Many hygroscopic (moisture-absorbing) materials can be used as relative-humidity sensors. Such materials absorb or lose moisture until a balance is reached with the surrounding air. A change in material moisture content causes a dimensional change, and this change can be used as an input signal to a controller.
Commonly used materials include:
- Human hair
- Wood
- Biwood combinations similar in action to a bimetallic temperature sensor
- Organic films
- Some fabrics, especially certain synthetic fabrics.
Humidity is read in "rh" or relative humidity. To obtain the rh, it is necessary to use two thermometers. One thermometer is a dry bulb, the other is a wet bulb. The device used to measure rh is the sling psychrometer. It has two glass-stem thermometers. The wet bulb thermometer is moistened by a wick attached to the bulb. As the dual thermometers are whirled, air passes over them. The dry and wet bulb temperatures are recorded.
Relative humidity is determined by:
- Graphs
- Slide rules
- Similar devices.
They also have low maintenance requirements.
Stationary Psychrometers: Stationary psychrometers take the same measurements as sling psychrometers.
They do not move. However, they use a blower or fan to move the air over the thermometer bulbs.
For approximate rh readings, there are metered devices. They are used on desks and walls. They are not accurate enough for use in engineering work. Humidistats, which are humidity controls, are used to control humidifiers. They operate the same way as thermometers in closing contacts to complete a circuit. They do not use the same sensing element, however.
Moisture Analyzers: It is sometimes necessary to know the percentage of water in a refrigerant. The water vapor or moisture is measured in parts per million. The necessary measuring instrument is still used primarily in the laboratory. Instruments for measuring humidity are not used here.
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