Relative Humidity Sensor Research Articles

Paper‐Based Electronics: Toward Sustainable Electronics

AbstractThe emergence of paper‐based electronic devices marks a significant leap forward in the design of flexible, lightweight, and eco‐friendly electronics. Paper‐based electronic sensors represent a transformative approach to creating flexible, lightweight, and environmentally friendly electronics. This review will discuss recent applications of paper‐based electronics, mainly in exploring emergent technologies employed in developing innovative sensors for chemical analysis. Furthermore, the role of paper‐based electronics in electrochemical, and physical sensing, specifically addressing relative humidity, temperature, pressure, and strain sensors will be commented. In addition, the integration of paper electronics in energy harvesting and storage is discussed, covering solar cells, tribogenerators, antennas, and supercapacitors. These advancements underscore the versatility and potential of paper‐based electronics in diverse applications, from wearable health monitors to sustainable energy solutions, paving the way for the future of recyclable and biodegradable electronic devices.

Effect of Surface Morphology on the Response Speed of Moisture Sensors

Relative humidity sensors are used for high-humidity measurement and moisture sensor or dew point sensors are used for low-humidity measurement (<1 ppmv). We fabricated moisture sensors using α-Al2O3 films as porous dielectric materials deposited by anodic spark deposition. In this study, a variety number of small pores has been studied to investigate the response speed of moisture sensors. Three different surface morphologies have been studied using scanning electron microscopy. One sample has the maximum number of small pores, one sample has the medium number of small pores, and the last sample has the minimum number of small pores. Small pores are defined as pore size ranging from 40 nm to 200 nm. We found that the sensor made as porous material with the maximum number of small pores has the fastest response speed, and that from the minimum number of small pores has the slowest response speed either from high humidity to low humidity or from low humidity to high humidity.

Evaluation of nature-based climate solutions for agricultural landscapes in the Galápagos Islands

The Galápagos Islands are highly vulnerable to climate and environmental change, and nature-based solutions can help local communities adapt local agricultural systems. Through a comparative analysis, we evaluated the effects of three land management strategies on soil temperature, soil water availability and storage, and carbon stocks in Santa Cruz Island (Galápagos Archipelago). We installed six monitoring sites that consisted of two replicates per pathway: (i) the avoided loss of tropical forest, (ii) the conservation of scattered trees and living fences in at-risk agroforestry system, and (iii) the increase in biomass after a reduction of the grazing intensity. The monitoring sites were equipped with a dense network of rain gauges, air temperature and relative humidity sensors, and capacitance/frequency probes that registered volumetric water content and soil temperature. After pedological characterization of the soil profiles, the soil physico-chemical and hydrophysical properties were determined in laboratory. Over a period of 30 months (July 2019 to December 2021), hydrometeorological and soil environmental data were collected.We assessed differences in soil temperature, moisture availability and soil organic carbon content between native forests, sites under traditional agroforestry and under passive restoration. Forest soils were 12 % cooler;, and soil moisture under forest was 20 % higher than in parcels with silvopastural management. Forest soils had a lower dry bulk density, lower saturated hydraulic conductivity and higher water retention capacity in comparison with the other two management types. In silvopastural systems, a decrease of grazing intensity had a positive effect on soil carbon stocks, that were about 50 % higher than in soils under traditional management. This study shows that avoided loss of tropical forest within an agricultural landscape is a promising strategy to mitigate increasing soil temperatures, agricultural drought, and decline in soil organic carbon content. Continued monitoring of the experimental sites is necessary to corroborate the findings of this investigation at longer temporal scales.

Full-Scale Comparison of Two Envelope Systems for Lightweight Wooden Framing in Cold Climates

Residential homes and apartments’ cooling and heating needs account for 63% of total building energy consumption. Improvements in the properties of building envelopes are among the best ways to reduce their energy consumption. The project’s general objective was to compare the performance of externally insulated and traditional envelopes of light wooden frame buildings at full scale. Two houses were constructed and equipped with relative humidity sensors and temperature probes to assess the physical properties of the building envelope. The first house was built according to the conventional method (insulation between the studs), and the second house was built according to the method with the insulation outside the wall (also known as the perfect wall). The results showed that external insulation effectively mitigates internal condensation risks by relocating dew points to the exterior surface, thereby enhancing structural durability and thermal stability. Thermographic imaging confirmed reduced thermal bridging and improved thermal performance in the externally insulated walls. Overall, this study supports, with a full-scale experiment, the adoption of external insulation as a viable strategy for enhancing energy efficiency, thermal comfort, and durability in residential buildings.

