Humidity Sensors

Week 8

2/19/13

Humidity Sensors

                Humidity sensors, or hygrometers, come in multiple forms depending on the desired measurement (absolute vs. relative), cost factor and level of accuracy.  These sensors are used to measure the moisture content in the air.  Measuring the relative humidity is most practical because the reading depends on an associated temperature; where a ratio of current moisture content to the maximum moisture content at that temperature is developed.  This makes the most sense as we are dealing with both moisture and ambient temperature when designing a room to be as comfortable as possible.  In this way, data from a hygrometer will be fed into an HVAC systems computer which will either humidify or dehumidify the air based on the requested conditions.

                Due to their ability to be used in multiple scenarios, low cost and relatively high and stable accuracy, the capacitive humidity resistor is the most commonly used type.  In this style, a film capacitor is sandwiched between a ceramic and a dielectric polymer.  The polymer will absorb or release moisture based on the relative conditions.  The addition or subtraction of water will change the capacitance of the capacitor which can be converted to a digital reading.  These sensors have an average accuracy of about +/- 2%.

                The least accurate humidity sensor is called the metal-paper coil and gives a good visual of the principle that makes the more accurate types work.  In this style, a piece of paper that has been saturated with salt is attached to a metal coil.  When humidity increases, the paper absorbs water and changes the shape of the coil.  These changes are calibrated so a dial can be used to provide relative readings.  Although the accuracy is limited to +/- 10%, these gauges are cheap and require no digital feedback to function.

 Brian did a good job in his post to point out the range of options from the most accurate to least accurate.  He refers to the idea that “primitive” methods of something physically changing such as the length of a hair have been superseded by more technological methods that use electrical properties and digital conversion.  In the end, however, these primitive methods still do the job that they were designed to do.

                Originally, I only thought of hygrometers being used in conjunction with HVAC systems for air condition quality purposes.  Jeanine’s post made me realize that their uses go far beyond just providing comfort.  Having a controlled humidity level in a hospital scenario is very important for controlling the spread of airborne germs and bacteria.  Industries like greenhouses can benefit by controlling the amount of moisture introduced to the plant life.  The uses obviously stretch much further than home comfort in thermostats.