Tutorial: Capacitive humidity sensors

09 October 2008

Airborne moisture content can affect critical aspects of production or product quality in many manufacturing processes. Measuring humidity can be tricky, but capacitive sensors have a high degree of precision and reliability. Peter Weelander, process industries editor, Control Engineering, explains.

Humidity is a broad term and needs to be specified. In most situations it means relative humidity (RH) and the reading is a percentage. So 50 per cent RH means the air in the sample is holding 50 per cent of the maximum amount of water vapour possible at this temperature. If the sample of air is warmed, the RH reading will go down, even though the amount of water vapour has not changed. Absolute humidity is the amount of vapour in a given volume of air, so it is usually specified as grams per cubic metre and not tied to a temperature. Dew point is the temperature at which the air becomes saturated and water vapour begins to condense, and is indicated in degrees Fahrenheit or Celsius.

Capacitive sensors measure RH by observing changes in capacitance between conductive electrodes. The dielectric constant of the sensor changes with humidity level in a way that can be measured. When an ambient temperature is also available, the other types of humidity readings can be calculated.

When evaluating a simple sensor or integrated humidity device, consider precision, humidity range, temperature range, dew point and hysteresis.

There are other operational issues that you should examine at the same time:
Durability and stability—Any unit can perform well in clean air, but smoke, chemical vapors, and other contaminants can degrade performance. Wide temperature or humidity swings can also have lasting effects.

Parallel temperature measurement—Some devices also include a temperature measurement, which can be very handy if you need it.

Sensor replacement or calibration—Most designs allow the sensor element itself to be removed for cleaning, calibration, or replacement. Some are easier than others.

Corresponding temperature measurement—While the humidity sensor may be very precise, if it is coupled with a less precise temperature device, calculations of dew point, absolute humidity, or logging values will be no better than the lower grade instrument.

Humidity sensors are available from a variety of suppliers. Configurations include naked sensors, handheld, wall mounted, and remote sensors.

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