Colorimetric polymer nanofilm-based time-temperature indicators for recording irreversible changes of temperatures in cold chain

Monitoring and recording subzero temperatures and humidity are essential activities for pharmaceutical, food-beverage, and cold storage industries, as product quality can be hampered during storage and transportation due to temperature disruptions. Traditional electronic subzero time-temperature indicators (TTIs) can be energy-inefficient, fragile, non-recyclable, and susceptible to data breaches and cyber-attacks. Hydrophilic colorimetric polymer nanofilms have been developed as colorimetric temperature and relative humidity (RH) sensors, however, the reversible color-changing behavior of these films significantly limited their application as TTIs, as cannot record temperature changes in the past. Herein, the first colorimetric polymer nanofilm-based TTI for recording an irreversible change of temperatures is reported. This device has shown quick color response in temperature ranges from 23 °C to -30 °C in fewer than 50 s. Remarkably, when the device experiences temperature disruption above a certain threshold time (tth), it shows irreversible color-changing behavior in response to the temperature change from subzero (-30 °C and -15 °C) to room temperature or above. It was demonstrated that tth, from minutes to days, of the TTI device can be precisely tuned by adjusting moisture absorber type and weight, interior RH, and storage temperature. Several field tests have demonstrated good versatility and applicability of the device.

Relative humidity sensor based on a nano-film deposited LPFG with hydrophilic polymer composite coating

A novel high-performance relative humidity (RH) sensor is proposed by using a hydrophilic polymer composite film-coated nano-film deposition deposited long-period fiber grating (LPFG). A high refractive index (RI) TiO2 nano-film is deposited on the LPFG as an inner layer. The film thickness is selected to maximize the surrounding refractive index (SRI) sensitivity of the grating when the SRI is approaching the cladding index. A composite film made of polyvinyl alcohol (PVA) and polyethylene glycol (PEG) is used as the an external humidity-to-RI transducer. The concentration and proportion of the PVA/PEG solutions are appropriately chosen to tune the refractive index (RI) RI of the polymer coating to the transition range during the RH varying from 60 % to 95 %. For the LPFG coated with two layers film, the a RH sensitivity up to 3.3 nm/% is achieved. The variation in RH is found to have a linear response. The proposed device offers a reliable alternative for higher RH environment sensing.

A monitoring platform based on electrical impedance and AI techniques to enhance the resilience of the built environment

Structural Health Monitoring (SHM) and early warning systems (EWSs) play a pivotal role in enhancing seismic resilience for both buildings and occupants. This paper introduces a monitoring platform that collects electrical impedance data from scaled concrete beams undergoing load and accelerated degradation tests. Artificial Intelligence (AI) algorithms are employed for predictive analysis, scrutinizing historical impedance data, and forecasting future trends. These algorithms adapt to environmental parameters, becoming valuable tools in data-driven decision-making processes. In particular, the study investigates concrete specimens in different test conditions, utilizing a distributed sensor network based on electrical impedance as well as temperature and relative humidity sensors. Real-time data are transmitted to a cloud infrastructure during accelerated degradation tests (both in water and in chloride-rich solution) and in room conditions. An AI-based forecasting approach using Prophet is proposed, ingesting electrical impedance and temperature data, and tested to predict electrical impedance corresponding to approximately 10 % of the time series balancing responsiveness with predictive accuracy, crucial for effective EWS operations and management requirements. The performance of the tested models is evaluated employing metrics such as Mean Average Error (MAE), Root Mean Square Error (RMSE), Mean Absolute Percentage Error (MAPE), and correlation. The proposed approach surpasses statistical methods and deep learning techniques, reporting a MAPE always lower than 3.20 % and a correlation higher than 81.65 % (in wet-dry cycles in water these values are 0.65 Ω and 91.85 %, respectively). This proves to be a promising step towards transparent SHM, which integrates AI models facilitating self-monitoring and early maintenance prediction, thus enhancing the resilience of the built environment.

