Machine Design

Sensor Sense: Humidity sensors

Relative humidity, or RH, is the percentage of moisture in the air referenced to the maximum level of moisture the air can hold at that temperature and pressure.

Edited by Robert Repas

Relative humidity sensors come in both capacitive and resistive versions. Humidity changes the dielectric constant of the insulating layer between two metal plates in capacitive sensors, while creating fluctuations in the impedance of resistive sensors.

Early humidity sensors used natural materials like silk or hair as the mechanical sensing element; the material shrank when dry because of low relative humidity and expanded when the humidity was high. Electronic sensors have replaced most mechanical systems today because of their accuracy, dependability, and lower costs.

Electronic RH sensors typically fall into the resistive or capacitive category. Capacitive RH sensors are built on a substrate of glass, ceramic, or silicon. A dielectric layer made from a thin polymer film or a metal oxide is placed between two metal electrodes. The surface electrode is porous so water vapor in the air reaches the dielectric layer yet still protects it from contamination and exposure to condensation.

At 50% RH, the capacitance of the sensor is typically 100 to 500 pF. The change in capacitance is directly proportional to the change in humidity, typically 0.2 to 0.5 pF for every 1% change. Capacitive RH sensors work best from 5 to 95% RH with an overall accuracy of ±2%. The response time is usually 30 to 60 sec for a 63% RH step change.

Many of the techniques used in semi-conductor manufacturing give capacitive RH sensors minimal long-term drift and hysteresis. The thin-film devices may include signal-conditioning circuits on the substrate to produce near-linear output voltages for humidity measurement.

Resistive RH sensors measure the change in electrical impedance of a hygroscopic material such as a conductive polymer, salt, or treated substrate. Hygroscopic means the material readily absorbs water — in this case from the air. Placed between two noble metal electrodes, the conductivity of the material changes with the amount of water absorbed. A zero-centered ac voltage is used to measure the conductivity to prevent polarization of the sensor. The resulting current flow is then converted to a dc value by the signal-conditioning circuits of the sensor.

Nominal operating temperatures for resistive RH sensors range from 40 to 100°C. They should have a life expectancy of over five years, though exposure to chemical vapors and other contaminants like oil mist may cause premature failure. With proper temperature compensation, resistive RH sensors can reach accuracies of ±0.1%.

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