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3746 KiB

Open AccessArticle

Fiber Optic Thermo-Hygrometers for Soil Moisture Monitoring

by Marco Leone, Sofia Principe, Marco Consales, Roberto Parente, Armando Laudati, Stefano Caliro, Antonello Cutolo and Andrea Cusano

Sensors 2017, 17(6), 1451; https://doi.org/10.3390/s17061451 - 20 Jun 2017

Cited by 25 | Viewed by 9632

Abstract

This work deals with the fabrication, prototyping, and experimental validation of a fiber optic thermo-hygrometer-based soil moisture sensor, useful for rainfall-induced landslide prevention applications. In particular, we recently proposed a new generation of fiber Bragg grating (FBGs)-based soil moisture sensors for irrigation purposes. [...] Read more.

This work deals with the fabrication, prototyping, and experimental validation of a fiber optic thermo-hygrometer-based soil moisture sensor, useful for rainfall-induced landslide prevention applications. In particular, we recently proposed a new generation of fiber Bragg grating (FBGs)-based soil moisture sensors for irrigation purposes. This device was realized by integrating, inside a customized aluminum protection package, a FBG thermo-hygrometer with a polymer micro-porous membrane. Here, we first verify the limitations, in terms of the volumetric water content (VWC) measuring range, of this first version of the soil moisture sensor for its exploitation in landslide prevention applications. Successively, we present the development, prototyping, and experimental validation of a novel, optimized version of a soil VWC sensor, still based on a FBG thermo-hygrometer, but able to reliably monitor, continuously and in real-time, VWC values up to 37% when buried in the soil. Full article

(This article belongs to the Special Issue Humidity Sensors)

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1896 KiB

Open AccessArticle

Development of High Sensitivity Humidity Sensor Based on Gray TiO₂/SrTiO₃ Composite

by Min Zhang, Shunhang Wei, Wei Ren and Rong Wu

Sensors 2017, 17(6), 1310; https://doi.org/10.3390/s17061310 - 7 Jun 2017

Cited by 19 | Viewed by 4685

Abstract

A gray TiO₂/SrTiO₃ composite nanocrystalline sensor with narrow band-gap was successfully prepared through a facile wet chemical method. The precursor was calcined in N₂ flow under atmospheric pressure and thereafter, a humidity sensor based on the composite was fabricated. [...] Read more.

A gray TiO₂/SrTiO₃ composite nanocrystalline sensor with narrow band-gap was successfully prepared through a facile wet chemical method. The precursor was calcined in N₂ flow under atmospheric pressure and thereafter, a humidity sensor based on the composite was fabricated. The sensor showed high resistive sensitivity and varied by more than four orders of magnitude with an increase in relative humidity (RH) from 11% to 95%. The response and recovery time were about 3.1 s and 76 s, respectively with maximum hysteresis at 1% RH. In comparison with pure SrTiO₃ and black TiO₂, the gray composite based device exhibits a higher sensitivity. These results demonstrate the potential of gray TiO₂/SrTiO₃ for humidity sensing applications. Full article

(This article belongs to the Special Issue Humidity Sensors)

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3180 KiB

Open AccessArticle

Thermochemical Humidity Detection in Harsh or Non-Steady Environments

by Devon Bridgeman, Francis Tsow, Xiaojun Xian, Qinan Chang, Yongming Liu and Erica Forzani

Sensors 2017, 17(6), 1196; https://doi.org/10.3390/s17061196 - 24 May 2017

Cited by 3 | Viewed by 4546

Abstract

We present a new method of chemical quantification utilizing thermal analysis for the detection of relative humidity. By measuring the temperature change of a hydrophilically-modified temperature sensing element vs. a hydrophobically-modified reference element, the total heat from chemical interactions in the sensing element [...] Read more.