The dependence of PurpleAir sensors on particle size distribution and meteorology and their relationship to integrating nephelometers

The National Park Service (NPS) has a need to measure real-time PM2.5 and haze levels across its many park units to inform the public of unhealthy air and track visual degradation of park scenic vistas. To fulfill this need, the NPS deployed PurpleAir monitors (PA) in several park units. PA monitors are inexpensive monitors promoted as particle counters capable of measuring particle size distributions. Each PA is equipped with two Plantower (PMS) particle counters and a temperature, relative humidity (RH) and pressure sensor. In 2022, the NPS initiated a field study to evaluate PA limitations, uncertainties, and how the measurements relate to particle light scattering. The study was conducted in Fort Collins, CO where five nephelometers, particle sizing instruments, semi-continuous ion concentration measurements, and seven PAs were deployed. One nephelometer and two PA's sampled air from an RH controlled chamber with RH ≤ 40%. The PA temperatures were accurate, but the PA RH measurements were systematically underreported RH by ∼28% and it is recommended to scale these data by 1.35. The PMS reported size distributions are shown to be invalid. PA instruments mounted without protection from wind exhibited biases of ±160% for wind speeds exceeding 5 mph. Hourly CH1 data were precise, with errors of ∼100% at low concentrations of 30 #/dl and 15% for concentrations ≥1000 #/dl. Normalizing all PMS sensors to each other reduced the errors to 20% and 8% for low and higher particle concentrations respectively. The reported total particle count, i.e. ≥0.3 μm channel (CH1), was biased low, and the bias decreased linearly with mean scattering diameter (MSD). The ratio of measured ≥0.3 μm to PA CH1 channel increases linearly with MSD. The overall normalized mean deviation (NMD) between the two measurements is 121.8%. The CH1 channel also increases as a function of RH primarily due to the growth of hygroscopic aerosols. On average, the relationship between the atmospheric scattering coefficient (bsp) and the PMS ≥0.3 μm CH1 channel is described by the equation bsp = 0.015 × CH1 for MSD levels less than 0.35 μm. The PMS CH1 response decreases linearly with MSD. The NMD between ambient PMS derived bsp and measured ambient bsp is 27.0%. Recommendations are made for PA network deployment.

Assessing the performance of three experimental methods to investigate air path inside wall assemblies

Air leakages in buildings’ envelopes can lead to an over-consumption of energy and to additional issues such as moisture damage and poor indoor environment. The leakage air flow rate is usually assessed at the building scale using a pressurization test and leakage locations are commonly detected by infrared thermography or smoke. However, little is known about the air paths within wall assemblies, despite the need for detailed experimental data to validate numerical models for the coupled heat, moisture and pollutants transfers. One of the reasons is the difficulty to find adapted and robust experimental methods.In this paper, three experimental laboratory approaches are tested on simple light-wall assembly configurations: the implementation of three-dimensional grids of thermocouples and relative humidity sensors within the wall, and the use of a micro-particle tracer. The results are compared with each other as well as with numerical simulations.It was found that thermocouple grid has limited intrusiveness, good accuracy and response time but results are subject to flow fluctuations. Moreover, heat capacity and conductive transfer within materials complicate the correlation between heat and air transfers. Humidity sensors grid avoids the latter issue but is intrusive, overestimating air dispersion. The micro-particle tracer has the advantage of being non-intrusive and less impacted by flow fluctuations but its ability to track the air flow is strongly linked with the insulation filtration properties and the flow velocity. Overall this method seems to provide the best results but it is also the most complex and time-consuming one.

High-Performance Humidity Sensor Capable for Monitoring Low-Moisture Levels in Energy Industries

Humidity sensors are extensively used in industrial processing and environmental control. There are two categories of humidity sensors: High-humidity (relative humidity) measurement and Low-humidity (moisture) measurement. The proper units for low-humidity measurement are dew point and parts per million in volume (ppmv) or parts per billion in volume (ppbv). It is very important to monitor moisture levels in natural gas during processing and transportation in pipelines. It is of critical importance to monitor moisture levels in lithium-ion battery manufacturing. In these cases, the moisture levels are limited to be less than -50°C dew point or 40 ppmv. The conventional relative humidity sensors cannot be used because of their low sensitivity. The dew point sensors on the market are polymer-based and gamma-phase-alumina based. The polymer-based sensors have low sensitivity, which can only accurately detect moisture levels of -60°C dew point and higher. The gamma-phase alumina sensors suffer from long-term drift due to phase change.We present novel high-performance humidity sensors based on porous alpha-phase alumina and silicon dioxide nanostructures, which are capable of detecting moisture levels as low as -100°C dew point or 13 ppbv with excellent long-term stability. The porous alpha-phase alumina films were produced by anodic-spark-deposition on aluminum substrates in which electric sparks were produced due to high-voltage anodization (>120V). Local high-temperature (about 2200°C) at the points of sparks converted the amorphous alumina into alpha-phase alumina (Sapphire). It provides an extremely stable structure for humidity sensors. Hydrophilic silicon dioxide films deposited by atomic layer deposition was used as sensing materials. The sensors’ sensitivity, temperature coefficient, response speed, and long-term stability were studied in details. The sensors have great promise for monitoring moisture levels in natural gas industry and lithium-ion battery manufacturing.

