Capacitive Relative Humidity Research Articles

Soil total suction sensing using fiber-optic technology

Measurement of the soil water suction is important for investigating geotechnical problems and mitigating associated risks; however, conventional high-range suction measurement techniques have some limitations for accurate, long-term, and quasi-distributed suction measurement in the field. Fiber-optic humidity sensors provide a viable solution due to their unique properties. Here, we present a novel microfabricated fiber-optic suction sensor for measuring the suction of water in unsaturated soils. We evaluated the performance of the sensor on four bentonite–sand mixtures by comparing it with two laboratory suction measurement methods (chill-mirror hygrometer, salt solution vapor equilibrium) and a capacitive relative humidity sensor. We determined that this sensor has a measurement range of 5–300 MPa with a root mean square error of less than 3.5 MPa, and a response time of 2–9 min over the tested suction variation range (6.5–80.5 MPa). The results of the evaporation model test showed that three fiber-optic sensors installed at different depths of bentonite could effectively capture changes in temperature, relative humidity in soil pores, and total suction during evaporation. Together, these results demonstrate the great potential of the fiber-optic suction sensor for in-situ, long-term, and quasi-distributed suction monitoring.

Uncertainty analysis of polymer-based capacitive relative humidity sensor at negative temperatures and low humidity levels

Capacitive relative humidity sensors, widely used in industrial and environmental applications, should be interchangeable and ideally have parametrizable response to relative humidity changes. While the effects of temperatures in the range between 0 °C and 50 °C are negligible, this is not the case for negative values. Therefore, this effect must be taken into consideration when parameterizing the output of the capacitive humidity sensors for temperatures below the freezing point of water. In this paper, we present our investigations on the temperature effect for one type of such sensors when using it at subzero temperatures. The experiments were conducted at temperatures of −10 °C, −20 °C and −30 °C. Additionally, we have tested the sensor performance at relative humidity levels low enough for most sensors not exhibit their best performances. Direct and inverse calibration methods were used and the uncertainty analysis of the devices under test is discussed.

Radiation effect on the polymer-based capacitive relative humidity sensors

The sensitivity of polymer-based capacitive relative humidity (RH) sensors after irradiation with neutrons, electrons and protons was measured. Degradation consists of the decreasing of the upper RH limit that can be measured. At the same time, low RH-level sensitivity is almost stable. After 30 krad of absorption dose, RH cut off is equal to 85% of max value, after 60 krad–40%. Degradation reduces after annealing which indicates high radiation sensitivity of the internal circuit in comparison to RH-sensing polymer film.

Uncertainty Analysis of Polymer-Based Capacitive Relative Humidity Sensor at Negative Temperatures and Low Humidity Levels

Uncertainty Analysis of Polymer-Based Capacitive Relative Humidity Sensor at Negative Temperatures and Low Humidity Levels

The Synthesis and Fabrication of Polyvinyl Alcohol Nanofibers Based Capacitive Relative Humidity Sensor

The Synthesis and Fabrication of Polyvinyl Alcohol Nanofibers Based Capacitive Relative Humidity Sensor

Synthesis and Fabrication of Polyvinyl Alcohol Nanofibers Based Capacitive Relative Humidity Sensor


 Abstract
 Capacitive humidity sensor based on Polyvinyl-alcohol (PVA) electrospun nanofibers was fabricated. PVA nanofibers were synthesized through versatile electrospinning technique. The synthesized nanofibers were heat treated and characterized via Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope (SEM) and thermogravimetric Analyzer (TGA) for structural, morphological and thermal properties. The fibers of admirable morphological structure were selected and deposited over interdigitated alumina electrodes for the investigation of Relative Humidity (RH) sensing characteristics. The variation in capacitance of the sensor with RH was measured 48pf at 32-92% RH. The dynamic response study confirmed the durability and stability of the sensor. The material exhibited quick response and recovery time and takes 13.27 seconds to measure the maximum RH value. Thus, the proposed sensing material has the potential of possible application in humidity sensing devices.

