Thursday, July 5, 2012

Various Kinds of Humidity Sensors

Humidity is the amount of water vapors present in the air. There are certain terminologies that are often used to calculate the humidity content in the air.

Absolute humidity. This is the ratio of water to air that is the total amount of water vapors present in the air at given time.

Relative humidity: This is the ratio of total quantity or air that is present in the form of gaseous vapors to the total quantity of moisture that air can absorb into it.

Specific humidity - this is the specific amount of water vapors in specific amount of air.

Humidity is measured in gram per cubic centimeter (g/cm3). On day to day basis we generally talk about relative humidity. Humidity content in atmosphere is measured by the humidity monitor that consists of humidity sensors and humidity controllers. There can be different kind of sensors that can be based on different principles. Here is a brief description of various humidity monitoring devices

Hygrometer/Psychrometer: In this system a pair of similar thermometer is used one of which is kept wet while the other one is kept dry. Difference between their values is recorded. Considering the ambient value of dry one and relative difference in their temperature, the moisture content in the air is calculated.

Bulk Polymer Resistive Sensor: this is an electric humidity sensor using capacitors and resistors. A grid of interlocked electrodes are placed on an insulating substrate a humidity sensitive salt is embedded on polymer resin and a coating of mixture is applied on electrodes. The resin is coated with protective coating that is permeable to water vapors. As water permeates, ionization begins and polymer ions become mobile. The impedance of sensor is measure and percent relative humidity is calculated.

Capacitive Sensor: In this kind of humidity monitoring systems porous electrodes are placed in parallel position on a substrate. The dielectric material in the middle of electrodes absorbs water form the air with changing humidity. The capacitance of sensor changes due to change in humidity. This kind of sensors can be used at high altitudes and is suitable to calculate low humidity values.

Bang-bang controller: This is also known as on-off control that switches between two extreme states, 0 and 1. This is mostly used to control and minimize the hysteresis effect. If the humidity levels are indicated to be above the given level, the switch shifts its state to trigger a cooling device.

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

Proportional controller: This is a linear feedback apparatus. The quantity of control measure imposed is directly proportional to the amount of error.

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

These sensors help maintain and control humidity levels of storages and electronic equipment rooms where increased humidity can cause hazards.

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Charlie McCoy is a freelance writer who has written many articles on Freezer temperature Monitoring, Warehouse temperature monitoring and Warehouse Humidity monitor.

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