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Special Issue "Humidity Sensors: Advances in Reliability, Calibration and Application"

A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Physical Sensors".

Deadline for manuscript submissions: closed (30 December 2018).

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Guest Editor
Prof. Dr. Peter W. McCarthy

University of South Wales, Pontypridd, UK
Website | E-Mail
Interests: clinical technology; neurophysiology; clinical biomechanics
Guest Editor
Prof. Dr. Zhuofu Liu

Harbin Univesity of Science and Technology, Harbin, Heilongjiang Province, China
Website | E-Mail
Interests: signal and information processing; biomedical engineering
Guest Editor
Dr. Vincenzo Cascioli

Murdoch University, School of Health Professions, Perth, Australia
Website | E-Mail
Interests: assessment of comfort and discomfort during prolonged sitting

Special Issue Information

Dear Colleagues,

Humidity detection has come a long way since the use of wet bulb psychrometry. Many electronic sensor-based methods are becoming available for detecting and reporting relative humidity. However, as one moves further away from the physical measurement of any property, the issues of reliability and accuracy of calibration become increasingly important. In the case of humidity, the property of the sensor that enables the measurement to be made can also be the property that can lead to problems with the calibration and sensitivity, as well as recovery of the sensor: all of which can limit uptake and application of the sensors.

The rapid recent growth in research aimed at finding the best method of sensing humidity is in proportion with the range of potential needs across a diverse series of fields from biomedical, agriculture, pharmaceutical to industrial such as in the semiconductor industry and food processing. Therefore, never has the need been greater for study and refinement of these sensors. We consider this a good point to bring together research into the uses (potential and actual) and look at the issues surrounding this measurement.

The aim of this Special Issue is to present some of the possibilities that humidity sensors offer, focusing on the different configurations that can be used and novel applications in any field. In addition, selected high quality papers from IFOST 2018 (http://www.ifost2018.org/IFOST2018/) will be invited for further consideration regarding publication in this Special Issue. Reviews presenting a deep analysis of specific problems such as calibration and use will also be considered.

We welcome original research papers and review articles on humidity sensor technology, their applications and comparison between types.

Prof. Dr. Peter W. McCarthy
Prof. Dr. Zhuofu Liu
Dr. Vincenzo Cascioli
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Sensors is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Humidity sensor
  • Calibration/reliability
  • Moisture detection
  • Temperature artefacts
  • Nanomaterials
  • High sensitivity structures, interferometers
  • Rapid response
  • Recovery rates
  • Printed humidity sensors
  • SPR/LMR/LSPR
  • Miniature humidity sensors
  • RFID
  • Thermal compensation
  • On-field application
  • Embedded/wearable/mobile humidity sensors
  • Wireless humidity sensors
  • Medical/healthcare
  • Food/environmental
  • Humidity profile mapping

Published Papers (12 papers)

