Pros and Cons:
Capactive type sensors are very linear and hence can measure RH from 0% to 100%, but require complex circuit and also need regular calibration. Resistive type sensors find difficulty in measuring low values (below 5%RH) , the change is impedance is too high and hence it is difficult to control the dynamics, temperature effects the properties significantly. However, advances in electronics can mitigate the problems of temperature effects and high impedance change.
Capacitive RH sensors dominate both atmospheric and process measurements and are the only types of full-range RH measuring devices capable of operating accurately down to 0% RH. Because of their low temperature effect, they are often used over wide temperature ranges without active temperature compensation. Thermoset polymer-based capacitive sensors, as opposed to thermoplastic-based capacitive sensors, allow higher operating temperatures and provide better resistivity against chemical liquids and vapors such as isopropyl, benzene, toluene, formaldehydes, oils, common cleaning agents, etc.
Other Humidity Sensing Mechanisms:
An electrolyte is formed by absorption of water and the current level obtained is proportional to the moisture content.
A volume of moist air is exposed to a drying agent and subsequently weighed. The weight corresponds to the moisture.
Attenuation of the transmitted signal varies as the amount of water content increases is an indication of the moisture content in the medium.
4. Dry-and Wet bulb temperature
Psychrometer gives relative humidity estimates based on Dry-and Wet bulb temperature measurement
5. Dew Point
Dew Point hygrometers measure dew-point temperature by detecting dew formation on a cooler base.