Open access peer-reviewed chapter

Humidity Sensors, Major Types and Applications

Written By

Jude Iloabuchi Obianyo

Submitted: 03 January 2021 Reviewed: 22 April 2021 Published: 04 January 2023

DOI: 10.5772/intechopen.97829

From the Edited Volume

Humidity Sensors - Types and Applications

Edited by Muhammad Tariq Saeed Chani, Abdullah Mohammed Asiri and Sher Bahadar Khan

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Abstract

The need for humidity sensors in various fields have led to the development and fabrication of sensors for use in industries such as the medical, textile, and laboratories. This chapter reviewed humidity sensors, major types and applications with emphasis on the optical fiber, nanobricks, capacitive, resistive, piezoresistive and magnetoelastic humidity sensors. While optical fiber sensors are best for use in harsh weather conditions, the nanobricks sensors have excellent qualities in humidity sensing. Capacitive sensors make use of impedance and are more durable than the equivalent resistive sensors fabricated with ceramic or organic polymer materials and have short response and recovery times which attest to their efficiency. Piezoresistive sensors have fast response time, highly sensitive and can detect target material up to one pictogram range. Magnetoelastic sensors are very good and can measure moisture, temperature and humidity between 5% and 95% relative humidity range. It was concluded that sensors have peculiar applications.

Keywords

  • Humidity sensors
  • Major types
  • Applications
  • A Review

1. Introduction

1.1 Humidity

Humidity is a measure of quantity of water vapor present in the air or a gas. Humidity is a general terminology and is used to measure the amount of water vapor in any given environment. Measurement of this parameter is important in the environment because many appliances in the industries cannot be stored under certain range of humidity. High humidity entails high atmospheric moisture content, hence condensation which depends on the atmospheric temperature resulting in corrosion of metals and similar features in industries.

The industries that could be affected include hospitals, textile industries, laboratories, storage rooms for computers, food processing industries, art museums, shopping malls, libraries, exhibition centers, pharmaceutical stores. For these reasons, humidity sensors are widely used in numerous fields of human endeavor such as for weather forecasting, food processing, health care etc. It is good practice to measure humidity from time to time for maintenance of optimal environmental conditions suitable for products in order to prevent damage.

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2. Humidity sensors

Humidity is measured with a device known as humidity sensor and different types are available in the market such as the, optical, gravimetric, capacitive, resistive, piezoresistive and magnoelastic sensors etc. Each of these humidity sensors have peculiar applications which depend on the design and suitability for a given environment. Humidity sensors are electronic equipments that can be used to measure humidity in any given environment. Being electronic devices, they transform information to equivalent electrical signal. Humidity sensors have very wide applications, functionality and appear in different sizes. Sometimes, they are incorporated in handheld devices as in smart phones.

2.1 Optical fiber humidity sensors

Being a recent technology in sensor development, optical fiber sensors have peculiar advantages such as lightweight, chemical stability, handy, non-susceptible to electromagnetic fields and the ability to communicate two or more signals over a common channel [1]. Because of these advantages, optical fiber sensors essentially is a dependable technology that can be applied in many fields such as industries, medicine, and structural health monitoring [2, 3, 4]. Apart from its excellent performance in measurement of humidity, Its application can be extended to the measurements of angle, refractive index, temperature, acceleration, pressure, breathing rate and oxygen saturation [5, 6, 7, 8, 9, 10].

Optical sensors exhibit optimal performance in harsh environmental conditions. Measurement of relative humidity, influence of coating thicknesses and temperature with the use of polyimide-recoated optical fiber Bragg gratings showed that relative humidity (RH) and temperature (T) sensitivities were SRH=2.21±0.10×106%RH1 and ST=7.79±0.08×106K1, these indicate a RH sensitivity of 44.2% and a T sensitivity of 86.6% [11]. Apart from the use of optical fiber sensors in measuring humidity, it can equally be used as a temperature sensing device because of its high sensitivity of 1.04 × 10−3°C−1 with a linearity of 0.994, this device has root mean square of 1.48°C an indication of 2% relative error [7].

Embedment of optic fiber sensors in metallic materials such as nickel- and copper-coated fiber Bragg grating (FBG) shields sensors from environmental influence and gives rise to better performance. Though, Copper-coated sensors lost their sensitivity to temperature and strain in the process of embedment using tungsten inert gas (TIG) welding due to thermal and mechanical strain, while nickel-coated sensors were less sensitive to the same effects and showed better sensor performance [12].