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Research

Open AccessArticle
In-Depth Investigation into the Transient Humidity Response at the Body-Seat Interface on Initial Contact Using a Dual Temperature and Humidity Sensor
Sensors 2019, 19(6), 1471; https://doi.org/10.3390/s19061471
Received: 30 January 2019 / Revised: 20 February 2019 / Accepted: 21 March 2019 / Published: 26 March 2019
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Abstract
Relative humidity (RH) at the body-seat interface is considered an important factor in both sitting comfort and generation of health concerns such as skin lesions. Technical difficulties appear to have limited research aimed at the detailed and simultaneous exploration of RH and temperature [...] Read more.
Relative humidity (RH) at the body-seat interface is considered an important factor in both sitting comfort and generation of health concerns such as skin lesions. Technical difficulties appear to have limited research aimed at the detailed and simultaneous exploration of RH and temperature changes at the body-seat interface; using RH sensors without the capability to record temperature where RH is recorded. To explore the causes of a spike in RH consistently produced on first contact between body and seat surface, we report data from the first use of dual temperature and RH (HTU21D) sensors in this interface. Following evaluation of sensor performance, the effect of local thermal changes on RH was investigated. The expected strong negative correlation between temperature and RH (R2 = −0.94) supported the importance of considering both parameters when studying impact of sitting on skin health. The influence of sensor movement speed (higher velocity approach: 0.32 cm/s ± 0.01 cm/s; lower velocity approach: 0.17 cm/s ± 0.01 cm/s) into a static RH region associated with a higher local temperature were compared with data gathered by altering the rate of a person sitting. In all cases, the faster sitting down (or equivalent) generated larger RH outcomes: e.g., in human sitting 53.7% ± 3.3% RH (left mid-thigh), 56.4% ± 5.1% RH (right mid-thigh) and 53.2% ± 2.7% RH (Coccyx). Differences in size of RH change were seen across the measurement locations used to study the body-seat interface. The initial sitting contact induces a transient RH response (duration ≤ 40 s) that does not accurately reflect the microenvironment at the body-seat interface. It is likely that any movement during sitting would result in similar artefact formation. As a result, caution should be taken when investigating RH performance at any enclosed interface when the surfaces may have different temperatures and movement may occur. Full article
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Open AccessArticle
Determination of Optimal Measurement Points for Calibration Equations—Examples by RH Sensors
Sensors 2019, 19(5), 1213; https://doi.org/10.3390/s19051213
Received: 26 February 2019 / Revised: 6 March 2019 / Accepted: 6 March 2019 / Published: 9 March 2019
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Abstract
The calibration points for sensors must be selected carefully. This study uses accuracy and precision as the criteria to evaluate the required numbers of calibration points required. Two types of electric relative humidity (RH) sensors were used to illustrate the method and the [...] Read more.
The calibration points for sensors must be selected carefully. This study uses accuracy and precision as the criteria to evaluate the required numbers of calibration points required. Two types of electric relative humidity (RH) sensors were used to illustrate the method and the standard RH environments were maintained using different saturated salt solutions. The best calibration equation is determined according to the t-value for the highest-order parameter and using the residual plots. Then, the estimated standard errors for the regression equation are used to determine the accuracy of the sensors. The combined uncertainties from the calibration equations for different calibration points for the different saturated salt solutions were then used to evaluate the precision of the sensors. The accuracy of the calibration equations is 0.8% RH for a resistive humidity sensor using 7 calibration points and 0.7% RH for a capacitance humidity sensor using 5 calibration points. The precision is less than 1.0% RH for a resistive sensor and less than 0.9% RH for a capacitive sensor. The method that this study proposed for the selection of calibration points can be applied to other sensors. Full article
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Open AccessArticle
Humidity Sensors with Shielding Electrode Under Interdigitated Electrode
Sensors 2019, 19(3), 659; https://doi.org/10.3390/s19030659
Received: 14 December 2018 / Revised: 30 January 2019 / Accepted: 31 January 2019 / Published: 6 February 2019
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Abstract
Recently, humidity sensors have been investigated extensively due to their broad applications in chip fabrication, health care, agriculture, amongst others. We propose a capacitive humidity sensor with a shielding electrode under the interdigitated electrode (SIDE) based on polyimide (PI). Thanks to the shielding [...] Read more.
Recently, humidity sensors have been investigated extensively due to their broad applications in chip fabrication, health care, agriculture, amongst others. We propose a capacitive humidity sensor with a shielding electrode under the interdigitated electrode (SIDE) based on polyimide (PI). Thanks to the shielding electrode, this humidity sensor combines the high sensitivity of parallel plate capacitive sensors and the fast response of interdigitated electrode capacitive sensors. We use COMSOL Multiphysics to design and optimize the SIDE structure. The experimental data show very good agreement with the simulation. The sensitivity of the SIDE sensor is 0.0063% ± 0.0002% RH. Its response/recovery time is 20 s/22 s. The maximum capacitance drift under different relative humidity is 1.28% RH. Full article
