Humidity Sensors

Of all the sensors used in a typical building, humidity sensors were what I knew the least about before writing this post.  I had a general idea of how the other types of sensors worked, but couldn't even really make assumptions as to how humidity sensors worked.  The first thing I learned with this post is that humidity sensors are called hygrometers.  I did figure that, like other sensors, hygrometers used a combination of temperature and pressure, or change in electrical properties for electronic sensors to determine the moisture in the atmosphere. 

When searching the internet I came across a list of typical types of hygrometers used in the industry; metal-paper coil, hair tension, chilled mirror dewpoint, capacitive humidity, resistive, and thermal conductivity. The metal-paper coil and the hair tension hygrometers seem to be the most "primitive" form of sensor, being that it uses physical properties of materials and calibration to display humidity level on a dial.  The metal-paper coil hygrometer works like a bimetallic thermometer, only instead of two different metals, a salt infused strip of paper is attached to a coil, causing the coil to change shape.  A hair tension hygrometer works by measuring the change in tension in a human or animal hair with the hair shortening as humidity increases.  Both of those types of hygrometers are calibrated to display relative humidity. 

The other types of humidity sensors use the change in a materials electrical properties to determine the relative humidity.  Most of the time a type of salt or electrical polymer is used in these hygrometers.