Handbook of Humidity Measurement, Volume 2

Electronic and Electrical Humidity Sensors

Ghenadii Korotcenkov

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781351400558
• Edition: 1
• Topic: Ciencias físicas
• Subtopic: Química

Because of unique water properties, humidity affects many living organisms, including humans and materials. Humidity control is important in various fields, from production management to creating a comfortable living environment. The second volume of The Handbook of Humidity Measurement is entirely devoted to the consideration of different types of solid-state devices developed for humidity measurement. This volume discusses the advantages and disadvantages about the capacitive, resistive, gravimetric, hygrometric, field ionization, microwave, Schottky barrier, Kelvin probe, field-effect transistor, solid-state electrochemical, and thermal conductivity-based humidity sensors.

Additional features include:

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• Provides a comprehensive analysis of the properties of humidity-sensitive materials, used for the development of such devices.

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• Describes numerous strategies for the fabrication and characterization of humidity sensitive materials and sensing structures used in sensor applications.

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• Explores new approaches proposed for the development of humidity sensors.

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• Considers conventional devices such as phsychometers, gravimetric, mechanical (hair), electrolytic, child mirror hygrometers, etc., which were used for the measurement of humidity for several centuries.

Handbook of Humidity Measurement, Volume 2: Electronic and Electrical Humidity Sensors provides valuable information for practicing engineers, measurement experts, laboratory technicians, project managers in industries and national laboratories, as well as university students and professors interested in solutions to humidity measurement tasks as well as in understanding fundamentals of any gas sensor operation and development.

Section III

Electronic and Electrical Humidity Sensors and Basic Principles of Their Operation

10	Capacitance-Based Humidity Sensors

10.1 BASIC PRINCIPLES OF OPERATION

Capacitance-type humidity sensors are a huge part of existing sensor types in both research and industry, as they offer significant advantages in terms of simplicity of fabrication, sensitivity, and low-power operation (Ishihara and Matsubara 1998; Kummer et al. 2004; Lee and Lee 2005; Chatzandroulis et al. 2011). They dominate in atmospheric and process measurements and are the only types of full-range relative humidity (RH) measuring devices capable of operating accurately down to 0% RH. According to estimations (Rittersma 2002), capacitive RH sensors represent more than 75% of the available humidity sensors on the market. Capacitive humidity sensors are commercially available from, for example, Sensirion (www.sensirion.com), Vaisala (www.vaisala.com), and Humirel (www.humirel.com).

In the simplest case, a capacitance-type sensor is made of two parallel plates. In such structure, the capacitance between the two electrodes is given by

$C={\epsilon }_r{\epsilon }_0\frac{A}{d}$

(10.1)

where ε_r and ε₀ are the relative and vacuum permittivity constants, respectively, A is the plate surface area, and d is the plate distance. From this equation, it is evident that only three ways exist to effect a change in the capacitance of that device: (1) alter the distance d between the two plates, (2) alter the overlapping area A between the two plates, and (3) change the dielectric permittivity between the plates (Ishihara and Matsubara 1998). This means that capacitive sensors can detect only those gases and vapors that affect these parameters. Water vapor can exert such influence and therefore, by measuring the change in the capacitance, the presence of the water vapors in the air can be detected. The principles of operation and more detailed description of the constructions of capacitance gas and chemical sensors can be found in Ishihara and Matsubara (1998) and Chatzandroulis et al. (2011).

10.1.1 HUMIDITY SENSORS OF PERMITTIVITY-TYPE

It is worth noting that the most common humidity sensors of the capacitive type are sensors of the permittivity type (i.e., sensors in which the variable parameter is the permittivity of the space between the electrodes). The simplest version of such sensors is air-gap sensors (Ford 1948; Fraden 2004; Zarnik and Belavic 2012; Choi and Kim 2013). As is known, the dielectric constant of air increases with increasing air humidity (see Table 10.1). According to Lea N. (1945), moisture in the atmosphere changes the electrical permittivity in air according to the equation:

An analysis of the possibilities of designing a humidity sensor based on this effect was carried out in Choi and Kim (2013). Simulation has shown that humidity sensors can be developed that they will be high-speed and reliable. However, the same analysis indicates that the sensors of this type do not have a high sensitivity, since the change in permittivity under the influence of humidity is insignificant (Table 10.1). With the growth of temperature, this influence increases, but to a very small degree. It must be taken into account that the temperature change is also accompanied by a change in the permittivity of the air (see Table 10.2), commensurate with the change in ε under the influence of humidity. All this means that the development of humidity measurement devices based on such sensors sharply increases the requirements of stability and sensitivity of devices, as well as minimization of parasitic capacitances. Another problem of such sensors is the low dielectric permeability of air. Thus, to achieve a capacitance value acceptable for measurement, the gap between the electrodes must be very small and the area very large, which creates significant difficulties in the implementation of such sensors. Another option to increase the sensitivity is to increase the pressure (Choi and Kim 2013).

