Reversible and Fast Responsive Optical Fiber Relative Humidity Sensor Based on Polyelectrolyte Self-Assembly Multilayer Film

A high-sensitivity optical fiber relative humidity sensor based on microsphere WGM resonator

This paper reports a high-performance humidity sensor made using a novel cellulose nanofiber (CNF)–silver nanoparticle (AgNP) sensing material. The interdigital electrode pattern was printed via reverse-offset printing using Ag nano-ink, and the sensing layer on the printed interdigitated electrode (IDE) was formed by depositing the CNF-AgNP composite via inkjet printing. The structure and morphology of the CNF-AgNP layer are characterized using ultraviolet–visible spectroscopy, an X-ray diffractometer, field emission scanning electron microscopy, energy-dispersive X-ray analysis, and transmission electron microscopy. The humidity-sensing performance of the prepared sensors is evaluated by measuring the impedance changes under the relative humidity variation between 10 and 90% relative humidity. The CNF-AgNP sensor exhibited very sensitive and fast humidity-sensing responses compared to the CNF sensor. The electrode distance effect and the response and recovery times are investigated. The enhanced humidity-sensing performance is reflected in the increased conductivity of the Ag nanoparticles and the adsorption of free water molecules associated with the porous characteristics of the CNF layer. The CNF-AgNP composite enables the development of highly sensitive, fast-responding, reproducible, flexible, and inexpensive humidity sensors.

A novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings is presented. Tests in a controlled environment indicate that the sensor has a linear, reversible, and accurate response behavior at 10-90% RH and at 13-60 degrees C. The RH and temperature sensitivities were measured as a function of coating thickness, and the thermal and hygroscopic expansion coefficients of the polyimide coating were determined.

Effect of interdigital electrode gap on the performance of SnO2-modified MoS2 capacitive humidity sensor

We propose a novel relative humidity (RH) sensor based on a hollow core-photonic bandgap fiber (HC-PBF) using a wavelength scan and a reference technique in this paper. The wavelength scan of water vapor absorption peak around 1368.59 nm is realized by injecting sawtooth modulated current to a DFB laser diode. The humidity level is determined by the voltage difference (ΔV) between the reference signal and absorption signal at the peak of water vapor absorption. The reference signal is used not only as a zero absorption baseline but also to reduce the influence of intensity noise from DFB laser source. We have experimentally implemented the wavelength scanning modulation scheme with a HC-PBF sensing head of 5 cm and a small gap between SMF and HC-PBF to allow air diffusion into air holes of HC-PBF. High humidity sensitivity of 3.02 mV/1% RH over the range from 0 to 90% RH is achieved without the use of any coating material for humidity sensing.

Water vapor adsorption is the least studied form of non-rainfall water inputs but is likely the most common one in arid and hyper-arid areas. It is determined by the magnitude of the downward gradient in water vapor pressure between the atmosphere and the soil; the surface area of the adsorbing soil; and the penetration depth of water vapor adsorption. Water vapor adsorption was measured using micro-lysimeters and profiles of relative humidity (RH) sensors in both loess and sand in the Negev desert, Israel, over the summers of 2021 and 2022. The RH sensor array allowed measurement of detailed changes in water content in the soil profile and provided an unprecedented insight into processes governing water vapor adsorption dynamics under arid conditions in-situ. The RH sensors significantly underestimated total water vapor adsorption, indicating that a finer array is needed to capture the full process. However, even with the current array, extremely small changes in water content were captured. With these measurements we explored the three main factors contributing to water vapor adsorption. The onset of a downward vapor pressure gradient coincided with the arrival of the sea breeze, indicating that the sea breeze is the primary source for water vapor adsorption in the uppermost soil layer. Water vapor adsorption was higher in loess than in sand, due to its finer texture and larger surface area. The most important finding of this research is that the dominant mechanism for water vapor flow under natural arid conditions (relative humidity in the soil (RHs)

We report a detailed study of a miniature optical relative humidity (RH) sensor based on a polymer-infiltrated photonic crystal fiber interferometer. The sensor shows a high sensitivity to RH variations with a change in its reflected power of about 12 dB for a humidity change of 84% RH. The sensor has the advantages of a very compact length of 1 mm, and an end-type probe configuration makes it suitable for monitoring humidity in microenvironments. The repeatability, long-term stability, measurement accuracy, and temperature dependence of the sensor are studied in this paper. The observed low thermal sensitivity of the sensor suggests that temperature compensation may not be needed if it is used in normal environments. The response time of the sensor is found to be 400 ms for a change in RH of ~ 30% RH. The fast response time suggests that the sensor can potentially be used as a human breath rate monitor in a clinical situation.