Attachment of fiber optic sensors on metallic structures have proved to be important in order to enhance the durability of sensors since non-embedded sensors are prone to damage. Laser cladding technology was used as an alternative in embedding metal coated fiber optics in which fiber Bragg grating is incorporated. This device minimized the high thermal and mechanical strain usually generated with TIG welding. It was shown that this sensor gave room for satisfactory results for strain and temperature measurements, and was favorably compared with the conventional gauge used in calibration of sensors [13].

Design is an important factor in performance of humidity sensors since different materials exhibit different magnitudes of sensitivity to humid environment. Performance of a humidity sensor is measured by its detection range. Magnesium oxide (MgO) humidity sensor designed with micro-arc oxidation (MAO) technology gave better performance than semiconductor humidity sensors which are usually prone to narrow detection ranges and poor sensitivities for detection. It has the advantage of using both impedance and capacitance as response signals, giving an output of 150 in the low relative humidity (RH) range (11.3–67% RH) when impedance was the response signal, and an output 120 at high humidity range of (67–97.3% RH) with capacitance as the response signal [14].

2.2 Nanobricks humidity sensors

Mesoporous tungsten oxide nanobricks (WO3 NBs) have proved to be excellent sensors because of its sensitivity to ammonia gas, volatile organic compounds and humid environment at room temperature [15]. It has a 75% response which is on the high side, a 15-day operational stability at 100 ppm ammonia concentration and extremely-high response/recovery time of 8/5 s. Generally, this family of sensors exhibit 32% resistance response at 20% RH with fast response/recovery time of 10/8 s, high specific surface area of the monoclinic crystal structure is instrumental to the high sensitivity of this device.

Nanobricks sensors such as Ag-CuO/rGO (5 at % Ag) have shown maximum response of approximately 52% RH humidity when operated at approximately 22°C. This sensor made of pure Ag-CuO nanostructures containing varied quantities of Ag produced by hypothermal method is highly sensitive to the presence of NO2 between a temperature range of 22–100°C [16].

Carbon nanotubes invented by [17] have extraordinary electrical and mechanical properties and for this reason are found in a number of applications [18, 19]. Due to their notable electrical characteristic, many researchers have studied the application of this material as a sensor not only because of its eminent electrical feature, but the reduced costs of manufacture which makes them very attractive for use by sensing industries [20]. Suspended aligned nanotubes beams was generated to asses its humidity sensing characteristics. The device was produced at room temperature using epoxy-based highly functional photoresist referred as SU8, and for 15–98% RH range, [20] reported a threefold better performance for suspended devices which exhibited improved sensitivity and little hysteresis at 10 humidity cycles when compared with non-suspended device. This implies that suspension of carbon nanotubes beam improved the sensitivity of the sensor, and the sensitivity factor is given by the expression;

S=RHR0R0×100E1

where RH is the resistance at the measured value of resistance, and R0 is value of baseline resistance.

Zinc stannate (ZnSnO3) nanoparticles prepared by chemical precipitation have shown to be good as humidity sensing device. The X-ray power diffraction of this sensor presented ZnSnO3 as having a perovskite phase with orthorhombic structure and a minimum of 4 nm crystalline size. Comparative study of ZnSnO3 pellet annealed at 600°C for 1:4 weight ratio and pure SnO3 by [21] indicate that ZnSnO3 is more sensitive to humidity than SnO3 when subjected to the same environmental conditions. They concluded that zinc stannate nanoparticles showed a maximum sensitivity of 3 GΩ/% RH when compared with the equivalent SnO3 used in this study.

In another research, it has been reported by [22] that synthesized Fe3O4-Nps with porous structure has shown exceptional qualities in humidity sensing. Characterization of prepared Fe3O4-Nps was done with the aid of X-ray diffraction, (XRD), transmission electron microscope (TEM) and vibration sample magnetometer (VSM), which revealed the spherical pores structure. With this, magnetic nanostructures (MNs) are highly valued by researchers and humidity sensors industries because they posses vital characteristics that make them to be widely applied.

