Capacitive Humidity Sensor Research Articles

High-Sensitivity Paper-Based Capacitive Humidity Sensors for Respiratory Monitoring

Paper-based humidity sensors have attracted considerable research attention due to their low cost, flexibility, and biodegradability. The problem of current paper-based humidity sensors is that they cannot simultaneously satisfy high sensitivity and fast response/recovery speed. To solve this problem, this article presents a new type of paper-based capacitive humidity sensor. The sensor comprises interdigitated electrodes (IDEs) and graphene oxide (GO) as the moisture-sensitive material. A sensing model with better performance was obtained by optimizing the IDE structure. The effect of different types of paper substrates on the performance of the sensor was investigated systematically, and an approach for selecting paper substrates in sensor design was formulated. The performance of the sensor was further improved by optimizing the parameters of the sensitive GO film and the operating frequency. Results show that the sensor achieves ultrahigh sensitivity to humidity (maximum of 32 146%) at an operating frequency of 100 Hz and combines high sensitivity (8504%) with fast response/recovery time (170/40 s) at an operating voltage of 500 Hz. Furthermore, the results of preliminary respiratory-monitoring experiments show that the proposed sensor exhibits excellent reliability in respiratory monitoring. It can accurately record the respiratory rate and respiratory depth of the subject in each breathing pattern. A portable respiratory-monitoring system was fabricated, which confirmed the excellent respiratory-monitoring performance and practical potential of the proposed sensor.

Uncertainty analysis of polymer-based capacitive relative humidity sensor at negative temperatures and low humidity levels

Capacitive relative humidity sensors, widely used in industrial and environmental applications, should be interchangeable and ideally have parametrizable response to relative humidity changes. While the effects of temperatures in the range between 0 °C and 50 °C are negligible, this is not the case for negative values. Therefore, this effect must be taken into consideration when parameterizing the output of the capacitive humidity sensors for temperatures below the freezing point of water. In this paper, we present our investigations on the temperature effect for one type of such sensors when using it at subzero temperatures. The experiments were conducted at temperatures of −10 °C, −20 °C and −30 °C. Additionally, we have tested the sensor performance at relative humidity levels low enough for most sensors not exhibit their best performances. Direct and inverse calibration methods were used and the uncertainty analysis of the devices under test is discussed.

Highly Sensitive Interdigitated Capacitive Humidity Sensors Based on Sponge-Like Nanoporous PVDF/LiCl Composite for Real-Time Monitoring

In this study, a sponge-like poly(vinylidene fluoride) (PVDF)/lithium chloride (LiCl) nanocomposite-entrenched interdigitated capacitive (IDC) sensor was developed for real-time humidity-sensing applications. Here, we demonstrated a sponge-like nanoporous structure ranging from 200 nm to 2 μm size holes, the PVDF/LiCl structure fabricated on an interdigitated capacitor (IDC) electrode functioning as a high-performance sensor because of the presence of ionized LiCl. The nanoporous PVDF/LiCl composite-based humidity sensor exhibited a high sensitivity of 12.6 nF/% relative humidity (RH), a linearity of 0.990, and a low hysteresis of 2.6% in the range of 25-95% RH. The composite film exhibited a response time of 17.7 s, a recovery time of 21 s, and an intensified increase of 8.02 nF/s (a decrease of 6.7 nF/s). The sensor designed demonstrates ultra-high sensing characteristics with 10 times higher sensitivity, i.e., 12.678.96 pF/%RH as compared to other polymer-based composite humidity sensors. Owing to the sensing performance and portability, the proposed nanoporous PVDF/LiCl composite-based IDC sensor is expected to be a promising platform for a wide range of humidity-sensing applications, including real-time breath monitoring and non-contact sensing.

Heatstroke risk informing system using wearable perspiration ratemeter on users undergoing physical exercise

We constructed an informing system to users for the heatstroke risk using a wearable perspiration ratemeter and the users’ thirst responses. The sweating ratemeter was constructed with a capacitive humidity sensor in the ventilated capsule. The timing point for informing heatstroke risk was decided to change from positive to negative on the second derivative of sweating curve. In addition, a wearable self-identification and -information system of thirst response was constructed with a smartphone. To evaluate the validity of wearable apparatus, we aimed to conduct human experiments of 16 healthy subjects with the step up and down physical exercises. The blood and urine samples of the subjects were collected before and after the 30-min physical exercise. The concentrations of TP, Alb, and RBC increased slightly with the exercise. In contrast, the concentrations of vasopressin in all subjects remarkably increased with the exercise. In almost subjects, they identified their thirst response until several min after the informing for heatstroke risk. In conclusion, the wearable ratemeter and self-information system of thirst response were suitable for informing system of heatstroke risk. The validity of timing point for informing heatstroke risk was confirmed with changes in the thirst response and concentrations of vasopressin in blood.