A facile nanocasting synthesis of mesoporous Ag-doped SnO2 nanostructures with enhanced humidity sensing performance

This study presents the development of a high-performance resistive humidity sensor based on a cetyltrimethylammonium bromide (CTAB)-assisted tin oxide (SnO₂) nanostructured thin film integrated with a Poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) (PEDOT: PSS)/SnO₂ heterojunction. The sensor design incorporates CTAB at varying weight percentages (0%, 6%, 11%, 16%, and 20%) during the hydrothermal synthesis of SnO₂ to regulate crystal growth, morphology, and surface area. The sample with 20 wt% CTAB (SnO-5) exhibited a flower-like stacked nanostructure, confirmed via field emission scanning electron microscopy (FESEM), which significantly enhanced water molecule adsorption and charge transport pathways. X-ray diffraction (XRD) analysis confirmed the tetragonal rutile phase of SnO₂ with decreasing crystallite size from 12.2 nm (nm) to 4.8 nm as CTAB concentration increased. The incorporation of PEDOT: PSS, a p-type conducting polymer, onto the SnO₂ layer via spin coating formed a p–n heterojunction, which improved charge separation and reduced recombination, thereby enhancing electrical conductivity and sensor performance. Electrochemical impedance spectroscopy (EIS) and current-voltage (J-V) measurements demonstrated that SnO-5 exhibited a low internal resistance (1.1 kilo ohms (kΩ)), a minimal cut-in voltage (0.071 Volts (V)), and a high current response (2.645 micro Amps.(µA)), indicating efficient carrier transport. The optimized SnO-5 sensor achieved a high sensitivity of 85.7%, a rapid response time of 14 s (s), and a quick recovery time of 7 s, with low hysteresis (1.60%) across a broad humidity range (5–97% Relative Humidity (RH)), outperforming several existing humidity sensing platforms. The synergistic effects of CTAB-induced nanostructuring and heterojunction engineering played a pivotal role in improving moisture interaction, charge mobility, and structural stability. Furthermore, to validate real-time application feasibility, machine learning (ML) algorithms were implemented to model and predict sensor behavior. Among the tested models, Random Forest (RF) Regression achieved the highest predictive accuracy (R² = 0.99), confirming the sensor’s robustness and reproducibility in dynamic environments. The proposed sensor’s outstanding performance, in combination with ML-enhanced evaluation, positions it as a promising candidate for next-generation humidity monitoring systems in industrial, environmental, and biomedical applications, including respiratory diagnostics and non-invasive health monitoring.

The presence of water can have adverse effects on the performance of mechanical and electrical hardware. Standing or condensed water can cause shorting or grounding issues in the short term and can accelerate corrosion in the long term. High humidity can also accelerate corrosion as well as cause swelling of electrical components, seals, or composite materials. Water exposure data can be used to actively intervene prior to degradation(e.g. to fix a water leak) or to calculate the remaining life of a system that has been exposed to water or high humidity. The U.S. Army has recognized the importance of environmental monitoring and failure prediction in weapon systems to ensure readiness, to enhance safety, and toimprove weapon performance. Since 2000, Picatinny Arsenal has sponsored the development of specialized environmental monitoring sensors with on-board diagnostics in various new weapon/ammunition systems. In this article we present an effort at the Pacific Northwest National Laboratory to develop an asset health monitor to assess water concentration inside the storage container of an artillery shell. Desiccant is used in the container to maintain the water vapor concentration below 5000 parts per million; a higher water concentration would indicate container leakage or the need to replace the desiccant. Various types of water sensors, including standing water indicators, corrosion indicators, and humidity sensors, were identified and categorized according to type of water exposure, accuracy, and response time. A commercially available relative humidity sensor was identified as a cost effective approach to measuring water vapor concentration inside the container. Relative humidity can be converted to waterconcentration using the water vapor saturation pressure at ambient temperature. Using diurnal temperature simulations, it was determined that the accuracy of the relative humidity sensor would be unsatisfactory in high temperature/low humidity situations where a small change in relative humidity results is a large change in water vapor concentration. To overcome this obstacle, a strategic sampling technique was developed to increase the accuracy of the water concentration measurement that is based on relative humidity sensor data. The relative humidity sensorwas integrated into the Remote Readiness Asset Prognostic/Diagnostic System (RRAPDS), which is an asset health monitor developed by PNNL to measure environmental conditions in munition containers. RRAPDS will be secured to the container with a probe that extends into the container to monitor internal temperature and humidity. RRAPDS includes a microcontroller to process environmental data and execute diagnostic routines. Thispaper will describe the approach that was developed to estimate water concentration using relative humidity sensor data and describe the sampling technique used to maximize the accuracy of the water vapor concentration assessment.