A measurement method for lossy capacitive relative humidity sensors based on a direct sensor-to-microcontroller interface circuit

A new time-domain measurement method for determining the capacitance and resistance values of lossy relative humidity capacitive sensors is presented. The method is based on a direct sensor-to-microcontroller interface for microcontrollers with internal analog comparators and timers. The interface circuit consists only of four reference resistors (two reference resistors if a microcontroller includes a voltage reference source), a given sensor and a microcontroller. A systematic error correction algorithm based on a correction dictionary and the M-multiple measurement approach are also proposed in the paper. Experimental investigations were carried out using a prototype device based on an 8-bit ATXmega32A4 microcontroller. The experimental research confirmed that the relative errors of measurement of capacitance introduced by the interface circuit are less than 0.71% (for capacitance values 100–286pF), and the relative errors of measurement of resistance are less than 0.74% (for resistance values 1–10MΩ).

Characterization of On-Foil Sensors and Ultra-Thin Chips for HySiF Integration

The characterization of multiple mechanically flexible passive and active electronic components, namely on-foil temperature sensor, relative humidity sensor, and ultra-thin microcontroller unit (MCU) integrated circuit (IC) is presented. These components are readily available for Hybrid System-in-Foil (HySiF) integration, which combines the merits of the high-performance silicon chips with the large-area electronics. The 60-nm thin-film resistance temperature detector (RTD) is fabricated on different substrates and the extracted temperature coefficient ranges from 0.002 to 0.003( $\Omega/\Omega$ )/°C. Furthermore, the capacitive relative humidity sensor is fabricated by utilizing a polymeric sensing dielectric material that is compatible with those polyimides used in ultra-thin chip embedding. The $1-\mu \text {m}$ thick sensor achieves a fast adsorption rise time of $\text {54 ms}$ and a spontaneous desorption fall time of $\text {750 ms}$ . For flexible ICs packaging and HySiF integration, microcontroller chips are back-thinned to a thickness of about ${30}~\mu \text {m}$ . The characterization of the MCU after the thinning process step showed undegraded performance of the integrated analog-to-digital converter in terms of linearity. However, a drop in the on-chip temperature sensor sensitivity is observed for the thin chips and significant degradation in the period jitter of the high-frequency RC oscillator is measured. Finally, continuity tests are performed to validate the functionality of the ultra-thin MCUs when embedded using the Chip-Film Patch (CFP) flexible package.

Oxidized Carbon Nanohorn-Hydrophilic Polymer Nanocomposite as the Resistive Sensing Layer for Relative Humidity

This study presents relative humidity (RH) sensing response of resistive sensors composed of nanocomposites with oxidized single-wall carbon nanohorns (SWCNHs) and hydrophilic polymers in different ratios at room temperature. The poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) and polyvinylpyrrolidone (PVP) materials were selected to be the hydrophilic polymers. Two types of nanocomposites with PVP were synthesized, one using SWCNHs/VP at 1/2 (w/w) ratio and another with SWCNHs/PVP in a 1/1 (w/w) ratio. The sensing structure is composed of a Si substrate, a SiO2 layer, and interdigitated (IDT) electrodes, on which the sensing layer is deposited via the drop-casting method. The sensor has a response comparable to that of a commercially available, state-of-the-art, capacitive relative humidity sensor, being characterized by a rapid response time and excellent stability with time. Two sensing mechanisms are considered and analyzed. The first involves the rapid swelling of the host hydrophilic polymer at higher relative humidity levels. The second involves a decrease in the number of holes due to interaction with water molecules, which act as electron donors.

All-Inkjet-Printed Humidity Sensors for the Detection of Relative Humidity in Air and Soil—Towards the Direct Fabrication on Plant Leaves

ABSTRACTWe demonstrate the fabrication, by exclusive means of inkjet-printing, of capacitive relative humidity sensors on flexible, plastic substrate. These sensors can be successfully used for the measurement of relative-humidity in both air and common soil. We also show that the same technique may be used for the fabrication of the same type of sensors on the surface of the leaves of Elægnus Ebbingei (silverberry).Our results demonstrate the suitability of leaves as substrate for printed electronics and pave the way to the next generation of sensors to be used in fields such as agriculture and flower farming.