We present a new method of chemical quantification utilizing thermal analysis for the detection of relative humidity. By measuring the temperature change of a hydrophilically-modified temperature sensing element vs. a hydrophobically-modified reference element, the total heat from chemical interactions in the sensing element can be measured and used to calculate a change in relative humidity. We have probed the concept by assuming constant temperature streams, and having constant reference humidity (~0% in this case). The concept has been probed with the two methods presented here: (1) a thermistor-based method and (2) a thermographic method. For the first method, a hydrophilically-modified thermistor was used, and a detection range of 0–75% relative humidity was demonstrated. For the second method, a hydrophilically-modified disposable surface (sensing element) and thermal camera were used, and thermal signatures for different relative humidity were demonstrated. These new methods offer opportunities in either chemically harsh environments or in rapidly changing environments. For sensing humidity in a chemically harsh environment, a hydrophilically-modified thermistor can provide a sensing method, eliminating the exposure of metallic contacts, which can be easily corroded by the environment. On the other hand, the thermographic method can be applied with a disposable non-contact sensing element, which is a low-cost upkeep option in environments where damage or fouling is inevitable. In addition, for environments that are rapidly changing, the thermographic method could potentially provide a very rapid humidity measurement as the chemical interactions are rapid and their changes are easily quantified. Full article

(This article belongs to the Special Issue Humidity Sensors)

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3860 KiB

Open AccessArticle

Autonomous Sensors for Measuring Continuously the Moisture and Salinity of a Porous Medium

by Xavier Chavanne and Jean-Pierre Frangi

Sensors 2017, 17(5), 1094; https://doi.org/10.3390/s17051094 - 11 May 2017

Cited by 14 | Viewed by 4572

Abstract

The article describes a new field sensor to monitor continuously in situ moisture and salinity of a porous medium via measurements of its dielectric permittivity, conductivity and temperature. It intends to overcome difficulties and biases encountered with sensors based on the same sensitivity [...] Read more.

The article describes a new field sensor to monitor continuously in situ moisture and salinity of a porous medium via measurements of its dielectric permittivity, conductivity and temperature. It intends to overcome difficulties and biases encountered with sensors based on the same sensitivity principle. Permittivity and conductivity are determined simultaneously by a self-balanced bridge, which measures directly the admittance of sensor electrodes in medium. All electric biases are reduced and their residuals taken into account by a physical model of the instrument, calibrated against reference fluids. Geometry electrode is optimized to obtain a well representative sample of the medium. The sensor also permits acquiring a large amount of data at high frequency (six points every hour, and even more) and to access it rapidly, even in real time, owing to autonomy capabilities and wireless communication. Ongoing developments intend to simplify and standardize present sensors. Results of field trials of prototypes in different environments are presented. Full article

(This article belongs to the Special Issue Humidity Sensors)

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10223 KiB

Open AccessArticle

A Capacitive Humidity Sensor Based on an Electrospun PVDF/Graphene Membrane

by Daniel Hernández-Rivera, Grissel Rodríguez-Roldán, Rodrigo Mora-Martínez and Ernesto Suaste-Gómez

Sensors 2017, 17(5), 1009; https://doi.org/10.3390/s17051009 - 3 May 2017

Cited by 73 | Viewed by 10250

Abstract

Humidity sensors have been widely used in areas such as agriculture, environmental conservation, medicine, instrumentation and climatology. Hydrophobicity is one of the important factors in capacitive humidity sensors: recent research has shown that the inclusion of graphene (G) in polyvinylidene fluoride (PVDF) improves [...] Read more.

Humidity sensors have been widely used in areas such as agriculture, environmental conservation, medicine, instrumentation and climatology. Hydrophobicity is one of the important factors in capacitive humidity sensors: recent research has shown that the inclusion of graphene (G) in polyvinylidene fluoride (PVDF) improves its hydrophobicity. In this context, a methodology to fabricate electrospun membranes of PVDF blended with G was developed in order to improve the PVDF properties allowing the use of PVDF/G membrane as a capacitive humidity sensor. Micrographs of membranes were obtained by scanning electron microscopy to analyze the morphology of the fabricated samples. Subsequently, the capacitive response of the membrane, which showed an almost linear and directly proportional response to humidity, was tested. Results showed that the response time of PVDF/G membrane was faster than that of a commercial DHT11 sensor. In summary, PVDF/G membranes exhibit interesting properties as humidity sensors. Full article

(This article belongs to the Special Issue Humidity Sensors)

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12942 KiB

Open AccessArticle

Humidity Sensor Based on Bragg Gratings Developed on the End Facet of an Optical Fiber by Sputtering of One Single Material

by Joaquin Ascorbe, Jesus M. Corres, Francisco J. Arregui and Ignacio R. Matias

Sensors 2017, 17(5), 991; https://doi.org/10.3390/s17050991 - 29 Apr 2017

Cited by 18 | Viewed by 4824

Abstract

The refractive index of sputtered indium oxide nanocoatings has been altered just by changing the sputtering parameters, such as pressure. These induced changes have been exploited for the generation of a grating on the end facet of an optical fiber towards the development [...] Read more.

