Relative humidity sensor with optical fiber Bragg gratings

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

Characterization of an ORMOCER®-coated FBG sensor for relative humidity sensing

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.

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)

A relative humidity (RH) sensor based on a seven-core fiber (SCF) coated with graphene oxide (GO) is proposed. The sensing structure consists of a part of the SCF sandwiched between two parts of no-core fibers (NCFs). The SCF was corroded by hydrofluoric acid until the six outer cores were exposed to air and then coated with a layer of GO. The two NCFs play splitting and coupling roles because of the core diameter mismatch. The center core of the SCF acts as the reference arm, and the outer six cores of the SCF act as the sensing arm to construct a Mach–Zehnder interferometer. When GO absorbs water molecules, its refractive index changes. Consequently, the phase difference between the center core and outer core modes varies, and the resonant dip moves. The experimental results indicate that the RH sensitivity can reach 0.165 nm/%RH in the range of 30% to 100%, and the corresponding linear fitting coefficient is 99.9%.

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.

This study proposes a novel optical fiber Bragg grating (FBG)-based load cell sensor using 3-D printing technology. It explores the impact of the infill density and pattern on the FBG’s temperature and mechanical strain sensitivity. Two FBGs were used, one for strain measurement, FBG L, and the other for temperature compensation, FBG T. Conducted measurements show that the strain-sensitive FBG L and strain-free FBG T exhibited a similar average temperature sensitivity value of around 10.5 pm/°C with a good linearity value of 99.92%. This will also allow the unstrained FBG T to effectively compensate for FBG L’s temperature changes during load measurement. Under varied load conditions, load cell sensors with lower infill density, such as 20%, displayed more significant average wavelength shift and increased strain sensitivities compared to those with higher infill density, such as 80%, due to having more voids and hollow sections within its actuator. This facilitates easier deformation, thus inducing more significant stretching on FBG L. Regarding the infill pattern, the gyroid pattern outperformed the tri-hexagon in both average wavelength shift and strain responses, showing its suitability for precise measurements under diverse loads. Therefore, this work demonstrates the critical role of the infill density and pattern in the performance of an efficient load cell sensor using FBGs.

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.

Relative humidity (RH) sensors have been fabricated with polyaniline-encased multiwall carbon nanotube (MWCNT) matrix. MWCNT matrices were grown on oxidized Si substrates by catalytic chemical vapour deposition technique. The MWCNT matrices were made hydrophilic by acid treatment. The sensing properties of MWCNTs were improved by encasing the acid-treated MWCNT matrix with PANI. PANI was synthesized via in situ chemical oxidation technique using three acid dopants, named as 5-sulfosalicylic acid, 4-nitrobenzoic acid and hydrochloric acid (HCl). A growth time variation of the PANI was also performed from 1.5 to 2.5 h. The time-varying sensor resistance was recorded with RH change, and RH sensing properties of the corresponding sensors were investigated with the measured data. Doping variation exhibited that CNT–PANI sensor, fabricated with HCl dopant, showed the highest response (42.6% at 90% RH). Synthesis time variation of PANI from 1.5 to 2.5 h exhibited that with an increase in synthesis time, the sensor responsivities degraded from 42.6 to 19.5% at 90% RH. Excellent repeatability, linear response behaviour (R2 = 0.992), low hysteresis loss (± 2.68%) were exhibited by the fabricated RH sensing devices. A facile transfer technology was developed in order to transfer the MWCNT on PET substrate for fabrication of flexible RH sensor. A practical application on human respiration monitoring was demonstrated with the wearable and flexible RH sensors. The humidity sensing mechanism and electron transfer process were schematically explained with the aid of energy band diagram.

A long-period grating (LPG) fiber optic sensor has been developed for monitoring the relative humidity levels and toxic chemicals, especially the chemical warfare agents. The principle of operation of this sensor is based on monitoring the refractive index changes exhibited by the reactive coating applied to the surface of the LPG region in response to analytes. Specific interaction of the analyte with the thin film polymer coating produces as the output a wavelength shift that can be correlated with the concentration of the analyte. Thin polymer coating for relative humidity sensor is made of carboxymethylcellulose (CMC) covalently bound to the surface of the fiber. Coating for chemical warfare agent detection employs metal nanoclusters imbedded in polyethylenimine (PEI) for specific reaction. The relative humidity level can be determined from 0% to 95% and the level of toxic chemicals can be determined is at least on the scale of 1 ppm. This small-size and low-cost LPG fiber optic sensor exhibited high sensitivity, rapid response, repeatability and durability. The goal of developing relative humidity sensor is to produce a fiber optic sensor-based health monitoring system for building, while the chemical sensor has found its application in point detection network for chemical warfare agent monitoring.

A high-performance fiber-optic relative humidity (RH) sensor was prepared by layer-by-layer self-assembly of polycation, poly (4-vinylpyridinium chloride), and polyanion, sodium alginate, on the surface of a thin-core fiber modal interferometer. The process of layer-by-layer self-assembly was monitored by UV-vis spectroscopy and surface profilometer. The morphology of the fabricated multilayer film was characterized by atomic force microscopy and field emission scanning electron microscopy. A fast, reversible, and linear response to RH with a high resolution of 0.1 nm/1% RH has been experimentally demonstrated by the proposed fiber-optic sensor.

We report the fabrication and characterization of a polycarbonate (PC) microstructured polymer optical fiber (mPOF) Bragg grating (FBG) humidity sensor that can operate beyond 100°C. The PC preform, from which the fiber was drawn, was produced using an improved casting approach to reduce the attenuation of the fiber. The fiber loss was found reduced by a factor of two compared to the latest reported PC mPOF [20] , holding the low loss record in PC based fibers. PC mPOFBG was characterized to humidity and temperature, and a relative humidity (RH) sensitivity of 7.31± 0.13 pm/% RH in the range 10–90% RH at 100°C and a temperature sensitivity of 25.86±0.63 pm/°C in the range 20–100 °C at 90% RH were measured.

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

Moisture is an essential parameter in the behaviour of capillary-porous construction materials such as timber and concrete. It affects liquid and gas transport phenomena, chemical and biological degradation processes, mechanical properties and, in the case of concrete, hydration. From a scientific point of view, moisture monitoring is essential in order to improve the understanding of material behaviour. Moisture measurements may increase the predictive accuracy of established material behaviour models. In practice, the increased knowledge of material behaviour improves structural maintenance planning. The main objective of this work is to propose a measurement method for non-destructive, in-depth moisture monitoring of construction materials. In this context, an intrinsic point and an averaging fibre optic relative humidity sensor have been developed and tested. The sensors are based on optical fibres that are coated with a hygroscopically swelling transducer polymer. When wet, the swelling of the coating strains the fibre (analogy with the hair hygrometer). The induced strain is assessed with conventional fibre optic strain sensing techniques such as fibre Bragg gratings and Michelson interferometry. Theoretical and experimental studies lead to a detailed understanding of the influence of humidity and temperature on the steady and transient state sensor behaviour. The sensors have an accurate, linear, reversible and reproducible response to relative humidity between 5 and 95 %RH and between 13 and 60 °C, at least. The sensor response time is in the order of 20 minutes. However, when packaged, the sensor responds slower. The temperature cross-sensitivity of the fibre Bragg grating sensor may be compensated with an additional non-hygroscopic grating, while the Michelson interferometric sensor provides an auto-compensation of temperature effects. Tests in mortar and timber samples demonstrate that the sensors preserve their sensing ability when embedded. These tests have also clarified the multiplexing potential of fibre Bragg gratings for forming a multi-point RH sensor. Multi-point sensors are particularly useful for profile measurements.

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.