Combined WiFi sensor for temperature and moisture of soil

Physical parameters, such as temperature and moisture of soil, are important indicators in agriculture. The current study focuses on the development and assembly of an autonomous sensor for soil moisture and temperature through the use of a standard Arduino d1 mini module, equipped with a capacitive humidity sensor. The developed sensor configuration is distinguished with the possibility to transmit data via WiFi communication network and renewable energy system with photovoltaic power panel, which allows the results to be transmitted wirelessly and the sensor to operate autonomously powered by solar energy.

Continuous and Real-Time Measurement of Plant Water Potential Using an AAO-Based Capacitive Humidity Sensor for Irrigation Control

Water potential measurement is an essential factor in determining water consumption management and recycling in the agricultural field. We report the development of a continuous water potential measurement system using sensors for water stress analysis in tomato plants with better irrigation plan feedback. The water potential sensor uses the capacitive sensing principle which measures humidity inside an anodic aluminum oxide (AAO) layer. An analog to digital converter with a wireless communication module system records the capacitance data of the sensing system. Calibration data of sensors derived from superabsorbent polymer (SP) and deionized water (DIW) mixtures can represent their water potential value. The method showed good matching of capacitance and water potential values above −7 MPa, matching the result obtained in tomato stem. The measurements were conducted for a few days with the sap flow and water potential sensors connected in series on a tomato stem. When sunlight is sufficient, sap flow increases; meanwhile, water potential decreases. The opposite phenomenon could be observed during the nighttime. With irrigation restricting conditions, both sap flow and water potential signal decrease, triggering the emergency watering signals. This continuous water potential sensing system can quantitatively monitor the plant stem’s water stress and set irrigation schedules to achieve high-quality products in the agricultural field.

Design and Optimization of Interdigital Capacitive Humidity Sensor with Highly Sensitive and Dynamic Response Time

Humidity sensors are widely used in various fields of life. In meteorological detection, the sensor must have high sensitivity and fast dynamic response time due to extreme environmental interference. However, the sensitive mechanism of the humidity sensor determines that the dynamic response time will inevitably be increased while improving the sensitivity, which undoubtedly creates difficulties for sensor design. This article takes the interdigitated capacitive humidity sensor as the research object and proposes an optimal design scheme for the sensor that considers high dynamic response time and sensitivity. By constructing the sensor’s theoretical mathematical model, the influence of each structure is analyzed. The theoretical model has been verified by finite element simulation to have an accuracy higher than 95%. The article constructs the sensor optimization objective equation based on this model. Through analysis, within the range of structural parameters set in the article, to improve the sensitivity and reduce the dynamic response time of the sensor, the width and spacing of the interdigital electrodes should have a minimum value of 3 μm and a maximum value of 14 μm, respectively. The thickness of the electrode layer and the moisture-sensitive layer should be flexibly adjusted according to the application to ensure the lowest value of the optimization objective function. To further improve the sensor’s performance, the article optimizes the electrode structure and heating strategy of the sensor heating layer, which not only enhances the uniformity of heat transfer but also increases the optimal heat transfer area by 6% compared with the traditional scheme.

Sustainable Printed Chitosan-Based Humidity Sensor on Flexible Biocompatible Polymer Substrate

Humidity is one of the most relevant physical parameters to sense and control for a wide range of commercial and industrial applications. Consequently, there is continuing demand for the development of innovative and sustainable humidity sensor solutions. Here, the development and characterization of fully additively manufactured, highly sensitive, resistive Chitosan-based humidity sensors on flexible thermoplastic polyurethane (TPU) foil, as well as on a glass carrier substrate are presented. The sensors unite aspects of sustainability and high performance in a broad humidity range (20–90%rH). The humidity response follows an exponential curve progression with relative changes in the resistance per %rH of 6.9% and 5.7% for the glass carrier sensor and the TPU sensor, respectively. In absolute values, this means that the Chitosan-based sensors are particularly sensitive in the low humidity range with a vast dynamic range (ten times larger compared to commonly used capacitive humidity sensors). The flexible sensor on the TPU substrate shows great stability even after repeated bending. In addition, the combination of flexible and biocompatible materials (TPU and Chitosan) with additive manufacturing technologies makes the sensor particularly sustainable while having great potential for a plethora of biomedical applications.

