Tutorial: Capacitive humidity sensors
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.
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% RH means the air in the sample is holding 50% of the maximum amount of water vapor possible at this temperature. If the sample of air is warmed, the RH reading will go down, even though the amount of water vapor has not changed. Absolute humidity is the amount of vapor in a given volume of air, so it is usually specified as grams per cubic meter and not tied to a temperature. Dew point is the temperature at which the air becomes saturated and water vapor begins to condense, and is indicated in degrees Fahrenheit or Celsius.
Traditionally, humidity was measured by using wet bulb/dry bulb thermometers, the stretching of silk or hair, or chilled mirrors to find the dew point. Such technologies still exist, but have largely been supplanted by more precise and less complicated techniques, including capacitive, resistive and thermal conductivity. In recent years, capacitive sensors have emerged as offering an excellent combination of precision, reliability, stability, and cost for measuring RH. This is largely due to major advances in manufacturing using techniques developed for sophisticated semiconductors.
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 these points where manufacturers or models will differ. These are generalities, so check with your suppliers for specifics:
Precision—Capacitive sensors can usually read with
Humidity range—Since RH is a percentage, it can be no lower than 0% and no higher than 100%. Some sensors can cover that completely, while others may lose a point or two at the extremes.
Temperature range—High temperatures of 180-200 °C are typical, with low ranges down to -40 °C.
Dew point—Units can usually cover ranges from -40-100 °C.
Hysteresis—Units that see wide swings should be capable of returning within 1-2%.
Your process requirements will dictate how sophisticated a unit you and its related cost. 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. Designs vary in their ability to tolerate specific conditions, so this is worth exploring with your supplier.
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.
You can also search online at the Control Engineering Supplier Search .
—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Process Instrumentation & Sensors Monthly
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