The refractive index of sputtered indium oxide nanocoatings has been altered just by changing the sputtering parameters, such as pressure. These induced changes have been exploited for the generation of a grating on the end facet of an optical fiber towards the development of wavelength-modulated optical fiber humidity sensors. A theoretical analysis has also been performed in order to study the different parameters involved in the fabrication of this optical structure and how they would affect the sensitivity of these devices. Experimental and theoretical results are in good agreement. A sensitivity of 150 pm/%RH was obtained for relative humidity changes from 20% to 60%. This kind of humidity sensors shows a maximum hysteresis of 1.3% relative humidity. Full article

(This article belongs to the Special Issue Humidity Sensors)

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7528 KiB

Open AccessArticle

A Cost-Effective Relative Humidity Sensor Based on Side Coupling Induction Technology

by Yingzi Zhang, Yulong Hou, Wenyi Liu, Huixin Zhang, Yanjun Zhang, Zhidong Zhang, Jing Guo, Jia Liu, Liang Zhang and Qiu-lin Tan

Sensors 2017, 17(5), 944; https://doi.org/10.3390/s17050944 - 25 Apr 2017

Cited by 38 | Viewed by 5440

Abstract

A intensity-modulated optical fiber relative humidity (RH) sensor based on the side coupling induction technology (SCIT) is presented and experimentally demonstrated. The agarose gel and the twisted macro-bend coupling structure are first combined for RH sensing applications. The refractive index (RI) of the [...] Read more.

A intensity-modulated optical fiber relative humidity (RH) sensor based on the side coupling induction technology (SCIT) is presented and experimentally demonstrated. The agarose gel and the twisted macro-bend coupling structure are first combined for RH sensing applications. The refractive index (RI) of the agarose gel increases with the increase of the RH and is in linear proportion from 20 to 80%RH. The side coupling power, which changes directly with the RI of the agarose gel, can strip the source noise from the sensor signal and improve the signal to noise ratio substantially. The experiment results show that the sensitivity of the proposed sensor increases while the bend radius decreases. When the bend radius is 8 mm, the sensor has a linear response from 40% to 80% RH with the sensitivity of 4.23 nW/% and the limit of detection of 0.70%. A higher sensitivity of 12.49 nW/% is achieved when RH raises from 80% to 90% and the limit of detection decreases to 0.55%. Furthermore, the proposed sensor is a low-cost solution, offering advantages of good reversibility, fast response time, and compensable temperature dependence. Full article

(This article belongs to the Special Issue Humidity Sensors)

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1239 KiB

Open AccessArticle

Performance Assessment of a Humidity Measurement System and Its Use to Evaluate Moisture Characteristics of Wheelchair Cushions at the User–Seat Interface

by Zhuofu Liu, Haifeng Cheng, Zhongming Luo, Vincenzo Cascioli, Andrew I. Heusch, Nadia R. Nair and Peter W. McCarthy

Sensors 2017, 17(4), 775; https://doi.org/10.3390/s17040775 - 5 Apr 2017

Cited by 7 | Viewed by 4033

Abstract

Little is known about the changes in moisture that occur at the body–seat interface during sitting. However, as increased moisture can add to the risk of skin damage, we have developed an array of MEMS (Micro-Electro-Mechanical System) humidity sensors to measure at this [...] Read more.

