A hygrometer is based on an element that has properties that change based on the level of water vapor in the air. Some of these sensors are simple providing results accurate enough for general applications, while others are more complex, allowing for more precise measurements. Knowing the humidity level of an environment is important for a variety of processes, from predicting the weather to operating precision electronic equipment.
What is Humidity?
Humidity can be stated in two different terms: absolute humidity and relative humidity. Absolute humidity is stated in units of grams per cubic meter. Absolute humidity is dependent on a variety of factors, including temperature and pressure, so it is not as useful to engineers as relative humidity. Relative humidity is the ratio of the actual vapor pressure at a given temperature to the maximum of saturation vapor pressure at the same temperature. This value is stated as a percent value. When the value is 100%, water droplets condense from the atmosphere.
Types of Hygrometers
There are five general categories of hygrometers:
- Capacitive –Capacitive hygrometers are based on the principle that moisture in the air changes its electrical permittivity.
- Conductive – Conductive hygrometers are based on an element that changes its resistivity based on atmospheric humidity.
- Optical – Optical hygrometers are complex sensors that include a mirror whose surface temperature is regulated by a thermoelectric heat pump. The temperature is adjusted until condensation appears on the mirror, changing the mirror’s reflective properties.
- Oscillating – An oscillating hygrometer is similar to an optical hygrometer, but in this case the mass of the mirror plate is measured. When water droplets condense on the plate, the mass of the plate increases, changing its resonant frequency.
- Fiber Optic – Fiber optic hygrometers consist of a glass fiber inscribed with a fiber Bragg grating and coated with a hygroscopic coating. As the coating absorbs water, it expands, placing a strain on the fiber and changing the critical frequency of the Bragg grating within the fiber optic sensor.
These humidity sensors are a far cry technologically than the traditional method to measure humidity: measuring the change in length of a piece of cat gut or human hair under tension.
Applications for Hygrometers
Many manufacturing processes are sensitive to humidity levels, and require monitoring. These include concrete curing and coating applications. Humidity levels are also critical to museums and other conservators, since high humidity levels can degrade paper and a variety of other mediums and cause mildew growth. Human skin can be damaged by humidity levels that are too low, so locations such as hospitals attempt to balance a humidity level that protects skin yet does not promote bacterial growth.
Humidity is one of the more difficult properties to measure, and humidity sensors are often some of the most complex sensors available. This complexity is necessary to provide accurate measurements.
Susan Kristoff - Susan Kristoff is mechanical engineer by trade, but has a diverse set of professional and personal interests. The glue that binds all of ...
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