Humidity Sensors

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

The capacitive humidity sensor consists of a hygroscopic dielectric material (polymer) sandwiched between a pair of electrodes, forming a small capacitor. Most capacitive sensors use a polymer as 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 by the evaluation electronics.

By definition, relative humidity is also a function of both ambient temperature and water vapor pressure. There is therefore a relationship between relative humidity, the amount of moisture present in the sensor, and the sensor capacity. This relationship is the basis for the operation of a capacitive humidity meter.

Polymer humidity sensors offer numerous advantages that make them a preferred choice for many applications. One of the biggest advantages is their high sensitivity, which enables precise humidity measurements. In addition, these types of sensors are characterized by a fast response time, so they can quickly react to changes in the environment. Another advantage is the wide measurement range in which these sensors can operate, from 0...100% relative humidity. Their long lifespan is also an important feature, especially when the sensors are used in temperate environments. In addition, polymer humidity sensors are inexpensive compared to dew point sensors, which makes them attractive for a wide range of applications. Another advantage is their high stability, which ensures reliable performance over long periods of time.

Despite their many advantages, polymer sensors also have some disadvantages that should be considered when selecting and using them. One major disadvantage is their sensitivity to environmental factors such as temperature fluctuations and chemical influences. These factors can affect the accuracy and lifespan of the sensor. Another problem is the possible drift caused by aging of the polymer, which results in a decrease in accuracy over time, necessitating regular calibration. In addition, polymer humidity sensors are susceptible to contamination by dust, dirt or chemical vapors, which can reduce their performance. In extreme environments, such as very high or very low humidity, the reliability of the sensor can also decrease. The response time can be longer under certain conditions, such as in very humid or contaminated environments. The temperature dependence of the sensors can also influence the measurement results, so that additional compensation measures may be necessary.

The performance of a polymer humidity sensor can be affected by various environmental conditions. Temperature fluctuations influence the sensitivity and accuracy of the sensor, as it is temperature-dependent and may require additional compensation. Chemical contaminants, such as corrosive gases or vapors, can attack the materials and degrade measurement performance. Dust and dirt on the sensor assembly result in reduced responsiveness and inaccurate readings. Very high humidity or condensing conditions can saturate the surface and cause malfunction or slower response time. Contamination by organic or inorganic substances, such as oils or chemical residues, can also alter the sensor properties and cause measurement drift. Finally, polymer aging during long-term use in difficult environments can gradually reduce accuracy.

Polymer sensors can be replaced, but the following points must be observed: Make sure that the replacement sensor is compatible with the existing hardware or electronics. After replacement, it is important to calibrate the new sensor to ensure accurate measurements and minimize deviations. Calibration ensures that the sensor delivers correct values. The replacement should be carried out under clean conditions and with care to avoid contamination or damage, avoiding direct contact with the sensitive elements as much as possible.

Industrial high-temperature humidity sensor HygroMer HT-1: The HT-1 sensor is the improved version of its predecessor, the industrial temperature humidity sensor IN-1. It can withstand higher temperatures of up to 190°C (IN-1 170°C) and a dew point of up to 90°Cdp and has a very high chemical resistance. Meteo & Outdoor Humidity sensor HygroMer HM1-SK: The HM1-SK is the first sensor from Rotronic to use an additional paint protection layer and new connection technology for applications with temporary high humidity and increased resistance to salt, fungicides, herbicides and pesticides. Chemical Sterilization Humidity sensor HygroMer HH1-SK: The HH1-SK is a sensor that was developed for applications with hydrogen peroxide. Due to its unique design, the sensor has a high resistance to conventional sensors that come into contact with hydrogen peroxide. Water activity humidity sensor HygroMer WA-1: This sensor was developed to accurately measure water activity and, due to its special design, requires very little space compared to other sensors.

Special polymer or paint coatings offer an effective way to protect polymer humidity sensors. A thin protective layer can prevent the penetration of moisture, gases and particles without affecting the sensor's functionality. These coatings are often hydrophobic and chemically resistant, allowing them to repel a variety of contaminants and thus protect the sensor in demanding environments. In addition to coatings, filters and membranes are an important protective measure. Microporous membranes can keep out dust and dirt while still providing adequate air permeability. Specialized filters, such as those made of PTFE, are chemically resistant and allow air to pass through, but protect the sensor from harmful particles. These protection methods, combined with the choice of a suitable enclosure or regular maintenance, ensure that the sensor's performance remains stable over a long period of time, even in environments with high levels of gases, dust or dirt.