Humidity sensor

Humidity is the amount of water vapor in air or any other medium and is an intricate part of our eco system. Humidity sensor is usually used to measure the relative humidity in the air. It does that by measuring both the air temperature and the moisture present.
Relative Humidity (RH) is the basic unit that needs to be measured to ascertain the humidity. Since humidity is dependent on temperature and RH is the ratio between the amount of moisture in the air and the maximum amount of moisture the air can hold at that specific temperature, humidity measurement fluctuates with the temperature value.

There are three main technologies that employed by humidity sensors and they are based on capacitance, resistance/impedance and thermal conductivity.
Capacitive humidity sensor is made up of a thin film of a polymer or a metal oxide fixed between two electrodes. A porous metal electrode is used to coat the surface of the sensing area, and this prevents degradation of the device through contamination or over exposure to condensation present in the surroundings.
The change that occurs in the dielectric of the capacitive sensor has a directly proportional relationship with the relative humidity present in the nearby environment. The capacitance fluctuation is normally from 0.2 to 0.5 pF for a change of 1 percent in relative humidity. These sensors are characteristically known for the ability to operate at high temperatures and are also known for low temperature coefficient and a semi resistance to vapors of the chemical nature.
This technology is popular in the industry for commercial and weather applications.
Resistive/impedance humidity sensor measures the change in resistance or impedance of a humidity sensitive film such as acids, salts, polymer or ceramic films.
These sensors generally contain noble metal electrodes deposited on ceramic or glass substrate or consist of wire-wound electrodes around a plastic or glass cylinder. The electrode configuration of resistive/impedance humidity sensors uses interdigitated pattern and sensing films are deposited between gap of the two electrodes. To evenly coat the surface, sensing materials are sometimes suspended or dissolved in a liquid before coating process. The liquid suspension increases electrical conductivity and hence the overall efficiency of the sensor.
These sensors are typically employed for industrial or commercial purposes.
Thermal conductivity humidity sensor is also known as absolute humidity sensor. These sensors work based on the principle of quantifying the thermal conductivity difference between air with water vapor and dry air. They are made up of negative temperature coefficient thermistor elements intricately joined in a bridge circuit. One element is exposed to the surrounding while the other one is hermetically sealed within dry nitrogen.
This technology is usually employed in desserts where the climate is hot in the day and cool during night time. The change is different from that of a humid climate since the cooling and heating is less rapid in that environment.

Applications of Humidity Sensors
Humidity sensors are among the most frequently used sensors and have broad applications. They can be used for various applications from weather prediction, to wine storage and to protection of paintings and art works etc. These sensors are also used in commercial and office buildings to maintain a certain level of air quality and industry personnel utilizes this technology to ensure that moisture sensitive products are manufactured under optimal conditions and stay the same in storage.

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