Fiber optic relative humidity and temperature sensor with the cascaded Vernier effect based on the C-shaped cavity structure

The water-absorbent sensing film, coated on the surface of traditional optical fiber humidity sensors, often suffers from detachment issues. In this paper, we present what we believe to be a new fiber-optic cascaded Fabry-Perot interferometer sensor for detecting relative humidity (RH) and temperature, without the need for sophisticated instrumentation. The sensing structure comprises two sections of single-mode optical fibers and a C-shaped cavity between them. The C-shaped cavity is created by grinding the side of a hollow-core fiber with fiber optic abrasive paper. The Vernier effect arises from the cascaded interaction between the C-shaped cavity filled with ultraviolet optical glue (NOA61) and the subsequent single-mode fiber pigtail. The sensor exhibits a high RH sensitivity of 0.248 nm/%RH (35-95%RH) and an RH resolution of up to 0.08%RH. It also has high-temperature sensitivities of -1.091 nm/°C (25 - 65°C). Furthermore, simultaneous measurement of RH and temperature is achieved by establishing a dual parameter matrix, and the sensor's response time and recovery time for RH and temperature are within 300s. Therefore, this work provides a simple and cost-effective manufacturing process and the proposed RH and temperature sensor features a compact size, strong environmental adaptability, and significant potential for practical applications.

Teor de água de equilíbrio: Uso de sensores de umidade relativa do ar intergranular em silos

ABSTRACT Grains are hygroscopic materials and, during storage, they are subject to exchanging moisture with the environment according to temperature and relative air humidity, making it important to monitor these factors. Digital temperature and relative air humidity sensors appear as an alternative for monitoring the moisture of grain mass inside silos; they are simple to install and use. Digital sensors present outstanding precision in temperature measurements. The objective in this study was to assess the efficiency and applicability of intergranular relative air humidity sensors linked to digital temperature sensors in the thermometry system of silos storing soybeans. Two samples of soybeans were analyzed in the upper, middle and lower thirds of the silo, for grain mass temperature, intergranular relative air humidity, estimated equilibrium moisture content according to collected data, and moisture content of the sampled grains. Moisture contents obtained from sensor measurements using the hygroscopic equilibrium equation and determined using the oven method were compared. The equilibrium moisture content estimated by the data provided by the sensors did not differ by the Tukey test (p ≤ 0.05) from the moisture content determined by the oven method. Digital temperature and relative air humidity sensors have proven to be efficient, as they contribute to estimating the equilibrium moisture content with satisfactory precision.

Water transport and corrosion under insulation: Experimental investigations of drying in mineral wool

Corrosion under insulation (CUI) is a major risk factor for the material integrity of insulated equipment that is encountered i.a. in the chemical process industries. Undetected CUI has been the root cause for major accidents, including loss-of-site, loss-of-life, prolonged plant shutdown, and environmental pollution. Reducing CUI risk via labour-intensive inspection routines is associated with billions of dollars of annual maintenance costs worldwide. CUI is caused by water bypassing a weather shield, migrating through the insulation and contacting the equipment surface. The resulting corrosion rates are very high and the process is hidden from view. Surprisingly, the fundamental mechanisms that drive water migration through insulation have been largely unexplored. In this work we present novel experimental investigations of the evaporation and drying dynamics for a heated vertical pipe with preformed mineral wool insulation and metal cladding. Relative humidity and temperature sensors provide measurements of how the water evaporates and travels through the system during controlled water ingress experiments. The experimental data shows that the drying time of the insulation material is proportional to the amount of water added. We find that the drying process proceeds in two stages, where the length of the first drying regime is directly related to the diffusion-limited evaporation from the bulk liquid water, while the second drying time is influenced also by other factors such as the adsorption and desorption of water on the insulation material fibres. The findings have implications for how time of wetness, used in CUI risk-based inspection methods, can be obtained from humidity sensor data. The new insights into water transport in insulation enables a paradigm shift towards predictive CUI maintenance using humidity spot sensors.

