Air humidity has a direct influence on material parameters and determines the course of many technological processes. This is why accurate air humidity control is an important task.

Capacitive relative humidity (RH) sensors

Capacitive humidity sensors are widely used in modern industrial equipment, household appliances and telemetric meteorological data acquisition systems.

Such sensors structurally consist of a substrate on which a thin-film polymer or metal-oxide between two conducting electrodes is located. The sensing surface is covered with a porous metal electrode to protect it from contamination and condensation. The substrate is usually made of glass, ceramic or silicon. Incremental changes in the dielectric constant of a capacitive humidity sensor are almost directly proportional to the relative humidity of the ambient air. At 1% humidity fluctuation, the capacitance changes by 0.2-0.5 pF, and at 50% humidity (25°C), fluctuations can range from 100 to 500 pF.

Capacitive humidity sensors are characterized by a low temperature coefficient, the ability to operate at high temperatures (up to 200 ° C), the possibility of full recovery from condensate and moderate resistance to chemical vaporization. Sensor response times range from 30 to 60 s for a 63% humidity step.

Modern capacitive sensor technology has integrated many advances in semiconductor electronics to achieve minimal parameter drift and hysteresis during long-term operation. For example, thin-film capacitive sensors can integrate a monolithic signal amplifier chip on the substrate. Often, modern signal amplifiers have a CMOS oscillator to smooth the linear output signal.

The typical accuracy class of capacitive sensors is ±2% RH over a range of 5 to 95% for a two-point calibration. Note that capacitive sensors have operating distance limitations, the sensing element can be located away from the signal amplification circuitry to avoid parasitic effects of the interconnecting cable (the level of capacitance fluctuation of the sensor is not high). The distance should be less than 3 meters.

Direct replacement of sensors can be a problem if the sensor is not laser processed during manufacturing or if computerized sensor calibration is not used. Laser-treated sensors have an interchangeability value of ±2%.

Capacitive dew point sensors

Thin-film capacitive sensors are characterized by discrete signal changes at low relative humidity. Their operation is characterized by stability and minimal drift over the entire operating period. However, these sensors do not have a linear output when the relative humidity drops below a few percent. This feature of the sensors led to the development of a dew point measurement system that combines a capacitive sensor with a microprocessor circuit that stores calibration data in a non-volatile memory block. This approach to the problem has significantly reduced the cost of hygrometers and dew point transmitters used in air conditioning and telemetric meteorological data acquisition systems. The sensors are mounted on a chip that has a voltage output signal based on the relative humidity level. The microprocessor control memorizes the voltage level at 20 in the temperature range of -40...27°C. Reference values are confirmed with a NIST hygrometer using Peltier cooled mirror technology. The voltage level at dew point and freezing point is stored in the EPROM memory of the sensor. The microprocessor uses this data to calculate a linear relationship algorithm while simultaneously measuring the dry thermometer temperature and water vapor pressure. Once the water vapor pressure is determined, the dew point temperature is calculated from the thermodynamic relationship stored in the EPROM memory. Correlation with cooled mirror sensing technology is above ±2°C for dew point in the range -40 to -7°C and above ±1°C in the range -7 to 27°C. The long-term stability of the sensor is less than 1.5°C per year. Metrology instruments based on this principle are widely used in various applications due to their attractive price compared to instruments based on cooled mirror technology.

Resistive moisture sensors

Resistive humidity sensors detect changes in the electrical resistance of a hygroscopic medium (e.g. conductive polymer, salt or treated substrate).

Resistive sensors have a bifilar winding. Once coated with a hygroscopic polymer, their resistance is inversely proportional to humidity.

Typically, resistive sensors consist of metal electrodes overlaid on a substrate with a photoresistor or electrodes wound on a plastic or glass cylinder. The substrate is coated with a saline or conductive polymer. When it is dissolved or placed in a liquid substance, it covers the sensor evenly. In another case, the substrate may be treated with some chemical reagent, such as an acid. The sensor absorbs the water vapor and the ionic groups disintegrate, which increases the electrical conductivity. Response times for most resistive sensors range from 10 to 30 seconds for a 63% measurement step. The resistance range of a typical resistive element ranges from 1 kOhm to 100 megohms.

