Previous to writing this blog post, I could have assumed that humidity sensors calculated their values based upon other factors within the atmosphere such as temperature or pressure. When first searching the internet I found that the term "hygrometer" is the proper name for an object which measures humidity or moisture content. They do in fact utilize other factors such as temperature, pressure, and electrical changes in capacitance or resistance in the atmosphere to calculate the moisture content within a room.
I was actually astounded to find that there were so many types of hygrometers. The primary types being hair tension, metal-paper coil, chilled mirror dewpoint, capacitive, resistive, and thermal conductivity sensors. Each of these types uses a different factor in humidity production in order to determine the level of humidity in a space. For example, the hair tension sensor utilizes the shortening of human or animal hair with an increase in humidity to measure humidity (which explains bad hair days in high humidity!) The amount that the hair shortens directly translates to the amount of moisture in the air on the dial.
A major distinction in the types of sensor utilized is whether or not it is utilizing electrical datum. Chilled mirror dewpoint, capacitive, resistive, and thermal conductivity sensors all fall under this category. Thermal conductivity sensors, measure absolute humidity in the surrounding air rather than relative humidity. This is another distinction in the type that could be measured that had not crossed my mind.
In reading other students blog posts, I found Matthew's to be interesting because flow measurement of fluids is similar to the measurement of humidity in that there are several different factors that can be measured in order to obtain the same result.
Sources:
http://en.wikipedia.org/wiki/Hygrometer
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