Little is known about the changes in moisture that occur at the body–seat interface during sitting. However, as increased moisture can add to the risk of skin damage, we have developed an array of MEMS (Micro-Electro-Mechanical System) humidity sensors to measure at this interface. Sensors were first evaluated against traceable standards, followed by use in a cross-over field test (n = 11; 20 min duration) using different wheelchair cushions (foam and gel). Relative humidity (RH) was measured at the left mid-thigh, right mid-thigh and coccyx. Sensors were shown to be unaffected by loading and showed highly reliable responses to measured changes in humidity, varying little from the traceable standard (<5%). Field-test data, smoothed through a moving average filter, revealed significant differences between the three chosen locations and between the gel and foam cushions. Maximum RH was attained in less than five minutes regardless of cushion material (foam or gel). Importantly, RH does not appear to distribute uniformly over the body–seat interface; suggesting multiple sensor positions would appear essential for effectively monitoring moisture in this interface. Material properties of the cushions appear to have a significant effect on RH characteristics (profile) at the body–seat interface, but not necessarily the time to peak moisture. Full article

(This article belongs to the Special Issue Humidity Sensors)

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2086 KiB

Open AccessArticle

Fiber Bragg Grating Measuring System for Simultaneous Monitoring of Temperature and Humidity in Mechanical Ventilation

by Carlo Massaroni, Michele A. Caponero, Rosaria D’Amato, Daniela Lo Presti and Emiliano Schena

Sensors 2017, 17(4), 749; https://doi.org/10.3390/s17040749 - 2 Apr 2017

Cited by 63 | Viewed by 7683

Abstract

During mechanical ventilation, the humidification of the dry air delivered by the mechanical ventilator is recommended. Among several solutions, heated wire humidifiers (HWHs) have gained large acceptance to be used in this field. The aim of this work is to fabricate a measuring [...] Read more.

During mechanical ventilation, the humidification of the dry air delivered by the mechanical ventilator is recommended. Among several solutions, heated wire humidifiers (HWHs) have gained large acceptance to be used in this field. The aim of this work is to fabricate a measuring system based on fiber Bragg grating (FBG) for the simultaneous monitoring of gas relative humidity (RH) and temperature, intended to be used for providing feedback to the HWHs’ control. This solution can be implemented using an array of two FBGs having a different center wavelength. Regarding RH monitoring, three sensors have been fabricated by coating an FBG with two different moisture-sensitive and biocompatible materials: the first two sensors were fabricated by coating the grating with a 3 mm × 3 mm layer of agar and agarose; to investigate the influence of the coating thickness to the sensor response, a third sensor was developed with a 5 mm × 5 mm layer of agar. The sensors have been assessed in a wide range of RH (up to 95%) during both an ascending and a subsequent descending phase. Only the response of the 3 mm × 3 mm-coated sensors were fast enough to follow the RH changes, showing a mean sensitivity of about 0.14 nm/% (agar-coated) and 0.12 nm/% (agarose-coated). The hysteresis error was about <10% in the two sensors. The contribution of temperature changes on these RH sensors was negligible. The temperature measurement was performed by a commercial FBG insensitive to RH changes. The small size of these FBG-based sensors, the use of biocompatible polymers, and the possibility to measure both temperature and RH by using the same fiber optic embedding an array of two FBGs make intriguing the use of this solution for application in the control of HWHs. Full article

(This article belongs to the Special Issue Humidity Sensors)

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3691 KiB

Open AccessArticle

Distributed Humidity Sensing in PMMA Optical Fibers at 500 nm and 650 nm Wavelengths

by Sascha Liehr, Mathias Breithaupt and Katerina Krebber

Sensors 2017, 17(4), 738; https://doi.org/10.3390/s17040738 - 31 Mar 2017

Cited by 29 | Viewed by 5920

Abstract

Distributed measurement of humidity is a sought-after capability for various fields of application, especially in the civil engineering and structural health monitoring sectors. This article presents a method for distributed humidity sensing along polymethyl methacrylate (PMMA) polymer optical fibers (POFs) by analyzing wavelength-dependent [...] Read more.

Distributed measurement of humidity is a sought-after capability for various fields of application, especially in the civil engineering and structural health monitoring sectors. This article presents a method for distributed humidity sensing along polymethyl methacrylate (PMMA) polymer optical fibers (POFs) by analyzing wavelength-dependent Rayleigh backscattering and attenuation characteristics at 500 nm and 650 nm wavelengths. Spatially resolved humidity sensing is obtained from backscatter traces of a dual-wavelength optical time domain reflectometer (OTDR). Backscatter dependence, attenuation dependence as well as the fiber length change are characterized as functions of relative humidity. Cross-sensitivity effects are discussed and quantified. The evaluation of the humidity-dependent backscatter effects at the two wavelength measurements allows for distributed and unambiguous measurement of relative humidity. The technique can be readily employed with low-cost standard polymer optical fibers and commercial OTDR devices. Full article