2.3 Capacitive humidity sensors

Organic materials such as hydrophobic polymers are employed in the manufacture of capacitive humidity sensors. Capacitive humidity sensors are more durable than the equivalent resistive humidity sensors because it has the propensity for heavy water vapor condensation at high humidity level [23], and may operate comfortably at relatively higher temperature of about 200°C [24], with capacitance range of between 100 to 500 pF at intervals of 50% changes in humidity level [25]. Capacitive sensors are easily available in the market because of their better linearity characteristics when compared with the resistive type sensors [26, 27]. Capacitive thin-film humidity sensors are responsive to changes in RH because their sensors rely on the dielectric constant value of the active layers. Response time of capacity sensors is a function of three parameters namely the sensor design, materials propensity for sorption and desorption of water vapor, and the sensor temperature [28].

The most important thing in capacitive humidity sensors is the material it is made of. The hygroscopic characteristics of the hydrophobic elements in these sensors are instrumental to its ability to absorb water molecules from their surrounding and as a result, this should be born in mind during construction so as to produce a good configuration that would optimally utilize these properties for effective performance [29]. Three major factors affect the performance of capacitive humidity sensors and include; surface area of the electrode, dielectric material polarization and distance apart between the electrodes.

As earlier stated that humidity sensors have vary wide applications. In agriculture, there is need to measure the moisture/humidity levels to guide in decision making regarding the optimal ambient humidity condition that will be suitable for excellent crop yield.

Without doubt, water is very important in the physicochemical and mechanical characteristics of soil. Variations in soil moisture quantity can have significant influence on ecosystems, plant growth and biodiversity. Capacitive moisture sensors are used for sensing the moisture contents of soils in agricultural endeavors. Availability of soil moisture in the root zone is very crucial for plants development and growth which depends on the physical characteristics of soil and the surrounding environment with respect to climatological conditions [30].

The output of a capacitive moisture sensor is a function of the complex relative permittivity εr of the soil which is the dielectric medium given by the expression [31];

εr=εrjεr=εrjεrelax+σdc2πfε0,E2

where εr and εr are the real and the imaginary part of the permittivity respectively, σdc is the electrical conductivity, εrelax, is the molecular relaxation contribution (dipolar rotational, atomic vibrational, and electronic energy states, j is the imaginary number 1, and f the frequency. However, it has been reported that capacitive coplanar soil moisture sensor showed reliable relationship between output voltage and gravimetric water content [32].

Another type of humidity sensor are the ones made with two-dimensional materials. The two-dimensional (2D) materials show superior physical characteristics when compared with the corresponding one-directional materials on application of charge and heat to a planar layer [33]. The two dimensional nature of a material give a higher ratio of surface area to volume and hence, makes it easier for fabrication of sensing layers.

Some of these humidity sensors can neither be fully classified as belonging to either the capacitive or resistive humidity sensors because they make use of combined effects of capacitance and impedance in sensing the humidity of environment. Example of this is the humidity sensor based on two dimensional molybdenium diselenide (MoSe2). Material for this sensor is the transition metal dichalcogenides (TMDCs) which are very good in fabrication of sensors for applications in industries. This humidity sensor has been reported to be highly stable and effectively superfast in detecting the moisture level in its surroundings. At frequency of 1 kHz and between the temperature range of 20–30°C, impedance witnessed a hysteresis of less than 1.98% while the capacitance had a hysteresis of less than 2.36%, maximum error rates of −0.162% and −0.183% were observed based on the impedance and capacitance responses respectively recorded between humidity range of 0–90% RH. Impedance response (tres and recovery times trec for the sensor are −0.96 s and −1.03 s. The corresponding response and recovery times for capacitance are −1.87 s and 2.13 s respectively [34].

The conduction mechanism of the sensor is such that as MoSe2 nano-flakes responds to humidity, dielectric constant increases relative to the dry film, there exists a corresponding ionic flow through the sensor. When the sensor is exposed to humidity which invariably comes in contact with the hydroxyl ions, the thin films of MoSe2 tend to absorb water molecules thus establishing ionic conduction paths between MoSe2 nanoflakes, thereby reducing the overall sheet resistance.

In 2004, mechanically exfoliated 2D graphene was discovered which is a new innovation in nanotechnology with several advantages such as transparency, flexibility and high carrier mobility. Major limitations in applicability of 2D graphene is the virtually zero band gap of this material which consequently leads to obstruction of its need for fabrication of sensors due to low on/off ratio and long response and recovery times of approximately 10 s [35, 36, 37, 38, 39]. TMDCs is a two-dimensional semiconductor, less expensive and very fast in detection of humidity signals and has a high carrier mobility of about 500 cm2/Vs.

