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
- Biwood combinations similar in action to a bimetallic temperature sensor
- Organic films
- Some fabrics, especially certain synthetic fabrics.
All these have the drawbacks of slow response and large hysteresis effects. Accuracy also tends to be questionable unless they are frequently calibrated. Field calibration of humidity sensors is difficult.
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:
- Slide rules
- Similar devices.
Thin-film sensors are now available, which use a absorbent deposited on a silicon substrate such that the
resistance or capacitance varies with relative humidity. They are quite accurate in the range of ±3 to 5 percent.
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 to be used 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.
Btu Meters: The Btu is used to indicate the amount of heat present. Meters are especially designed to indicate the Btu in a chilled water line, a hot water line, or a natural gas line. Specially designed, they are used by skilled laboratory personnel at present.