made it possible to measure and control humidity easily.

What is Humidity?
The amount of water vapor in the air accounts for the humidity of air. Absolute humidity is the ratio of water to air, i.e it is the quantity of water in a particular volume of air. Relative humidity is the amount of water vapor that is present in a gaseous mixture of air and water. In simple terms, relative humidity is the ratio of the amount of water vapor present in air to the actual amount of water vapor, air can hold. Specific humidity is the amount of water vapor in a specific mass of air.

How to Measure Humidity?
Humidity is measured in grams per cubic meter (g/m³). Generally, when we talk about humidity in our day-to-day life, we refer to the relative humidity. Humidity monitors are instruments used for monitoring the humidity. These monitors are made up of humidity sensors and humidity controllers. There are monitors which make use of various sensors and controllers, that work on different principles to measure relative humidity.

Displacement Sensor: Prior to the electronic age, humidity was determined using common materials like horse hair, nylon and cellulose. A strand of any of these materials is attached between two holders or a strain gauze. As they expand and contract with the change in humidity, their length changes. This change in length is taken as a measure of relative humidity. Contamination possibility is negligible in these sensors however, hysteresis effect is prominent i.e the current value depends on the previous state of the sensor.

Hygrometer/Psychrometer: This instrument uses a pair of similar thermometers, one of which is kept in wet condition and the other one is kept dry i.e. at normal room temperature. When the water evaporates, the temperature of the wet bulb thermometer falls. The difference between the temperatures of the wet bulb thermometer and dry bulb thermometer is recorded. Considering the ambient temperature of the dry bulb thermometer and the difference value, humidity value is obtained from the humidity chart. During the process, both the thermometers have to be kept away from direct sunlight. The results may not be accurate for lower humidity values.

Bulk Polymer Resistive Sensor: Nowadays, electrical humidity sensors are manufactured using capacitors and resistors for humidity measurement. In the bulk polymer resistive sensor, a grid of interlocked electrodes (metal plates) is placed on an insulating ceramic substrate. A humidity sensitive salt is embedded in a polymer resin (polymer compound in a highly vicious state), and a coating of the mixture is applied on the electrodes. The resin is covered by a protective coating that is permeable to water vapor. As water permeates the coating, ionization process begins and the polymer ions become mobile within the resin. The electrodes are excited by passing an alternating current through them. The impedance (measure of opposition to the AC current) of the sensor is measured and the percent relative humidity is calculated. These sensors are not very effective at low humidity values.

Capacitive Sensor: In the capacitive sensor, porous electrodes arranged in parallel are placed on a substrate. The dielectric material placed between the electrodes absorbs water vapor from the air as per the change in humidity. The capacitance of the sensor changes due to the change in the dielectric constant value. Impedance value is obtained from the capacitance variation. Due to the polymer dielectric material, this sensor can be used in high temperatures. This sensor is suitable to calculate low humidity values.

Most of the electrical sensors used to measure humidity are the bulk polymer resistive sensor or the capacitive sensor.

Humidity controllers help to control and maintain the humidity level of places like storage rooms and electronic equipment rooms, where increased humidity can cause any hazards. Humidity controllers use many techniques to control the humidity.

Bang-bang controller: Also known as on-off control, it switches abruptly between two extreme states, 0 and 1. It is mostly used to control and minimize the hysteresis effect. If the humidity sensor indicates the humidity levels to be above the desired level, the switch changes its state to trigger a cooling device.

Proportional–integral–derivative controller (PID controller): It is a slightly complicated control loop feedback mechanism. It requires a real-time system feedback.

Proportional controller: It is a linear feedback mechanism. It is more complicated than bang-bang mechanism but less complicated than PID mechanism. The quantity of control measure imposed is directly proportional to the amount of error.

Fuzzy logic controller: In this controller, the values do not switch between the two limit states abruptly, but it can also range between the two limits.

There are many more humidity monitoring techniques that are used as per the requirements. Humidity was never considered a 'worry factor', and was never required to be controlled. But now, considering the adverse effect it can have on the surroundings, a need has arisen to measure it as well as control it.