A Guide to Humidity Sensors and Controllers

Global warming due to the Greenhouse effect, has brought about an imbalance in the Earth's environment. Due to the Greenhouse effect, greenhouse gases increase the temperature and humidity of the air. Water vapor, being a greenhouse gas, causes additional warming.

The process goes on in a never-ending loop, causing the temperature and humidity to rise continuously. Different monitoring devices like humidity sensors, monitors and controllers have made it possible to measure and control humidity easily.

Humidity is the amount of water vapor present in the 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.

Humidity is measured in grams per cubic meter (g/m³). There are special monitors used to find the humidity in the air. These monitors are made up of sensors and controllers. There is a large variety of these instruments, which are based on different principles to measure relative humidity.

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.

The change in length is taken as a measure of relative humidity. Possibility of contamination is negligible in these sensors, although hysteresis effect is prominent, i.e the current value depends on the previous state of the sensor.

This instrument uses a pair of similar thermometers, one of which is kept in wet condition, and the other 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 temperature of the wet bulb thermometer, and that of the dry bulb thermometer is recorded.

Considering the ambient temperature of the dry bulb thermometer, and the difference value, humidity value is obtained from the 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.

Nowadays, electrical 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 to 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) of the sensor is measured, and the percent relative humidity is calculated. These sensors are not very effective at low humidity values.

In capacitive sensors, porous electrodes arranged in parallel are placed on a substrate. The dielectric material placed between the electrodes absorbs water vapor from the air according to 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.

Bulk polymer resistive sensors, and capacitive sensors are the most widely used sensors.

Controllers help to control and maintain the humidity level of places like storage rooms, and electronic equipment rooms.

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

This is a slightly complicated control loop feedback mechanism. It requires a real-time system feedback.

This is a linear feedback mechanism. It is more complicated than bang-bang mechanism, but is not as complicated as PID mechanism. The quantity of control measure imposed is directly proportional to the amount of error.

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

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.