How do you measure humidity?
Jon Titus - February 06, 2013
My high school had a weather station on the roof so earth-science students could make
measurements each morning. We climbed up a ladder in the janitor's closet, went through a hatch
and out onto the flat roof. Relative humidity measurements posed a challenge because they required
a wet-bulb and a dry-bulb thermometer. The wet-bulb thermometer had a cloth covering on the bulb
that drew water by capillary action from a nearby water reservoir. The two temperatures let us find
the relative humidity on a chart. Of course, someone had to remember to fill the water reservoir and
measurements stopped during cold weather. After the principal ended our daily roof visits, we used
a sling psychrometer. In spite of its need for regular "refreshment," the wet-dry bulb technique
provides good accuracy. Measuring humidity with electronic sensors poses challenges, though.
I had a humidifier installed on our forced-air HVAC system at home and the technician left behind a
humidistat that some installations would use. So I took it apart to see how the sensing element
worked (see photo below). To my surprise, the sensing element comprised three lengths of a plastic
film (black arrow) woven between two end points. One end point provides spring tension. A hinged
"paddle" on the other end would actuate a switch (not shown) to turn the humidifier on or off based
on the relative-humidity level set with a knob. The knob turned a cam that controlled the tension in
the plastic film. Obviously the film stretches or contracts as humidity changes. (You can build a
simple hygrometer with a long piece of hair, which has similar expansion and contraction
characteristics.)
I got surprised when I looked at the plastic film because Honeywell, the humidifier manufacturer,
also sells humidity sensors, such as the HH6130 and HH6131. I have experimented with both and
they work well and provide an accurate measure of humidity, good to within a few percent.
The Honeywell sensors include a temperature sensor and provide a temperature-compensated
humidity measurement. An SPI or I2C port on an MCU can obtain the humidity and temperature
information. The raw data requires some basic math operations, but no lookup table or linearization
steps.
Many humidity sensors rely on a slightly hydroscopic porous material sandwiched between two
electrodes. As the humidity increases, the dielectric constant of the non-conductive material
changes, too, which in turn changes the capacitance between the electrodes. The sandwiched
material expands or contracts slightly, depending on the amount of water vapor in the surrounding
atmosphere.
I have two Humirel HS1101 relative-humidity sensors I hope to work with soon. This capacitive
sensor can change the frequency output of an oscillator built with a CMOS 555-timer IC. In the
Humirel reference circuit, the frequency ranges from about 7350 Hz at o-percent RH up to about
6030 Hz at 100-percent RH. So you don't get a wide frequency range, but a microcontroller could
count pulses over a fixed time and convert the count to a RH value. The slope of frequency vs.
humidity looks close to linear.
Other humidity-level sensors exist, but I doubt you can find others that offer the simple interfaces
provided by those described above.
References
1. "Relative Humidity Sensor, HS1100/HS1101," Humirel.
2. "Honeywell HumidIcon Digital Humidity/Temperature Sensors: HIH6130/6131 and HIH6120/6121
Series"
3. "I2C Communication with the Honeywell HumidIcon Digital Humidity/Temperature Sensors"