Ask Control Engineering
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Pulling out your hair to measure humidity
May 20, 2011
Dear Control Engineering: I recently ran across an antique humidity measuring device that was supposed to have used stretched human hair as the measuring element. Is that real or was the guy pulling my leg?
There are many techniques for measuring the amount of moisture in air, which we describe generically as humidity. While humidity has a major effect on our comfort level, it can also affect various manufacturing processes from making paper to pharmaceutical products.
Most measuring devices are designed to calculate relative humidity (RH), which is the amount of moisture in the air relative to the maximum at that temperature. So, if the RH at 75 °F is 50%, that means the present humidity level is 50% of the maximum amount of moisture the air can hold at that temperature. If the temperature goes up, the RH figure will go down even though the amount of moisture has not changed, because warmer air can hold more moisture.
Traditional measuring methods can involve human hair or strands of silk because they change length as they absorb or give up moisture. This characteristic contributes to bad hair days when it rains. For a more precise measurement, the traditional wet/dry bulb thermometer setup can be used, although this is maintenance intensive.
For industrial applications, a more common sensor technology is a capacitive humidity sensor, which is very precise and does not involve hair or water. It takes advantage of the fact that the dielectric constant of air changes with humidity.
Read a short tutorial on capacitive humidity sensors.
--Peter Welander, pwelander(at)cfemedia.com