Most resistive sensors use AC excitation voltage without DC bias to prevent polarization of the sensor. The resulting current is converted and rectified to a DC voltage signal for further amplification, linearization, or analog-to-digital conversion.

The nominal frequency ranges from 30 Hz to 10 kHz.

Resistive sensors are not fully resistive due to capacitive effects in the range over 10-100 megohms. The main advantage of resistive humidity sensors is their excellent interchangeability (typically ±2% RH), which allows a resistor to be used to calibrate the signal amplification circuitry at a fixed humidity level. This eliminates the need for humidity calibration standards. The accuracy of each resistive humidity sensor can be measured in a calibration vessel or with a specialized computer system. The operating temperature range of resistive humidity sensors is -40 to 100°C.

Under conditions of domestic and commercial operation, the service life of such sensors is more than 5 years, but exposure to chemical vapors and other contaminants (oil, for example) can lead to their early failure. Another disadvantage of resistive humidity sensors is their tendency to shift values when operating in condensate if a water-soluble coating is used. Resistive sensors have a significant temperature dependence when used in environments with large temperature variations (greater than 10°F). At the same time, temperature compensation circuitry can be added to the sensor design to increase its accuracy. Thus, the main advantages of resistive sensors are small size, low cost, interchangeability, and long-term stability.
 
The design of modern resistive sensors utilizes a ceramic coating to reduce the fusion of environmental conditions when condensation occurs. The sensors consist of a ceramic substrate with metal electrodes deposited using photoresistive technology. The surface of the substrate is coated with a conductive polymer (or mixed ceramic composition), and the sensor itself is placed in a protective plastic housing with a dust filter.

The binder material is ceramic powder suspended in a liquid medium. After the surface is coated and dried, the sensors are treated with high temperature. The result is a thick film coating, insoluble in water, which completely protects the sensor from condensation.

After exposure to water, the typical recovery time to a 30% ceramic backed sensor is 5-15 minutes, depending on air velocity.

The interchangeability of the sensors is less than 3% in the measuring range 15-95% RH. Accuracy is ±2%. When the sensor is used with signal amplification circuitry, the output voltage is directly proportional to the ambient relative humidity.

Thermally conductive absolute humidity sensors

These sensors measure absolute humidity by determining the difference between the thermal conductivity of dry air and air saturated with water vapor.

Thermally conductive sensors are often used to measure absolute humidity at high temperatures. Their operating principle is very different from resistive and capacitive sensors.

If the air or gas is dry, it has significant heat absorption capabilities. A typical example is a desert climate. The desert is very hot during the day, but the temperature drops dramatically at night due to the dry atmospheric climate. Conversely, a humid climate cannot cool as quickly because heat is retained by water vapor in the atmosphere.

Thermally conductive humidity sensors (or absolute humidity sensors) consist of two matched NTC thermistors connected in a bridge circuit. The output voltage of the bridge is directly proportional to the absolute humidity. One thermistor is hermetically sealed in dry nitrogen and the other is open.

When current flows through the thermistors, the thermal resistance increases the temperature to over 200°C. The heat dissipated from a sealed thermistor is greater than that of an open thermistor due to the difference in thermal conductivity between water vapor and dry nitrogen. Since the dissipated heat creates different operating temperatures, the resistance difference of the thermistors is proportional to the absolute humidity.

A simple resistor assembly gives an output voltage range of 0 - 130 g/cc at 60°C. Calibration is accomplished by placing the sensor in dry air or nitrogen and adjusting the output signal to zero. Absolute humidity sensors have a long service life, their operating temperature reaches 300°C, and the sensor housing is resistant to chemical vapors.

An interesting feature of thermally conductive sensors is that they react to any gas that has a different thermal conductivity than nitrogen. This will affect the measurement results. Typically, absolute humidity sensors are used in dryers, microwave ovens, and steamers.

In general, absolute humidity sensors have better resolution at temperatures above 200°F than capacitive and resistive humidity sensors. They can be used in applications where conventional humidity sensors are not acceptable. The typical accuracy of absolute sensors is 3g/cubic meter, which is about 5% relative humidity at 40°C or 0.5% at 100°C.