Apart from its high applicability in humidity sensing because of its exceptional physical properties and high carrier mobility, they are also used in gas, temperature, electronic [40, 41, 42, 43, 44, 45, 46, 47] and optoelectronics sensors [48, 49, 50, 51, 52].

2.4 Resistive humidity sensors

The working principle of resistive humidity sensors is the ability for the sensor to detect vapor in its surrounding which has direct influence on the electrical resistance of the sensing layer [53] In this sensor device, humidity increase give rise to increase in electrical conductivity which invariably lowers the system resistivity within 1 to 100 kΩ [54]. This sensor has subdivisions such as the electronic and ionic conduction types and are based on the mechanism by which the signals are received by the sensing material. The electronic type is made up of polyelectrolytes which respond to changes in water vapor in the surrounding by affecting the resistivity. The ionic conduction type depend on changes in the dielectric constant of the polymer dielectrics which gives rise to the two categories of resistive type sensors namely; the ionic-conduction [55, 56] and the electronic-conduction [57, 58] respectively.

Resistive humidity sensors can be fabricated with either ceramic or organic polymer materials. However, the ceramic type have shown to be advantageous over the organic polymer type evidenced from the performance of equivalent sensor fabricated with metal oxides materials [54]. Two methods are available for preparation of ceramic metal-oxides for applications in humidity sensors namely, the conventional method [59] and the advanced method [60], meant to impart porosity [61, 62, 63] to the device, a characteristic which enhances the efficiency of the sensor.

Innovations and application of internet of things have improved the characteristics of these humidity sensors. These characteristics include low power consumption, room-temperature operation, small size and compatibility with other platforms such as thermal and chemical compatibility during fabrication, easy selection from other sensors for use in calibration of the newly fabricated sensor [64, 65, 66, 67, 68].

For instance, resistive and capacitive humidity sensor was fabricated using the new bis(4-benzylpiperazine-1-carbodithioato-k2S,S′)nickel(II) complex, with Von Grotthuss as the conduction mechanism. Between humidity range of 30–90%, [69] reported that the resistance of the sensor decreased by two orders of magnitude. The result showed that at 30%RH, the resistance was 2.94×108Ω and at 90%RH, it was found to be 2.34×106Ω giving a range of 2.92×108. A hysteresis of 1.54% was observed with response and recovery times of 25 and 30 seconds. Response and recovery times of approximately 0.14s and 1.7s between a humidity range of 0–70% were reported by [70] after using resistive humidity sensor comprised of biopolymer-derived carbon thin film and carbon microelectrodes, evidencing the super-sensitivity of this sensor when compared with the bis(4-benzylpiperazine-1-carbodithioato-k2S,S′)nickel(II) complex. These short response and recovery times attest to the efficient performance of this sensor and is attributed to the shellac-derived carbon (SDC) film which enables sharp absorption and desorption equilibrium. This sensor is based on a shellac-derived carbon (SDC) active film deposited on sub-micrometer-sized carbon interdigitated electrodes (cIDEs) which is responsible for the optimization of the response and recovery times respectively. Characterization of this SDC-cIDEs-based humidity sensor revealed excellent dynamic range of between 0–90% RH, with a dynamic response of 50% and very high sensitivity of 0.54/% RH.

Nitrogen-doped layers incorporated in humidity sensors are unique and have proved to be excellent humidity sensor device. Example of this sensor is the configuration of nitrogen-doped reduced graphene oxide (nRGO) placed on a colourless polyimide film. This sensor has a detection range of 6.1% to 66.4% RH, the results also showed that a 1.36-fold better performance in terms of sensitivity is achieved when platinum nanoparticles are attached on the surface of nRGO as compared with the pure nRGO. These sensitivities are in the neighbourhood of 4.51% for the Pt-nRGO at 66.4% RH and 3.53% for the nRGO at 66.4% RH [71].

2.5 Piezoresistive humidity sensors

Cantilever-based nanomechanical sensors have two important qualities, fast response time, highly sensitive and real time and label-free detection ability. These important features have made these sensors to be very useful instruments in advanced detection of molecules and can be widely applied in numerous fields [72, 73, 74, 75]. These sensors can be used in process monitoring, gas sensors, in hospitals for diagnostic biosensing, and in detection of solvent vapours. One significant feature of this sensor is that they make use of poly-coated cantilever as their active layers [76, 77]. These sensors function as a result of the induced stress from the adsorption of molecules on the sensing layer. The advantage of nanomechanical sensors are their ability to detect target materials up to one trillionth of a gram range, but one major limitation to its application is the bulky size of the optical measurement system [78] that uses a piezoresistive sensor. The self-sensing method approach is usually employed to overcome this shortcoming [79].