(This article belongs to the Special Issue Humidity Sensors)

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1556 KiB

Open AccessArticle

Compliment Graphene Oxide Coating on Silk Fiber Surface via Electrostatic Force for Capacitive Humidity Sensor Applications

by Kook In Han, Seungdu Kim, In Gyu Lee, Jong Pil Kim, Jung-Ha Kim, Suck Won Hong, Byung Jin Cho and Wan Sik Hwang

Sensors 2017, 17(2), 407; https://doi.org/10.3390/s17020407 - 19 Feb 2017

Cited by 22 | Viewed by 8129

Abstract

Cylindrical silk fiber (SF) was coated with Graphene oxide (GO) for capacitive humidity sensor applications. Negatively charged GO in the solution was attracted to the positively charged SF surface via electrostatic force without any help from adhesive intermediates. The magnitude of the positively [...] Read more.

Cylindrical silk fiber (SF) was coated with Graphene oxide (GO) for capacitive humidity sensor applications. Negatively charged GO in the solution was attracted to the positively charged SF surface via electrostatic force without any help from adhesive intermediates. The magnitude of the positively charged SF surface was controlled through the static electricity charges created on the SF surface. The GO coating ability on the SF improved as the SF’s positive charge increased. The GO-coated SFs at various conditions were characterized using an optical microscope, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and LCR meter. Unlike the intact SF, the GO-coated SF showed clear response-recovery behavior and well-behaved repeatability when it was exposed to 20% relative humidity (RH) and 90% RH alternatively in a capacitive mode. This approach allows humidity sensors to take advantage of GO’s excellent sensing properties and SF’s flexibility, expediting the production of flexible, low power consumption devices at relatively low costs. Full article

(This article belongs to the Special Issue Humidity Sensors)

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4446 KiB

Open AccessArticle

Uncertainty Analysis in Humidity Measurements by the Psychrometer Method

by Jiunyuan Chen and Chiachung Chen

Sensors 2017, 17(2), 368; https://doi.org/10.3390/s17020368 - 14 Feb 2017

Cited by 21 | Viewed by 8450

Abstract

The most common and cheap indirect technique to measure relative humidity is by using psychrometer based on a dry and a wet temperature sensor. In this study, the measurement uncertainty of relative humidity was evaluated by this indirect method with some empirical equations [...] Read more.

The most common and cheap indirect technique to measure relative humidity is by using psychrometer based on a dry and a wet temperature sensor. In this study, the measurement uncertainty of relative humidity was evaluated by this indirect method with some empirical equations for calculating relative humidity. Among the six equations tested, the Penman equation had the best predictive ability for the dry bulb temperature range of 15–50 °C. At a fixed dry bulb temperature, an increase in the wet bulb depression increased the error. A new equation for the psychrometer constant was established by regression analysis. This equation can be computed by using a calculator. The average predictive error of relative humidity was <0.1% by this new equation. The measurement uncertainty of the relative humidity affected by the accuracy of dry and wet bulb temperature and the numeric values of measurement uncertainty were evaluated for various conditions. The uncertainty of wet bulb temperature was the main factor on the RH measurement uncertainty. Full article

(This article belongs to the Special Issue Humidity Sensors)

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3303 KiB

Open AccessArticle

High-Sensitivity and Low-Hysteresis Porous MIMType Capacitive Humidity Sensor Using Functional Polymer Mixed with TiO2 Microparticles

by Ming-Qing Liu, Cong Wang and Nam-Young Kim

Sensors 2017, 17(2), 284; https://doi.org/10.3390/s17020284 - 2 Feb 2017

Cited by 75 | Viewed by 6726

Abstract

In this study, a high-sensitivity and low-hysteresis porous metal–insulator–metal-type capacitive humidity sensor is investigated using a functional polymer mixed with TiO2 microparticles. The humidity sensor consists of an optimally designed porous top electrode, a functional polymer humidity sensitive layer, a bottom electrode, and [...] Read more.