$\kappa =1+\frac{211}{T}(P+\frac{48P_S}{T}\cdot H){10}^{–6}$

(10.2)

where T is the absolute temperature (in K), P is the pressure of moist air (in mmHg), P_s is the pressure of saturated water vapor (in mmHg) at temperature T, and H is the RH (in %).

TABLE 10.1
Dependence of the Dielectric Constant of Air on the Relative Humidity at Normal Temperature and Pressure

Relative Air Humidity, %	Dielectric Constant, ε
0 50 100	1.00058 1.00060 1.00064

TABLE 10.2
Temperature Influence on the Dielectric Constant of Air

Temperature
°C	K	Dielectric Constant, ε
–60 +20 +60	213 293 333	1.00081 1.00058 1.00052

In the case of using porous dielectrics between electrodes capable of accumulating water, the situation is much better. As is known, due to the polar structure of the H₂O molecule, water exhibits a very high permittivity ε_w = 80 at room temperature (Grange and Delapierre 1991). Dielectric materials usually have considerably smaller permittivity (see Table 10.3). This means that a change in the water content in the dielectric can give a much larger capacity change than a change in air humidity, especially if the dielectric used has a low dielectric constant and is porous (Khanna and Nahar 1984; Kim et al. 2000). The smaller the dielectric constant of the dielectric used is, and the greater is the proportion of space between the electrodes that the water occupies, the greater will be the effect, appearing in the increase in capacity. Based on this requirement, it becomes clear that high adsorption capacity and high porosity are important parameters of materials suitable for the development of capacitive humidity sensors. As can be seen from Table 10.3, polymers and some inorganic dielectrics meet these requirements.

At equilibrium conditions, the amount of moisture present in a hygroscopic material depends on both the ambient temperature and the ambient water vapor pressure. So, there is a relationship between RH, the amount of moisture present in the sensor, and the sensor capacitance. Capacitive sensors, as well as other absorption-based humidity sensors, typically show a nonlinear behavior as a function of RH (Rittersma 2002). This behavior can be described by the phenomenological equation:

$\frac{C_S}{C_0}={(\frac{{\epsilon }_w}{{\epsilon }_d})}^n$

(10.3)

TABLE 10.3
Dielectric Constants of Selected Materials

Material	Dielectric Constant, ε
Vacuum	1.0000 00
Air (1 atm)	1.0000 54
Air (100 atm)	1.0548
H2O (20°C)	80
Polymers
Polytetrafluoroethylene (Teflon)	2–2.1
Polyethylene	2.2–2.4
Polyamide	2.8
Polystyrene	2.6–3
Polyvinyl chloride	3.2
Nylon	4–5
Conductive metal oxides
SnO2	9.9
TiO2	86–173
SrTiO3 (Strontium titanate)	310
Other materials
Paper	2–4
Wood, dry	2–6
Si	11–12
Glass	3.7–10
SiO2	3.9–4.5
Alumina	9.1–11.5
Ethanol (25°C)	24.3

Source: Blythe, T. and Bloor, D., Electrical Properties of Polymers, Cambridge University Press, Cambridge, NY, 2005.

where ε _d and ε _w are the permittivity of the dielectric at a dry and wet state and n is a factor related to the morphology of the dielectric. This relationship is the basis of the operation of a capacitive humidity instrument. In real polymer-based humidity sensors, the change in a dielectric constant can achieve 30% when humidity changes within 0%–100% RH. In the absence of moisture, the dielectric constant of the hygroscopic dielectric material and the sensor geometry determine the value of capacitance C₀. It is important to note that the measurement is made from a large base capacitance; thus the 0% capacitance readings are made at a finite and measurable RH capacitance level. For example, the typical capacitance variability of the humidity-sensitive films is 0.2–0.5 pF for a 1% RH change, while this value is between 100 and 500 pF at 50% RH for the bulk capacitance at room temperature (Fontes 2005).

It is important to note that, in order to achieve a noticeable change in capacity, water in dielectric material must be in a free state, because only the free-water molecules have dielectric properties similar to those of liquid water, while the bound water exhibits icelike dielectric properties. According to Evans (1965) and Matzler and Wegmuller (1987), freshwater ice has a permittivity of 3.17–3.19, which is significantly less than that of water. With increasing temperature, the permittivity of ice increases in accordance with expression (Matzler and Wegmuller 1987):

${\epsilon }^{,}=3.1884+0.00091·T(Tin°C)$

(10.4)

Free water is usually understood as water held in capillaries. Taking into account the above, we come to the conclusion that the processes of capillary condensation should play an important role in the effects responsible for the sensitivity of capacitive humidity sensors. More detail description of the mechanism of water vapor interaction with polymers and ceramics one can find in Chapter 10 (Volume 1) and Chapter 2 (Volume 3) of our series.

10.1.1.1 Parallel Plate Structure

In classical capacitive sensors, a humidity-sensitive material is placed between the top and bottom electrodes (see Figure 10.1). This is the so-called parallel plate structure, which can be implemented on typical substrates of ceramic, glass, ...