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Open AccessArticle
A Laboratory Study on Non-Invasive Soil Water Content Estimation Using Capacitive Based Sensors
Sensors 2019, 19(3), 651; https://doi.org/10.3390/s19030651
Received: 1 October 2018 / Revised: 14 November 2018 / Accepted: 19 November 2018 / Published: 5 February 2019
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Abstract
Soil water content is an important parameter in many engineering, agricultural and environmental applications. In practice, there exists a need to measure this parameter rather frequently in both time and space. However, common measurement techniques are typically invasive, time-consuming and labour-intensive, or rely [...] Read more.
Soil water content is an important parameter in many engineering, agricultural and environmental applications. In practice, there exists a need to measure this parameter rather frequently in both time and space. However, common measurement techniques are typically invasive, time-consuming and labour-intensive, or rely on potentially risky (although highly regulated) nuclear-based methods, making frequent measurements of soil water content impractical. Here we investigate in the laboratory the effectiveness of four new low-cost non-invasive sensors to estimate the soil water content of a range of soil types. While the results of each of the four sensors are promising, one of the sensors, herein called the “AOGAN” sensor, exhibits superior performance, as it was designed based on combining the best geometrical and electronic features of the other three sensors. The performance of the sensors is, however, influenced by the quality of the sensor-soil coupling and the soil surface roughness. Accuracy was found to be within 5% of volumetric water content, considered sufficient to enable higher spatiotemporal resolution contrast for mapping of soil water content. Full article
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Open AccessArticle
A Fast Response−Recovery 3D Graphene Foam Humidity Sensor for User Interaction
Sensors 2018, 18(12), 4337; https://doi.org/10.3390/s18124337
Received: 9 October 2018 / Revised: 1 December 2018 / Accepted: 3 December 2018 / Published: 8 December 2018
Cited by 1 | PDF Full-text (1961 KB) | HTML Full-text | XML Full-text
Abstract
Humidity sensors allow electronic devices to convert the water content in the environment into electronical signals by utilizing material properties and transduction techniques. Three-dimensional graphene foam (3DGF) can be exploited in humidity sensors due to its convenient features including low-mass density, large specific [...] Read more.
Humidity sensors allow electronic devices to convert the water content in the environment into electronical signals by utilizing material properties and transduction techniques. Three-dimensional graphene foam (3DGF) can be exploited in humidity sensors due to its convenient features including low-mass density, large specific surface area, and excellent electrical. In this paper, 3DGF with super permeability to water enables humidity sensors to exhibit a broad relative humidities (RH) range, from 0% to 85.9%, with a fast response speed (response time: ~89 ms, recovery time: ~189 ms). To interpret the physical mechanism behind this, we constructed a 3DGF model decorated with water to calculate the energy structure and we carried out the CASTEP as implemented in Materials Studio 8.0. This can be ascribed to the donor effect, namely, the electronic donation of chemically adsorbed water molecules to the 3DGF surface. Furthermore, this device can be used for user interaction (UI) with unprecedented performance. These high performances support 3DGF as a promising material for humidity sensitive material. Full article
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Open AccessArticle
A Noninvasive TDR Sensor to Measure the Moisture Content of Rigid Porous Materials
Sensors 2018, 18(11), 3935; https://doi.org/10.3390/s18113935
Received: 30 September 2018 / Revised: 5 November 2018 / Accepted: 12 November 2018 / Published: 14 November 2018
Cited by 2 | PDF Full-text (2484 KB) | HTML Full-text | XML Full-text
Abstract
The article presents the potential application of the time domain reflectometry (TDR) technique to measure moisture transport in unsaturated porous materials. The research of the capillary uptake phenomenon in a sample of autoclaved aerated concrete (AAC) was conducted using a TDR sensor with [...] Read more.
The article presents the potential application of the time domain reflectometry (TDR) technique to measure moisture transport in unsaturated porous materials. The research of the capillary uptake phenomenon in a sample of autoclaved aerated concrete (AAC) was conducted using a TDR sensor with the modified construction for non-invasive testing. In the paper the basic principles of the TDR method as a technique applied in metrology, and its potential for measurement of moisture in porous materials, including soils and porous building materials are presented. The second part of the article presents the experiment of capillary rise process in the AAC sample. Application of the custom sensor required its individual calibration, thus a unique model of regression between the readouts of apparent permittivity of the tested material and its moisture was developed. During the experiment moisture content was monitored in the sample exposed to water influence. Monitoring was conducted using the modified TDR sensor. The process was additionally measured using the standard frequency domain (FD) capacitive sensor in order to compare the readouts with traditional techniques of moisture detection. The uncertainty for testing AAC moisture, was expressed as RMSE (0.013 cm3/cm3) and expanded uncertainty (0.01–0.02 cm3/cm3 depending on moisture) was established along with calibration of the applied sensor. The obtained values are comparable to, or even better than, the features of the traditional invasive sensors utilizing universal calibration models. Both, the TDR and capacitive (FD) sensor enabled monitoring of capillary uptake phenomenon progress. It was noticed that at the end of the experiment the TDR readouts were 4.4% underestimated and the FD readouts were overestimated for 12.6% comparing to the reference gravimetric evaluation. Full article
