Humidity measurement instruments and transducers test for absolute humidity, relative humidity, or dew point in air. Humidity measurement instruments typically operate in a range, from 0 to 100 percent humidity. They are sometimes combined with other sensing devices such as temperature sensors.
Humidity measurement instruments and sensors can assess a number of different factors. Absolute humidity, expressed as grams of water vapor per cubic meter volume of air, is a measure of the actual amount of water vapor or moisture in the air, regardless of the air’s temperature. Relative humidity, expressed as a percent, also measures water vapor, but relative to the temperature of the air.
The dew point temperature, which provides a measure of the actual amount of water vapor in the air, is the temperature to which the air must be cooled in order for the air to be saturated and dew to form. Due to the intertwining of atmospheric measurements, humidity measurement instruments are sometimes equipped with pressure and temperature sensors as well. Three main applications for humidity measurement instruments are judging moisture in gases or air, bulk solids or powders, or in fuels or other liquids.
There are many technologies for humidity measurement instruments. Capacitive or dielectric instruments include a material that absorbs moisture, which changes its dielectric properties and enhances its capacitance. Chilled mirror technology uses a mirror that is chilled to the point that moisture starts to condense on it. This temperature is the dew point. In electrolytic technology, moisture is proportional to the current needed to electrolyze it from a desiccant.
In resistivity or impedance style sensors, a material absorbs moisture, which changes its resistivity or impedance. In strain gage instruments, a material absorbs water, expands, and is measured with a strain gage. Psychrometers, often called wet/dry bulbs, measure relative humidity by gauging the temperature difference between two thermometers, one wet and one dry. One critical specification for these devices is the humidity or moisture range to be measured or the dew point range.
Humidity and moisture accuracy is expressed in terms of percentage of measurement. The dew point accuracy (a temperature reading) is expressed as a variance in temperature output. Outputs from humidity measurement instruments can be analog current, voltage, or frequency; digital, including computer signals; or a switch or an alarm.
They can have analog, digital, or video type displays and can have a number of different form factors. They can be standard sensors or transducers, or simple gauges or indicators. They can also be various types of instruments; handheld, bench top, or mounted.
In addition to pressure and temperature compensation, humidity measurement instruments can have a number of features to make them more useful or easier to use. These can include data logging, event triggering, self-testing, self-calibration, and battery power.
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