Investigation of the free-base Zr-porphyrin MOFs as relative humidity sensors for an indoor setting

Maintaining optimal relative humidity is paramount for human comfort. Therefore, the utilization of quartz crystal microbalance (QCM) as a relative humidity sensor platform holds significant promise due to its cost-effectiveness and high sensitivity. This study explores the efficacy of three free-base Zr porphyrin metal-organic frameworks (MOFs) - namely MOF-525, MOF-545, and NU-902 - as sensitive materials for QCM-based relative humidity sensors. Our extended experimental findings reveal that these materials exhibit notable sensitivity, particularly within relative humidity ranges of 40–100 %. However, we observe potential irreversible adsorption sites within the MOF-545 framework, hindering its ability to revert to its initial state after prolonged exposure. In light of this observation, we conduct periodic cycling experiments at relative humidity levels of 40–70 % to evaluate the measurement repeatability and feasibility of these sensors for indoor applications. Interestingly, the periodic cycling study demonstrates that MOF-545 shows promising repeatability, positioning it as a strong contender for indoor relative humidity sensing. In contrast, MOF-525 may necessitate extended desorption time, and NU-902 displays diminished sensitivity at low relative humidity levels. Nevertheless, a preliminary treatment of the MOF-545 QCM sensor may be necessary to address irreversible adsorption sites and uphold measurement repeatability, as only reversible adsorption sites are currently accessible. This study underscores the potential of MOF-based QCM sensors for effective relative humidity monitoring in indoor environments, thus facilitating improved comfort and environmental control.

Enhancing hygrothermal monitoring of wet construction with digital twins

The development of digital building twins in the Architecture, Engineering, Construction and Operation (AECO) sector requires the adoption of adequate monitoring strategies for building components. This article presents a proposal and validation of a digital twin-enabled hygrothermal monitoring procedure applied to lightweight concrete using an IoT Arduino-based prototype. This study conducted tests to measure moisture in three types of lightweight concrete for over a year. Temperature and relative humidity sensors were embedded in the lightweight concrete samples. The recorded data were compared to four in-situ surface moisture meters and gravimetric analysis. Although surface moisture meters have different reference scales, the qualitative assessment is similar. The embedded sensors preserved their performance levels during the entire testing period, even those exposed to relative humidity levels near 100 %. The continuous monitoring of temperature and relative humidity in non-structural construction elements, using low-cost sensors embedded during construction, is proven to be viable.

A Review of Impedance Spectroscopy Technique: Applications, Modelling, and Case Study of Relative Humidity Sensors Development

Impedance Spectroscopy (IS) is a general term for the technique referring to small-signal measurements of the linear electrical response of a domain of interest. This method is based on the analysis of the system’s electrical response to yield helpful information about its domain-dependent physicochemical properties (generally, the analysis is carried out in the frequency domain). Nowadays, there are many areas of application where IS can be used to evaluate or enhance the development of emerging products and processes. As a contribution to this field of research, this paper presents relevant theoretical–practical aspects of the interpretation and analysis of the electrical behavior of materials based on IS and IS modelling. The work starts by historically introducing IS and then goes through different domains of application of the technique, such as Materials Science and correlated areas. Afterwards, an introduction to IS usage for constructing equivalent electrical circuits is presented, aiming at modelling the materials’ electrical behavior, followed by examples from the literature that use the two possible circuit development approaches, the series and the parallel association of circuit elements. Lastly, the authors present a case study of their most recent efforts of a circuit model development of relative humidity (RH) sensors based on heterogeneous mixed metal oxide (MMO) nanostructures, used to understand and identify existing contributions to the overall electrical response of the sensors to moisture; in their case, the electrical response of the MMO sensors was modelled with a high level of superposition between the experimental and fitted data, using a parallel combination of circuit elements, which is an unconventional one.