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Open AccessArticle
Online Moisture Measurement of Bio Fuel at a Paper Mill Employing a Microwave Resonator
Sensors 2018, 18(11), 3844; https://doi.org/10.3390/s18113844
Received: 1 October 2018 / Revised: 4 November 2018 / Accepted: 6 November 2018 / Published: 9 November 2018
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Abstract
This paper investigates online moisture measurement of biofuel employing a strip line cavity resonator at approximately 366 MHz, attached above and below the conveyor belt. An existing sensor design is modified for the factory assembly, and the correct operation has been tested prior [...] Read more.
This paper investigates online moisture measurement of biofuel employing a strip line cavity resonator at approximately 366 MHz, attached above and below the conveyor belt. An existing sensor design is modified for the factory assembly, and the correct operation has been tested prior to this paper with a small number of measurement points and collected reference samples (n = 67). The purpose is now to concentrate on the accuracy of the measurement and increase the number of measurement points (n = 367). The measurements were made in 5 different lots, and the thickness and moisture properties of the biomaterial mat were varied between minimum and maximum levels by adjusting the settings of the belt filter press that presses pulp slush into a mat. In order to further reduce inaccuracy, at the maximum one standard deviation was allowed from the average height of the equivalent water layer for each dataset, and consequently the number of samples was reduced to 235. A linear fit and a parabola fit were determined for thickness of the equivalent water layer vs. the relative resonant frequency shift: R2 = 0.82 and R2 = 0.78. Full article
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Open AccessArticle
A Novel Method and an Equipment for Generating the Standard Moisture in Gas Flowing through a Pipe
Sensors 2018, 18(10), 3438; https://doi.org/10.3390/s18103438
Received: 3 September 2018 / Revised: 7 October 2018 / Accepted: 8 October 2018 / Published: 13 October 2018
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Abstract
When inert gas containing water molecules flows into a metal pipe, the water molecules cannot exit instantaneously from the outlet of the pipe but are captured at adsorption sites on the inner surface of the pipe until most of the sites are occupied. [...] Read more.
When inert gas containing water molecules flows into a metal pipe, the water molecules cannot exit instantaneously from the outlet of the pipe but are captured at adsorption sites on the inner surface of the pipe until most of the sites are occupied. A theoretical model and a subsequent experiment in this article show that the delay time depends on the amount of moisture level; the higher the moisture-level, the shorter the delay time. Based on the result, we propose a new method and its implementation to the validation of a standard moisture generation to be used in the field measurement such as in factories and pipe lines. Full article
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Open AccessArticle
Consumer Grade Weather Stations for Wooden Structure Fire Risk Assessment
Sensors 2018, 18(10), 3244; https://doi.org/10.3390/s18103244
Received: 14 August 2018 / Revised: 14 September 2018 / Accepted: 25 September 2018 / Published: 27 September 2018
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Abstract
During January 2014, Norway experienced unusually cold and dry weather conditions leading to very low indoor relative humidity (RH) in inhabited (heated) wooden homes. The resulting dry wood played an important role in the two most severe accidental fires in Norway recorded since [...] Read more.
During January 2014, Norway experienced unusually cold and dry weather conditions leading to very low indoor relative humidity (RH) in inhabited (heated) wooden homes. The resulting dry wood played an important role in the two most severe accidental fires in Norway recorded since 1923. The present work describes testing of low cost consumer grade weather stations for recording temperature and relative humidity as a proxy for dry wood structural fire risk assessment. Calibration of the weather stations relative humidity (RH) sensors was done in an atmosphere stabilized by water saturated LiCl, MgCl2 and NaCl solutions, i.e., in the range 11% RH to 75% RH. When calibrated, the weather station results were well within ±3% RH. During the winter 2015/2016 weather stations were placed in the living room in eight wooden buildings. A period of significantly increased fire risk was identified in January 2016. The results from the outdoor sensors compared favorably with the readings from a local meteorological station, and showed some interesting details, such as higher ambient relative humidity for a home close to a large and comparably warmer sea surface. It was also revealed that a forecast predicting low humidity content gave results close to the observed outdoor weather station data, at least for the first 48 h forecast. Full article
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Open AccessArticle
Design and Implementation of an Infrared Radiant Source for Humidity Testing
Sensors 2018, 18(9), 3088; https://doi.org/10.3390/s18093088
Received: 26 June 2018 / Revised: 27 August 2018 / Accepted: 11 September 2018 / Published: 13 September 2018
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Abstract