In this study, a high-sensitivity and low-hysteresis porous metal–insulator–metal-type capacitive humidity sensor is investigated using a functional polymer mixed with TiO2 microparticles. The humidity sensor consists of an optimally designed porous top electrode, a functional polymer humidity sensitive layer, a bottom electrode, and a glass substrate. The porous top electrode is designed to increase the contact area between the sensing layer and water vapor, leading to high sensitivity and quick response time. The functional polymer mixed with TiO2 microparticles shows excellent hysteresis under a wide humidity-sensing range with good longterm stability. The results show that as the relative humidity ranges from 10% RH to 90% RH, the proposed humidity sensor achieves a high sensitivity of 0.85 pF/% RH and a fast response time of less than 35 s. Furthermore, the sensor shows an ultra-low hysteresis of 0.95% RH at 60% RH, a good temperature dependence, and a stable capacitance value with a maximum of 0.17% RH drift during 120 h of continuous test. Full article

(This article belongs to the Special Issue Humidity Sensors)

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9470 KiB

Open AccessArticle

Standardization, Calibration, and Evaluation of Tantalum-Nano rGO-SnO₂ Composite as a Possible Candidate Material in Humidity Sensors

by Subbiah Karthick, Han-Seung Lee, Seung-Jun Kwon, Rethinam Natarajan and Velu Saraswathy

Sensors 2016, 16(12), 2079; https://doi.org/10.3390/s16122079 - 7 Dec 2016

Cited by 37 | Viewed by 7538

Abstract

The present study focuses the development and the evaluation of humidity sensors based on reduced graphene oxide—tin oxide (rGO-SnO₂) nanocomposites, synthesized by a simple redox reaction between GO and SnCl₂. The physico-chemical characteristics of the nanocomposites were analyzed by [...] Read more.

The present study focuses the development and the evaluation of humidity sensors based on reduced graphene oxide—tin oxide (rGO-SnO₂) nanocomposites, synthesized by a simple redox reaction between GO and SnCl₂. The physico-chemical characteristics of the nanocomposites were analyzed by XRD, TEM, FTIR, and Raman spectroscopy. The formation of SnO₂ crystal phase was observed through XRD. The SnO₂ crystal phase anchoring to the graphene sheet was confirmed through TEM images. For the preparation of the sensors, tantalum substrates were coated with the sensing material. The sensitivity of the fabricated sensor was studied by varying the relative humidity (RH) from 11% to 95% over a period of 30 days. The dependence of the impedance and of the capacitance with RH of the sensor was measured with varying frequency ranging from 1 kHz to 100 Hz. The long-term stability of the sensor was measured at 95% RH over a period of 30 days. The results proved that rGO-SnO₂ nanocomposites are an ideal conducting material for humidity sensors due to their high sensitivity, rapid response and recovery times, as well as their good long-term stability. Full article

(This article belongs to the Special Issue Humidity Sensors)

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11114 KiB

Open AccessReview

Recent Developments in Fiber Optics Humidity Sensors

by Joaquin Ascorbe, Jesus M. Corres, Francisco J. Arregui and Ignacio R. Matias

Sensors 2017, 17(4), 893; https://doi.org/10.3390/s17040893 - 19 Apr 2017

Cited by 202 | Viewed by 12550

Abstract

A wide range of applications such as health, human comfort, agriculture, food processing and storage, and electronic manufacturing, among others, require fast and accurate measurement of humidity. Sensors based on optical fibers present several advantages over electronic sensors and great research efforts have [...] Read more.

A wide range of applications such as health, human comfort, agriculture, food processing and storage, and electronic manufacturing, among others, require fast and accurate measurement of humidity. Sensors based on optical fibers present several advantages over electronic sensors and great research efforts have been made in recent years in this field. The present paper reports the current trends of optical fiber humidity sensors. The evolution of optical structures developed towards humidity sensing, as well as the novel materials used for this purpose, will be analyzed. Well-known optical structures, such as long-period fiber gratings or fiber Bragg gratings, are still being studied towards an enhancement of their sensitivity. Sensors based on lossy mode resonances constitute a platform that combines high sensitivity with low complexity, both in terms of their fabrication process and the equipment required. Novel structures, such as resonators, are being studied in order to improve the resolution of humidity sensors. Moreover, recent research on polymer optical fibers suggests that the sensitivity of this kind of sensor has not yet reached its limit. Therefore, there is still room for improvement in terms of sensitivity and resolution. Full article

(This article belongs to the Special Issue Humidity Sensors)

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