Probes and Sensors for Measurement of Relative Humidity

Humidity sensors measure the humidity or water content in the air or other environments. The most common types and how they work are listed here: Capacitive humidity sensor These sensors measure the change in capacitance between two electrodes as the relative humidity varies. Between the electrodes is a thin film polymer which reacts to the humidity. These sensors are accurate and stable and are therefore the most commonly used type of sensor. Resistance humidity sensor These sensors measure the resistance between two electrodes. The resistance changes depending on the moisture content, allowing the moisture value to be determined. These sensors are less expensive but less accurate than capacitive humidity sensors. Dew point sensor These sensors determine the dew point at which moisture condenses from the air. They can work in different ways and observe the condensation point or by optical measurement on a cooled mirror. The sensors are very expensive and are designed for applications with the highest accuracy of humidity measurements.

The capacitive humidity sensor consists of a hygroscopic dielectric material (polymer), which is placed between a pair of electrodes and forms a small capacitor. Most capacitive sensors use a polymer as the dielectric material with a typical dielectric constant of 2 to 15. The polymer used reacts with the water vapor particles in the room and a humidity value is generated with the evaluation electronics.

Communication and data transfer are often still carried out in analog form. The trend to switch to digital solutions is on the rise, as digital processing has several advantages: - Digital sensors provide precise measurements and are less susceptible to signal distortion - Digital communication means that power consumption can be reduced and accumulators and batteries last longer - Cost-efficient integration into existing systems

Rotronic humidity sensors are among the most accurate sensors on the market and achieve an accuracy of up to +/- 0.5 %rH at room temperature. Humidity sensors are often not exposed to laboratory conditions, but to everyday conditions in which we do not have clean air and other gas mixtures are present. These gas mixtures can have an influence on the measuring accuracy. Rotronic AG - A PST Brand is the only industrial humidity sensor manufacturer that can provide its customers with a pollution list with concentrations of various gas mixtures.

Digital sensors provide more precise measurement readings because they are less susceptible to analog signal distortion.

There are various methods for calibrating and adjusting a humidity sensor: Calibration and adjustment with calibration box and humidity standards -> Beginner Calibration in air: Despite its apparent simplicity, calibration in air can be difficult, mainly due to temperature gradients. In order to correctly calibrate and adjust the temperature measurements provided by a probe, you must be able to meet the following requirements: Both the probe and the reference thermometer should be supplied with the same flow of air. The pre-fitted filter caps used to protect the sensors should be carefully removed from the probe. The air velocity at the sensor should be within the limits of 200 to 500 feet/minute (1 to 2.5 meters/second). Any comparison between two instruments with a velocity below 200 feet/minute may not be valid. Air velocities above 500 feet/minute may damage the unprotected humidity sensor. The temperature of the air stream should be virtually constant. Calibrating and adjusting the humidity If possible, calibrate and adjust the probe at a temperature of 18 to 25 °C. During calibration Avoid exposing the probe to high humidity (60% relative humidity and above) for a prolonged period of more than 30 minutes. The use of a humidity generator is probably the fastest method for calibrating and adjusting the humidity at several values. When using a humidity generator for calibration, ensure that the samples are immersed as fully as possible in the generator to reduce the influence of room temperature on the humidity sensor.

Communication and data transfer are often still carried out in analog form. The trend to switch to digital solutions is on the rise, as digital processing has several advantages: • Digital sensors provide precise measurements and are less susceptible to signal distortion • Digital communication means that power consumption can be reduced and accumulators and batteries last longer • Cost-efficient integration into existing systems