A novel way to measure humidity through testing the emissivity of an area radiant source is presented in this paper. The method can be applied in the environment at near room temperature (5~95 °C) across the relative humidity (RH) range of 20~90% RH. [...] Read more.
A novel way to measure humidity through testing the emissivity of an area radiant source is presented in this paper. The method can be applied in the environment at near room temperature (5~95 °C) across the relative humidity (RH) range of 20~90% RH. The source, with a grooved radiant surface, works in the far infrared wavelength band of 8~12 μm. The Monte-Carlo model for thermal radiation was set up to analyze the V-grooved radiant surface. Heat pipe technology is used to maintain an isothermal radiant surface. The fuzzy-PID control method was adopted to solve the problems of intense heat inertia and being easily interfered by the environment. This enabled the system to be used robustly across a large temperature range with high precision. The experimental results tested with a scanning radiant thermometer showed that the radiant source can provide a uniform thermal radiation capable of satisfying the requirements of humidity testing. The calibration method for the radiant source for humidity was explored, which is available for testing humidity. Full article
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Open AccessArticle
Reliability Modeling for Humidity Sensors Subject to Multiple Dependent Competing Failure Processes with Self-Recovery
Sensors 2018, 18(8), 2714; https://doi.org/10.3390/s18082714
Received: 30 June 2018 / Revised: 6 August 2018 / Accepted: 16 August 2018 / Published: 18 August 2018
Cited by 1 | PDF Full-text (3536 KB) | HTML Full-text | XML Full-text
Abstract
Recent developments in humidity sensors have heightened the need for reliability. Seeing as many products such as humidity sensors experience multiple dependent competing failure processes (MDCFPs) with self-recovery, this paper proposes a new general reliability model. Previous research into MDCFPs has primarily focused [...] Read more.
Recent developments in humidity sensors have heightened the need for reliability. Seeing as many products such as humidity sensors experience multiple dependent competing failure processes (MDCFPs) with self-recovery, this paper proposes a new general reliability model. Previous research into MDCFPs has primarily focused on the processes of degradation and random shocks, which are appropriate for most products. However, the existing reliability models for MDCFPs cannot fully characterize the failure processes of products such as humidity sensors with significant self-recovery, leading to an underestimation of reliability. In this paper, the effect of self-recovery on degradation was analyzed using a conditional probability. A reliability model for soft failure with self-recovery was obtained. Then, combined with the model of hard failure due to random shocks, a general reliability model with self-recovery was established. Finally, reliability tests of the humidity sensors were presented to verify the proposed reliability model. Reliability modeling for products subject to MDCFPs with considering self-recovery can provide a better understanding of the mechanism of failure and offer an alternative method to predict the reliability of products. Full article
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Open AccessArticle
Humidity Measurement in Carbon Dioxide with Capacitive Humidity Sensors at Low Temperature and Pressure
Sensors 2018, 18(8), 2615; https://doi.org/10.3390/s18082615
Received: 27 June 2018 / Revised: 3 August 2018 / Accepted: 6 August 2018 / Published: 9 August 2018
PDF Full-text (2917 KB) | HTML Full-text | XML Full-text
Abstract
In experimental chambers for simulating the atmospheric near-surface conditions of Mars, or in situ measurements on Mars, the measurement of the humidity in carbon dioxide gas at low temperature and under low pressure is needed. For this purpose, polymer-based capacitive humidity sensors are [...] Read more.
In experimental chambers for simulating the atmospheric near-surface conditions of Mars, or in situ measurements on Mars, the measurement of the humidity in carbon dioxide gas at low temperature and under low pressure is needed. For this purpose, polymer-based capacitive humidity sensors are used; however, these sensors are designed for measuring the humidity in the air on the Earth. The manufacturers provide only the generic calibration equation for standard environmental conditions in air, and temperature corrections of humidity signal. Because of the lack of freely available information regarding the behavior of the sensors in CO2, the range of reliable results is limited. For these reasons, capacitive humidity sensors (Sensirion SHT75) were tested at the German Aerospace Center (DLR) in its Martian Simulation Facility (MSF). The sensors were investigated in cells with a continuously humidified carbon dioxide flow, for temperatures between −70 °C and 10 °C, and pressures between 10 hPa and 1000 hPa. For 28 temperature–pressure combinations, the sensor calibration equations were calculated together with temperature–dependent formulas for the coefficients of the equations. The characteristic curves obtained from the tests in CO2 and in air were compared for selected temperature–pressure combinations. The results document a strong cross-sensitivity of the sensors to CO2 and, compared with air, a strong pressure sensitivity as well. The reason could be an interaction of the molecules of CO2 with the adsorption sites on the thin polymeric sensing layer. In these circumstances, an individual calibration for each pressure with respect to temperature is required. The performed experiments have shown that this kind of sensor can be a suitable, lightweight, and relatively inexpensive choice for applications in harsh environments such as on Mars. Full article
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