Annealing Temperature Effects on Humidity Sensor Properties for Mg0.5W0.5Fe2O4 Spinel Ferrite.

The effects of annealing temperature on the structural, physical and humidity sensing properties of stoichiometric Mg0.5W0.5Fe2O4 spinel ferrite are investigated. In order to highlight the influence of sintering temperature on the structural, magnetic and electrical properties, ferrite samples were sintered for 2 h at 850 °C, 900 °C, 950 °C, 1000 °C and 1050 °C and the physical properties and humidity influence on magnesium-tungsten ferrite materials were analyzed. X-ray diffraction investigations confirmed the formation of magnesium-tungsten ferrite in the analyzed samples. SEM micrographs revealed the influence of annealing temperature on the microstructures of the samples and provided information related to their porosity and crystallite shape and size. This material, treated at different temperatures, is used as an active element in the construction of capacitive and resistive humidity sensors, whose characteristics were also investigated in order to determine the most suitable sintering temperature.

Printed Humidity Sensors from Renewable and Biodegradable Materials

AbstractIncreasing environmental concerns raised by the accumulation of electronic waste draws attention to the development of sustainable materials for short‐lived electronics. In this framework, printed capacitive humidity sensors and temperature resistive detectors composed exclusively of biodegradable materials: shellac, carbon‐derived particles, and egg‐albumin are reported. The sensor platform comprises interdigitated electrodes serving as a capacitive transducer for humidity sensing, and a serpentine used as a resistive temperature detector. Both the interdigitated and serpentine electrodes are manufactured by screen‐printing carbon ink on a shellac substrate. The humidity sensors are constructed by drop‐coating egg albumin on the interdigitated carbon electrodes and the temperature detector is prepared by encapsulating the serpentine design with shellac. Shellac is shown to be a biodegradable alternative to hydrophilic cellulose‐derived substrates, with the capacitive humidity sensors demonstrating a sensitivity of 0.011% RH−1. The response and recovery times on shellac are 12 and 20 times faster than on cellulose‐based substrate, and the serpentine resistive temperature detectors have a temperature coefficient of 5300 ppm K−1. At the end of their service‐life, the sensors produced are home compostable and can be environmentally friendly disposed, potentially enabling their future use for sustainable and environmentally friendly smart‐packaging, agricultural sensing, or point‐of‐care testing.

Experimental Measurement of Diffusion Coefficient of Polyimide Film for Capacitive Humidity Sensors.

Polyimide (PI) film is widely used as the key component of the capacitive humidity sensor, whose diffusion coefficient has a significant impact on the sensor's dynamic characteristics, but is rarely discussed. This paper provides a test method and processes for effective diffusion coefficients of water molecules in self-synthesis PI films. The films were formed by four ingredients (PMDA-ODA, BPDA-ODA and BPDA-BAPP, PMDA-BAPP) with PI acid concentrations of 23%, 20%, 17% and 15%, and tested in temperatures of 20 °C, 35 °C and 50 °C, respectively. The results indicated that BPDA-BAPP film was good as a moisture sensitive film, whose average effective diffusion coefficient was 2.709 × 10-14 m2/s. The temperature of the environment had a significant effect on the humidity-sensitive properties, but the PI acid concentration effect could be indirect.

A Cellulose Nanofiber Capacitive Humidity Sensor with High Sensitivity and Fast Recovery Characteristics

Humidity sensors with high sensitivity and fast response characteristics are of great interest for researchers. In this work, capacitive humidity sensors were fabricated using ionic liquid/cellulose nanofibers (CNFs) as the composited sensing film. The porous CNFs are beneficial for preparing sensing films via a solution process, and the ionic liquid could be uniformly dispersed in the films. The humidity-sensing performance of the as-prepared sensors was investigated. The optimized sensor showed a high response (27.95 pF/% RH) in a wide humidity range (11–95% RH) and a fast response speed in the adsorption process (the recovery time was only ~1 s). The high response of the sensors was attributed to the polarization at the interface between the electrolyte and the metal electrode, while the fast recovery was due to the rapid desorption of water molecules on the sensing films. Finally, the application of the obtained sensors in human breath monitoring was explored.