Humidity sensors based on flexible sensitive nanomaterials are very attractive in noncontact healthcare monitoring. However, the existing humidity sensors have some shortcomings such as limited sensitivity, narrow relative humidity (RH) range, and a complex process. Herein, we show that a tin sulphide (SnS) nanoflakes-based sensor presents high humidity sensing behaviour both in rigid and flexible substrate. The sensing mechanism based on the Schottky nature of a SnS-metal contact endows the as-fabricated sensor with a high response of 2491000% towards a wide RH range from 3% RH to 99% RH. The response and recovery time of the sensor are 6 s and 4 s, respectively. Besides, the flexible SnS nanoflakes-based humidity sensor with a polyimide substrate can be well attached to the skin and exhibits stable humidity sensing performance in the natural flat state and under bending loading. Moreover, the first-principles analysis is performed to prove the high specificity of SnS to the moisture (H2O) in the air. Benefiting from its promising advantages, we explore some application of the SnS nanoflakes-based sensors in detection of breathing patterns and non-contact finger tips sensing behaviour. The sensor can monitor the respiration pattern of a human being accurately, and recognize the movement of the fingertip speedily. This novel humidity sensor shows great promising application in physiological and physical monitoring, portable diagnosis system, and noncontact interface localization.

A method for standardizing the manufacturing process of integrated temperature and humidity sensor based on fiber Bragg grating

This study developed a humidity sensor by synthesizing ZrO2 films on porous silicon (PSi) via spray pyrolysis at two substrate temperatures (300 °C and 400 °C). This is crucial as the performance of humidity sensors is strongly dependent on the microstructure and morphology of the sensing film. XRD and FESEM analysis confirmed the cubic crystal nanostructure of the synthesized ZrO2. The FTIR spectra show the presence of Si–O–Si, Si–OH and Zr–O sites at the surface of the ZrO2/PSi composite film. The ZrO2 sample prepared at 300 °C exhibited larger specific surface area and smaller particle size. Thus, the humidity sensor fabricated based on the sample prepared at 300 °C shows higher sensitivity. The humidity sensing of ZrO2 device was measured at room temperature over relative humidity (RH) of 25%,35% and 75%. The results showed that the UV irradiation strong influences the response characteristics of the sensor because it becomes a source of heat stimulation, especially at low humidity level. The fabricated ZrO2 sensor exhibited highest sensitivity of 492.66% observed when the humidity is 75% RH + UV irradiation, with faster response-recovery time in the range of (40–100) µs compared to contemporary sensors. These results confirm the applicability of ZrO2 nanoparticles on PSi as a potential material for the fabrication of simple and high performance humidity sensors.

We propose and demonstrate a relative humidity (RH) sensor based on a polarimetric fiber laser and beat frequency (BF) demodulation. The laser cavity of the proposed fiber laser sensor consists of a pair of fiber Bragg gratings (FBGs) and a high-birefringence erbium-doped fiber (Hi-Bi EDF) coated by a thin polyvinyl alcohol (PVA) film layer. This thin-film layer responds to humidity changes in the environment by swelling or contracting, inducing stress effects on the EDF. The stress effect, in turn, results in changes to the polarization mode beat frequency (PMBF) detected by the photodetector (PD) in the sensing system. The inherent high-birefringence property of the Hi-Bi EDF significantly enhances the sensitivity of the PMBF to humidity-induced stress. Humidity sensing experiments are conducted utilizing the proposed sensor. Experiment results show that the sensor has a high average humidity sensitivity of 13.9 MHz/%RH within the humidity range of 20.4%RH–36.0%RH. A stability test is also conducted, demonstrating that the proposed sensor achieves an accuracy of 0.65%RH.

A microfiber interferometer based on a single-mode tapered-no-core single-mode (STNCS) fiber structure coated with a thin layer of graphene oxide (GO) was proposed and demonstrated as a relative humidity (RH) sensor. The STNCS fiber structure has a strong evanescent field, which can sensitively response to changes in the surrounding refractive index. The moisture-sensitive material GO was used to form the film on the no-core fiber by a dip impregnation method. The effects of GO concentration and the tapered waist diameter on RH sensitivity were studied experimentally. The experimental results showed that a thinner waist diameter provides higher RH sensitivity. When the concentration of GO solution is 0.01 mg/mL, the STNCS fiber structure with a waist diameter of 2.9 μm performs better in RH sensing; a high sensitivity of 0.461 nm/%RH in the range of 40-98% was achieved. The sensor has good stability with wavelength fluctuations of ±0.009 nm and ±0.008 nm over 60 mins at RHs of 50% and 80%, respectively. The proposed fiber optic humidity sensor has a high sensitivity over a wide measurement range and offers good stability showing promising potential application in the field of RH sensing.