Highly miniaturized nanomechanical SI-polymer composite membrane-type internal-stress sensor (MIS) with piezoresistive elements have shown to be an excellent device for humidity detection. This sensor has a surface area of 500 μm2 and consists of a thin SI-polymer composite membrane supported by two piezoresistive beams. It has a relative resistance of 0.6% at a relative humidity of 58% and a response of 5.2 mV/% RH to 70% relative humidity range, a sensing resolution of 0.5% humidity and polymer expansion ratio, ϵp of 2.4×103. A perfectly correlated linear relationship existed between the sensor output and relative humidity at 19.5°C, while the Fast Fourier Transform (FFT) analysis of the sensor system for noise resolution at 60% RH was 2.5 mV/Hz [79].

2.6 Magnetoelastic humidity sensors

Uncrystallized alloys made up of Fe, Ni and Co have proved to be excellent magnetoelastic materials and are very good in fabrication of magnetoelastic humidity sensors. These materials are prone to change of shape on exposure to magnetic fields so that on application of mechanical stress, result in magnetization. The dual directional connection is instrumental to the functioning of this sensor device [80, 81, 82, 83]. It is designed in such a manner that sample vibration is directly proportional to the frequency of AC magnetic field applied to the sensor system.

Typical example of this sensor is the TiO2 nanotubes (TiO2-NTs) coated with Fe40Ni28Mo4B18 amorphous ferromagnetic ribbons as a humidity sensor, can be used to measure moisture values between the range of 5–95%. Measurement precision of this sensor is very high with low hysteresis. Sensor resonance frequency was approximately 3180 Hz which indicated highly significant change when compared with other magnetoelastic humidity sensors [84]. However, the resonance frequency shift of magnetoelastic sensors depend on two parameters expressed as in Eq. (3); [82, 85, 86, 87].

f=f2mME3

where m= the variation of sensor mass, M=mass of the magnetoelastic sensors (MES) prior to adsorption.

MES have been reported to be very good in the measurement of humidity and temperature [88, 89, 90, 91, 92] and mass [93, 94]. Different methods are available for sensing in the environment such as the use of flow cytometry, immunosensors, and gas chromatography in sensing of volatile organic compounds (VOC) though laborious, time consuming and expensive as limitations to its applications. Acoustic wave based sensors are better being highly sensitive in detection of VOCs, bacteria etc., though with very high sensitivities, it is a wireless sensing instrument and too expensive to afford [95, 96].

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3. Conclusions

Humidity sensors are very essential in industries because of the roles they play in giving information about the ambient environmental conditions for products storage. A knowledge which is very important in materials management since different products require certain amount of atmospheric moisture in its surroundings beyond or below which deterioration sets in. Therefore this chapter reviewed humidity sensors, its major types and applications in different fields. Emphasis was on the optical fiber, nanobricks, capacitive, resistive, piezoresistive and magnetoelastic humidity sensors respectively. Each of these humidity sensors have peculiar applications because while the optical fiber sensor perform excellently in harsh environmental conditions, the nanobricks are excellent sensors because of its sensitivity to ammonia gas, volatile organic compounds and humid environment at room temperature. While the nanobricks sensors operate more comfortably at room temperature, capacitive sensors can operate comfortably at relatively higher temperature of about 200°C and have better linearity characteristics when compared with the resistive type sensors.

Resistive humidity sensors fabricated with either ceramic or organic polymer materials have shown to be very good in humidity sensing as a result of increase in electrical conductivity that lowers the electrical resistivity with very short response and recovery times. Piezoresistive sensors are equally good in humidity sensing with low relative resistance and a perfectly correlated linear relationship which exist between the sensor output and relative humidity, while the magnetoelastic humidity sensors fabricated from alloys of iron, nickel and cobalt have wonderful performance in humidity sensing because of their high measurement precision and low hysteresis. These humidity sensors do not have universal applications because of their peculiarities since each of them is more suitable for sensing in a given environment and recommendation should be based on this notion.

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Written By

Jude Iloabuchi Obianyo

Submitted: 03 January 2021 Reviewed: 22 April 2021 Published: 04 January 2023