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7.3. Mechanical Hygrometers

Mechanical hygrometers use sensing elements relying on a mechanical property of the sensor varying with

humidity. The most common example is the animal hair hygrometer, which uses a piece of animal hair kept under

tension. As humidity increases the hair becomes more flexible and stretches. A strain gage monitors the

displacement caused by the hair stretching with a change in the moisture content of the air. The output of the strain

gage is directly proportional to the relative humidity and is usually indicated on a mechanical meter. Mechanical

hygrometers are generally compact, light weight, reliable, and inexpensive. Accuracies, however are in the ±10%

range.

7.4. Electronic Humidity Sensors

Electronic humidity sensors typically use either a change in resistance or capacitance to measure humidity. These

sensors have become a popular choice because technology advances have made them accurate, compact, stable

and low power. A capacitive sensor consists of two electrodes, separated by a dielectric. As the water vapor in air

increases or decreases, the sensor's dielectric constant changes producing a higher (or lower) capacitance

measurement corresponding to the humidity level. A resistive sensor consists of two electrodes, separated by a

conductive layer. As the humidity in the air increases (or decreases) the conductivity of the sensing layer changes,

altering the resistance between the two electrodes. New techniques for producing thin films have made these types

of sensors, accurate, stable, and easy to manufacture in large quantities. The choice of material assures fast

response times with little hysteresis. The accuracy of electronic sensors is limited by their drift over time caused by

wide variations in temperature and humidity or the presence of pollutants.

The Si70xx humidity senors use a MiM capacitor as the reference and perform a high-accuracy 24-bit conversion

using a sigma delta conversion approach. Each part is factory calibrated for capacitance to RH with a minimum

offset and slope correction. Later members of the family include non-linearity correction and temperature

compensation.

Silicon Laboratories humidity sensors use the capacitance change due to moisture absorption of a polyimide film to

sense humidity. The polyimide film is deposited over a metal finger capacitor and exposed to the ambient via an

opening in the package. The polyimide material and sensing capacitor has been selected for excellent stability. An

optional expanded polytetrafluoroethylene (ePTFE) hydrophobic filter provides protection against dust and most

liquids.

The polyimide film is thin (<5 µm) and very responsive to humidity (response time of less than 10 seconds). The

hydrophobic filter has little impact on the response time.

While the Si70xx sensors are largely conventional, mixed-signal CMOS integrated circuits, relative humidity

sensors in general, and those based on capacitive sensing using polymeric dielectrics in particular, have unique

application and use requirements that are not common to conventional (non-sensor) ICs. Chief among those are:

Each of these items is discussed in more detail in the following sections.