High sensitivity Mach-Zehnder interferometric fiber-optic humidity sensor based on multimode interference enhancement

Variations in humidity can significantly disrupt the proper functioning of precision instruments and meters across various industrial, medical, and scientific applications, thereby necessitating real-time humidity monitoring. Here, a high sensitivity fiber-optic relative humidity (RH) sensor based on Mach-Zender interferometer (MZI) is proposed and demonstrated. The humidity sensor consists of single-mode fiber (SMF), peanut structure (PS), and multi-mode fiber (MMF). A multi-mode interference (MMI) enhanced fiber-optic MZI was constructed using a SMF-PS-SMF-MMF-SMF-PS-SMF cascade structure to detect small changes in the refractive index (RI) and expansion force of gelatin film caused by humidity variation. PS, as a beam splitter and combiner of MZI, provides the possibility for simple, economical, and efficient manufacturing of the fiber-based part of humidity sensors. The addition of MMF will excite higher-order core and cladding modes, laying the foundation for achieving high-sensitivity fiber-optic humidity sensors. The fiber-optic RH sensor prototype demonstrate a high sensitivity of about 0.633 nm/% RH in the RH range of 50 % RH to 80 % RH, and a low temperature cross-sensitivity of 0.165 % RH/°C. This sensor has the advantages of safe and easy manufacturing process, low manufacturing cost, and high sensitivity, providing continuous high accuracy humidity measurement results for humidity sensing in different fields.

High sensitivity humidity sensor based on the U-shaped tapered no-core fiber coated with MXene.

Ti3C2Tx MXene is an emerging two-dimensional material that has good potential in relative humidity (RH) measurement because of its unique layer structure, strong hydrophilic nature, and large specific surface area. Here, a high-performance RH sensor integrating Ti3C2TX MXene nanosheets and U-shaped tapered no-core fiber (UTNCF) is proposed. The sensing principle is based on mode interference. The change of ambient RH leads to the change of the refractive index (RI) of Ti3C2Tx MXene, which eventually leads to the shift of the transmission spectrum of the sensing probe. The average sensitivity is 1.11 nm/%RH in the RH range of 45% to 80%, and the response time is 25 ms. The proposed micro-nano fiber RH sensor has the advantages of high sensitivity, fast response, good repeatability, and stability. In addition, the proposed sensor has a broad application prospect in human respiratory monitoring, industrial and agricultural production, and environmental monitoring.

Fabrication and Implementation of PVA Thin Film-based Relative Humidity Sensor

This paper presents the fabrication of polyvinyl alcohol (PVA)-based relative humidity sensors using copper interdigitated electrodes (IDEs). These sensors were fabricated by patterning copper IDEs onto FR4 fiberglass substrates and coating them with a thin film of PVA. By varying the number of IDE electrode pairs and the concentration of PVA, the sensors exhibited different resistance values, demonstrating an inverse relationship with the number of IDE electrodes and PVA concentration. Additionally, the thickness of the sensing layer, controlled by spin coating speed, affected the resistance of the sensors. The fabricated sensors achieved a humidity measurement range from 50% to 99% RH based on resistance changes. Furthermore, an interface circuit was designed to integrate the fabricated sensors with an ESP8266 microcontroller, achieving accurate measurements above 74% RH.

Microwave humidity sensor based on shorted split ring resonator with interdigital capacitance and α-Al2O3 nanoflakes/chitosan sensitive film for respiration monitoring

A microwave relative humidity (RH) sensor based on shorted split ring resonator with interdigital capacitance (SSRR-IDC) as a test platform and α-Al2O3 nanoflakes/chitosan (CS) composite as sensitive film is proposed for the detection of human respiration. The SSRR-IDC was designed by optimizing the finger spacing and width of the IDC. Electromagnetic simulation of the designed SSRR-IDC confirms a strong electric field distribution and high dielectric sensitivity in the IDC region. The porous α-Al2O3 nanoflakes/CS composite was coated onto the IDC region of the SSRR to obtain a microwave humidity sensor. Abundant hydrophilic groups and large specific surface area of the α-Al2O3 nanoflakes/CS contribute to the adsorption of water molecules. The proposed microwave humidity sensor shows excellent sensitivity in wide RH range, especially at high humidity condition. The working principle and the sensitivity enhancement mechanism of the proposed microwave sensor were explained by combining a circuit model analysis and the dielectric property and surface microstructure changes of the sensitive materials. Furthermore, the microwave sensor also demonstrates short response/recover time and outstanding repeatability, long-term stability, temperature stability, and selectivity. Additionally, the prepared microwave humidity sensor successfully records respiratory data from different volunteers. Changes in respiratory depth before and after drinking water of the volunteers can also be distinguished by this sensor.