Capacitive humidity sensors and capacitive moisture sensors are mostly based on the changes in the permittivity of the dielectric material. The permittivity of atmospheric air, some gases, and many solid materials are functions of their moisture contents and their temperature.
The main advantage of this type of sensor is that a relatively small change in humidity results in a large change in capacitance sufficient enough for a sensitive detection.
Capacitive humidity sensors enjoy wide dynamic ranges, from 0.1 ppm to saturation points. They can function in saturated environments for long periods of time, a characteristic that would adversely affect many other humidity sensors.
Their ability to function accurately and reliably extends over a wide range of temperatures and pressures. Capacitive humidity sensors also exhibit low hysteresis and high stability with minimal maintenance requirements.
These features make them viable for many specific operating conditions and ideally suitable for a system where uncertainty of unaccounted conditions exists during operations. There are many types of capacitive humidity sensors. In here, aluminum, tantalum, silicon, and polymer types are introduced.
Aluminum Capacitive Humidity Sensors
The majority of capacitive humidity sensors are based on aluminum oxide-type dielectric material. In these types of sensors, high-purity aluminum is chemically oxidized to produce a prefilled insulating layer of partially hydrated aluminum oxide, which acts as the dielectric. A water-permeable but conductive gold film is deposited onto the oxide layer, usually by vacuum deposition, which forms the second electrode of the capacitor.
In another type, the aluminum–aluminum oxide sensor has a pore structure as illustrated in Figure 1. The oxide, with its pore structure, forms the active sensing material. Moisture in the air reaching the pores reduces the resistance and increases the capacitance. The decreased resistance can be thought of as being due to an increase in the conduction through the oxide. An increase in capacitance can be viewed due to an increase in the dielectric constant. The quantity measured can be either resistance or capacitance or impedance. High humidity is best measured by capacitance because resistance changes are vanishingly small at high levels.
In addition to the kind of transducer design illustrated in Figure 1, there are many others available with a number of substantial modifications due to their particular properties, such as the increased sensitivity and/or the faster response.
Although most of these modifications can result in a change in physical dimensions or appearance, the basic sensing material—the aluminum oxide—remains the same.
In some versions, the oxide layer is formed by parallel tubular pores that are hexagonally packed and perpendicular to the plane of the base layer. These pores stop just before the aluminum layer, forming a very thin pore base.
Water absorbed in these tubules is directly related to the moisture content of the gas in contact with it. The porous nature of the oxide layer produces a large area for the absorption of water vapor.
At low humidity, the capacitance is due entirely to the mixed dielectric formed between the oxide, water vapor, and air. However, at higher humidity, parallel conductance paths through the absorbed water are formed down the pore surfaces.
Near saturation, this pore surface resistance becomes negligible, implying that the measured capacitance is virtually in between the very thin pore base and the aluminum core.
Tantalum Capacitive Humidity Sensors
In some versions of capacitive humidity sensors, one of the capacitor plates consists of a layer of tantalum deposited on a glass substrate. A layer of polymer dielectric is then added, followed by a second plate made from a thin layer of chromium.
The chromium layer is under high tensile stress such that it cracks into a fine mosaic structure that allows water molecules to penetrate inside the dielectric material. The stress in the chromium also causes the polymer to crack into a mosaic structure.
A sensor of this type has an input range of 0%–100% relative humidity (RH). The capacitance is 375 pF at 0% RH and a linear sensitivity of 1.7 pF per% RH. The error is usually less than 2% due to nonlinearity and 1% due to hysteresis.
Silicon Capacitive Humidity Sensors
In some capacitive humidity sensors, silicon is used as the dielectric. The structure and operation of silicon humidity sensors are very similar to the aluminum oxide types. Some silicon-type humidity sensors also use the aluminum base and a thin-film gold layer as the two electrodes.
The silicon dielectric has a very large surface area, which means that the sensitivity is still relatively large even if the sensing area is very small. This is an important feature with the increasing chances of miniaturization.
Both sensor types are now typically found as extremely small wafer-shaped elements, placed on a mechanical mount with connecting lead wires. The formation of porous silicon is a very simple anodization process, and since no elaborate equipment is needed, devices can be made relatively at low costs. Also, by controlling the formation conditions, the structure of the porous silicon can easily be modified so devices can be tailored to suit particular applications.
In both the silicon and the aluminum oxide capacitive humidity sensors, the radii of the pores in the dielectric are such that they are specifically suited for water molecules. Most possible contaminants are too large in size to pollute the dielectric.
However, contaminants can block the flow of water vapor into the sensor material, thus affecting the accuracy of the sensor.
For example, in dust contaminated streams, it may be possible to provide a simple physical barrier such as a sintered metal or plastic hood for the sensor heads. Many sensors come with specially designed casings to provide protection.
Polymer Capacitive Humidity Sensors
In some sensors, the dielectric consists of a polymer material that can absorb water molecules. The absorption of water vapor of the material results in changes in the dielectric constant of the capacitor. By careful design, the capacitance can be made directly proportional to percentage RH of the surrounding gas or atmosphere.
In general, an important key feature of capacitive humidity sensors is the chemical stability. Often, humidity sensing is required in an air sample that contains vapor contaminants (e.g., carbon monoxide) or the measurements are performed on a gas sample other than air (e.g., vaporized benzene).
The performance of these sensors, and in particular the silicon types, is not affected by many of these gases. Hydrocarbons, carbon dioxide, carbon monoxide, and CFCs do not cause interference.
However, the ionic nature of the aluminum oxide dielectric makes it susceptible to certain highly polar, corrosive gases such as ammonia, sulfur trioxide, and chlorine.
Silicon is inert; its stable nature means that these polar gases affect the sensor element to a far lesser degree.
Capacitive Moisture Sensors
Capacitive moisture measurements are based on the changes in the permittivity of granular- or powder-type dielectric materials such as wheat and other grains containing water. Usually, the sensor consists of a large cylindrical chamber (e.g., 150 mm deep and 100 mm in diameter), as shown in Figure 2.
The chamber is filled with samples under test. The variations in capacitance with respect to water content are processed. The capacitor is incorporated into an oscillatory circuit operating at a suitable frequency.
Capacitive moisture sensors must be calibrated if the samples are made from different materials, as the materials themselves demonstrate different permittivity and other properties. Accurate temperature is necessary as the dielectric constant may be highly dependent on temperature.
Most of these devices are built to operate at temperature ranges of 0°C – 50°C, supported by tight temperature compensation circuits. Once calibrated for a specific application, they are suitable for measuring moisture in the range of 0% – 40%.