Capacitive and Resistive Humidity Sensors Based on Flexible Nanocellulose Film for Wearable Electronics

Capacitive and Resistive Humidity Sensors Based on Flexible Nanocellulose Film for Wearable Electronics

MEDA HS: Relative humidity sensor for the Mars 2020 Perseverance rover

The Finnish Meteorological Institute (FMI) provides a relative humidity measurement sensor (HS) for NASA's Mars 2020 rover. The sensor is a part of the Mars Environmental Dynamic Analyzer (MEDA), a suite of environmental sensors provided by Spain's Centro de Astrobiología. The main scientific goal of the humidity sensor is to measure the relative humidity of the Martian atmosphere near the surface and to complement previous Mars mission atmospheric measurements for a better understanding of Martian atmospheric conditions and the hydrological cycle. Relative humidity has been measured from the surface of Mars previously by Phoenix and Curiosity. Compared to the relative humidity sensor on board Curiosity, the MEDA HS is based on a new version of the polymeric capacitive humidity sensor heads developed by Vaisala. Calibration of humidity devices for Mars conditions is challenging and new methods have been developed for MEDA HS. Calibration and test campaigns have been performed at the FMI, at University of Michigan and the German Aerospace Center (DLR) in Berlin to achieve the best possible calibration. The accuracy of HS and uncertainty of the calibration has been also analysed in detail with VTT Technical Research Centre of Finland. Assessment of sensor performance after landing on Mars confirms that the calibration has been successful, and the HS is delivering high quality data for the science community.

APPLICATION OF MULTIFACTOR DISPERSION ANALYSIS WITH THE PURPOSE OF QUALITATIVE ASSESSMENT OF THE INFLUENCE OF FACTORS ON THE SENSITIVITY OF THE CAPACITIVE HUMIDITY SENSOR

In order to qualitatively assess the influence of such factors as the thickness of the moisture-sensitive layer, the concentration of the NaCl salt solution as an adsorbing material and their combined effect on the sensitivity of the capacitive humidity sensor, a multivariate dispersion analysis was used. Capacitive humidity sensors served as experimental samples. Capacitive humidity sensors are made on a 0.7×0.9 mm sitall substrate. A copper film is applied to the surface of the sitall substrate, which forms the covers of the capacitive humidity sensors in the form of a meander with the appropriate geometry of 7.85·10-2×150·10-6×1.2·10-6 m. In such a design, the moisture-sensitive layer is hygroscopic salt, which serves as a dielectric. To create a moisture-sensitive film, solutions of hygroscopic salt NaCl with concentrations of 0.89 mol/l and 5.33 mol/l were used. They were applied to the surface of capacitive humidity sensors with a spray gun at a distance of 40-50 cm with thicknesses of 5.0 μm and 10.0 μm. A multifactor plan was developed in the case of two qualitative factors. The research results were obtained on the basis of the comparison of the factor variance, which is due to the influence of the factor, and the residual variance, which is due to the influence of random causes. Factorial and residual variance were compared using the value of Fisher’s test. Using multivariate dispersion analysis, it was found that the concentration of the NaCl salt solution has a significant influence on the response of the model, namely the sensitivity of the capacitive humidity sensor. In this case, the value of Fisher’s criterion observed in the experiment significantly exceeds the critical value of Fisher’s criterion, namely 22.77>4.75 (F>Fck). The influence of such factors as the thickness of the moisture-sensitive layer and the combined effect of the thickness of the moisture-sensitive layer and the concentration of the salt solution is insignificant, i.e. the difference in the response values ​​of the model is related to its random nature rather than to the change in the value of the factor. Using the dispersion analysis of the influence of the factor, it was proved that regardless of the design of the capacitive humidity sensors, the concentration of the salt solution used to create the moisture-sensitive layer significantly affects the sensitivity of the capacitive humidity sensor.

Garage-Fabricated, Ultrasensitive Capacitive Humidity Sensor Based on Tissue Paper.

The role of humidity sensors in different industries and field applications, such as agriculture, food monitoring, biomedical equipment, heating, and ventilation, is well known. However, most commercially available humidity sensors are based on polymers or electronic materials that are not degradable and thus contribute to electronic waste. Here, we report a low-cost, flexible, easy-to-fabricate, and eco-friendly parallel-plate capacitive humidity sensor for field applications. The sensor is fabricated from copper tape and tissue paper, where copper tape is used to create the plates of the capacitor, and tissue paper is used as a dielectric sensing layer. Along with the low cost, the high sensitivity, better response and recovery times, stability, and repeatability make this sensor unique. The sensor was tested for relative humidity (RH), ranging from 40% to 99%, and the capacitance varied linearly with RH from 240 pF to 720 pF, as measured by an Arduino. The response time of the sensor is ~1.5 s, and the recovery time is ~2.2 s. The experiment was performed 4–5 times on the same sensor, and repeatable results were achieved with an accuracy of ±0.1%. Furthermore, the sensor exhibits a stable response when tested at different temperatures. Due to the above advantages, the presented sensor can find ready applications in different areas.

