Introduction

Humidity is the presence of water in air. Humidity sensor or hygrometer is an instrument used for measuring moisture content in the atmosphere. In other words, it measures both air temperature and moisture as to get the ratio of actual moisture in the air to the highest amount of moisture air at that point. Humidity measurement instruments totally rely on measurement of some other instruments, for example temperature, pressure, mass, mechanical or electrical change in a substance, as moisture is absorbed. By calibration and calculation, these measured qualities can lead to measurement of humidity. Important things to keep in mind when selecting a humidity sensor include accuracy, repeatability, interchangeability, long-term stability, size, packaging, cost effectiveness, resistance to chemical and physical contaminants, and ability to recover from condensation. There are two types of sensors: absolute humidity sensors and relative humidity sensors.

Absolute Humidity Sensor

Absolute humidity is the measure of the mass of the water vapor present in specified volume.

AB = Mw/v

where AB is the absolute humidity (g/m3 or grains/ft3), Mw is the mass of water vapor (gram or grain) and v is the volume of air (m3 or ft3).

Relative Humidity Sensor

It is defined as ratio of the amount of moisture content of air to the maximum (saturated) moisture level that the air can hold at a same given temperature and pressure of the gas.

RH (%) = (Pv /Ps) *100

where Pv is the actual partial pressure of moisture content in air and Ps is the saturated pressure of moist air at the same given temperature (both in Bar or KPa).

Humidity sensors can be divided into two categories: Capacitive sensors and resistive humidity sensors.

Capacitive Sensor

A capacitive humidity sensor detects humidity based on a change of capacitance between two detection electrodes provided on a semiconductor substrate. Most capacitive sensors use plastic or polymer as the dielectric material, with a typical dielectric constant ranging from 2 to 15. The sensor element is built out of a capacitor. A polymer absorbs or releases water proportional to the relative environmental humidity, and thus changes the capacitance of the capacitor, this is called the dielectric. The dielectric which is measured by an electronic circuit gives relative air humidity. The capacitance type humidity sensor detects the humidity and can measures any changes in the electrostatic capacity of an element respective of ambient humidity.

Basic structure of capacitive type humidity sensor is shown below

On Alumina substrate, lower electrode is formed using gold, platinum or other material. A polymer layer such as PVA is deposited on the electrode. This layers senses humidity. On top of this polymer film, gold layer is deposited which acts as top electrode. The top electrode also allows water vapor to pass through it, into the sensing layer. The vapors enter or leave the hygroscopic sensing layer until the vapor content is in equilibrium with the ambient air or gas. Thus, capacitive type sensor is basically a capacitor with humidity sensitive polymer film as the dielectric.

Resistive Humidity Sensor

A resistive humidity sensor detects relative humidity by measuring the change in the resistance of an element corresponding to the ambient humidity. Most of the resistance type humidity sensors include an electrolytic, polymeric, Or on the other hand metallic oxide sensor component. An impedance humidity sensor changes depending on its electrical impedance as the humidity of the surrounding environment changes, the measured impedance is converted into humidity readings.

Thick film conductor of precious metals like gold, ruthenium oxide is printed and fabricated in the shape of comb to form an electrode. Then a polymeric film is applied on the electrode; the film thus act as a humidity sensing film, due to the existence of movable ions. Change in impedance occurs due to the change in the number of movable ions.

References

https://www.engineersgarage.com/articles/humidity-sensor

https://en.wikipedia.org/wiki/Humistor

https://en.wikipedia.org/wiki/Hygrometer

http://wikid.eu/index.php/Humidity_sensors_with_CMOSens%C2%AE_technology#Features