Low-Power Wearable Breath Monitoring Device With Humidity- and Ammonia-Sensing Functions (May 2022)

This article presents a wearable breath-monitoring device integrated with humidity- and ammonia-sensing functions. The breath-monitoring device was constructed using a piezoelectric cantilever array and hollow flexible polydimethylsiloxane (PDMS) membrane. Important parameters of breath, such as the respiratory rate and tidal volume, can be detected without actuated energy. The Nafion-based capacitive humidity sensor and ZnO nanoparticle-based resistance ammonia sensor were compactly integrated into the housing ring of a breath monitoring device. The fabricated humidity sensor demonstrated a faster response in the sensing range below 75% compared with a commercial product. The sensitivity of the fabricated ammonia sensor reached 0.16 ppm/mV and the sensor exhibited an approximately fourfold shorter response time compared with the commercial product. Two applications were tested for the developed device prototype. One was monitoring the humidity and ammonia of the environmental air along with breath parameters while breathing. The other was to examine the amount of breath air exhaled from the body of the subject. The total power consumed by the developed prototype device was approximately 46 mW, which is significantly lower than that of existing commercial products. This compact and lightweight prototype device demonstrates significant potential for wearable applications.

Dynamic Compensation Method for Humidity Sensors Based on Temperature and Humidity Decoupling.

Currently, integrated humidity sensors with fast-response time are widely needed. The most commonly used polyimide capacitive humidity sensor has a long response time, which is difficult to meet the need for a fast response. Most studies focusing on technology and materials have a high cost and are difficult to ensure compatability with the CMOS process. The dynamic compensation method can shorten the response time by only adding digital circuits or software processing. However, conventional compensation technology is not suitable for humidity sensors due to temperature coupling. This paper proposes a new dynamic compensation method for humidity sensors based on the decoupling of temperature factors by analyzing the coupling relationship between sensor dynamic characteristics and temperature. Simulations and experiments were used to verify the proposed method. The experimental results show that the proposed method reduces the humidity response time of the sensor by 85.6%. The proposed method can effectively shorten the response time of humidity sensors.

Fabrication of polyaniline/graphene oxide composites for implementation in humidity sensing.

This work reports the measurement of impedance variations under various humidity conditions at frequency ranges between 100 Hz and 5 MHz. An electrochemical polymerization process has been used in the synthesis including varying the mass ratios of graphene oxide (GO) in polyaniline. An electrochemical deposition method has been used to produce a sample film on an indium tin oxide glass slide. The percentage relative humidity (RH%) of the samples has been estimated to be 20-90%. Impedance and humidity had an inverse relationship, i.e. the impedance value decreased with an increase in humidity. In contrast with platinum capacitive humidity sensors (HS), the GO-based HS had a sensitivity of 75-99%, which was ~10-fold more than that of traditional sensors. With three different parameter weight % of GO, the frequency range have been 100 Hz to 5 MHz and RH% has been found to 20-90%. The HS showed a fast response and recovery time. Therefore, GO appears to be a useful material for building HS with high sensitivity for a comprehensive approach.

Highly sensitive and fully printable humidity sensor on a flexible substrate based on a zinc oxide and polyethylenimine composite

We report a highly sensitive and fully printable capacitive humidity sensor based on a zinc oxide (ZnO) and polyethylenimine (PEI) composite. The sensor has a simple structure, consisting only of a layer of the ZnO:PEI composite, coated using an ethanol solution, on a layer of silver inter-digital electrodes that have been pre-printed on a polyethyleneterephthalate substrate. The sensor with ZnO:PEI in the ratio of 2:1 by volume exhibits a response of 43 907 000% at maximum humidity, with a detection range of 15%–95% relative humidity, higher than other sensors fully made by wet-coating processes. Fourier transform infrared spectroscopy, atomic force microscopy, and scanning electron microscopy measurements suggest that the high response likely arises from the use of a hydrophilic polymer with a high dipole moment which facilitates dipole-dipole interactions with water molecules and from the highly granular morphology of the composite which leads to a high surface-to-volume ratio and more-numerous water adsorption sites. The fabricated sensor also demonstrates short response/recovery times (5 s/3 s), good repeatability over multiple humidification and desiccation cycles, and only 5% loss in response after being kept in the ambient for three weeks.