Identification of the Transient Response of a Capacitive Relative Humidity Sensor

This paper presents the results of identifying the transient response of a relative air humidity sensor. It is shown that the capacitive relative humidity sensor is an integrated microprocessor system that performs joint processing of the readings of the absolute humidity sensor and the temperature sensor. A mathematical model of the relative humidity measurement process is proposed. The transient responses of the absolute humidity sensing element and the temperature sensor are specified under the assumption that they correspond to first-order aperiodic factors. Based on the relationships of thermodynamics, Arden Buck equation, and given transient functions, analytical dependencies are obtained to identify the parameters of the transient response of the capacitive relative humidity sensor. The results of several field experiments are processed and analyzed in accordance with the proposed mathematical model. The effect of anomalous sensor readings is revealed and consists in the fact that with simultaneous stepwise changes in humidity and temperature, the sensor records a change in relative humidity with the opposite sign. It has been established that the cause of the anomalous results of relative humidity measurements is the presence of a large difference between the response time constant of the absolute humidity sensing element and that of the temperature sensor. Measures are proposed to prevent anomalous results of relative humidity measurements.

A measurement method for capacitive sensors based on a versatile direct sensor-to-microcontroller interface circuit

In the paper, there is presented a new time-domain measurement method for determining the capacitance values of capacitive sensors, dedicated, among others, to capacitive relative humidity sensors. The method is based on a versatile direct sensor-to-microcontroller interface for microcontrollers with internal analog comparators (ACs) and with precision voltage reference sources, e.g. digital-to-analog converters (DACs). The reference source can be replaced by a resistive divider attached to the negative input of the AC. The interface circuit consists only of a reference resistor Rr, a given capacitive sensor working as a voltage divider, and a microcontroller (its peripherals: AC, timer, DAC, I/O pins). A prototype of the proposed complete solution of a compact capacitive smart sensor based on an 8-bit ATXmega32A4 microcontroller has been developed and tested. The maximum possible relative inaccuracy of an indirectly measurable capacitance was analysed, and experimental research was also performed. The results confirmed that the relative errors of value determination for a capacitive sensor are less than ±0.06%, which corresponds to a capacitance measurement accuracy of less than 0.1 pF for a range of measured capacity values from 100 pF to 225 pF, which in turn corresponds to at least a 0.5% relative humidity resolution for commercial capacitive RH sensors (e.g. TE Connectivity HS1101LF and Philips H1).

The Measurement of the Internal Relative Humidity of Concrete at Early Ages

Abstract Measurement of the internal relative humidity (RH) in concrete can be used to describe the availability of water, which has implications on several properties including shrinkage and hydration rate. However, accurate measurement of the internal RH of concrete is challenging, especially at early ages. This article examines different methods of using capacitive sensors for measuring the internal RH of concrete. The capacitive RH sensors are typically placed inside a void that enables the RH of the air within the void to be measured. Condensation of water vapor (caused by variation in temperature of the sample) has been observed to result in unreliable measurements, calibration issues, sensor malfunction, and sensor damage. Sensors may be heated cyclically to evaporate the condensed water vapor; however, many of these sensors failed to operate properly. Continuously heated sensors maintain the sensor at a higher temperature than the ambient concrete to reduce the potential for the formation of condensation. The continuously heated sensor provided the best results and is a good candidate for future testing. Suggestions for the measurement of internal concrete RH and calibration of the sensors are provided.

Identification of the transient response of the capacitive relative humidity sensor

Identification of the transient response of the capacitive relative humidity sensor

Fabrication of a capacitive relative humidity sensor using aluminum thin films deposited on etched printed circuit board

A capacitive humidity-sensing device was created by thermal evaporation of 99.999% aluminum. The substrate used for the coating was etched double-sided printed circuit board. The etched printed circuit board serves as the dielectric of the capacitor while the aluminum thin films deposited on either side serve as the plates of the capacitor. The capacitance was measured before and after exposure to humidity. The device was then calibrated by comparing the readings of capacitance with that of the relative humidity sensor of the Vernier LabQuest2. It was found that there is a linear relationship between the capacitance and relative humidity given by the equation C=1.418RH+29.139 where C is the capacitance and RH is the relative humidity. The surface of the aluminum films is porous and it is through these pores that water is adsorbed and capillary condensation occurs, thereby causing the capacitance to change upon exposure to humidity.

Investigating soil water retention characteristics at high suctions using relative humidity control

A technique for controlling relative humidity (RH) is presented, which involves supplying a sealed chamber with a continuous flow of air at a computer-regulated RH. The desired value of RH is achieved by mixing dry and wet air at appropriate volumes and is measured for servo-control at three locations in the chamber with capacitive RH sensors and checked with a sensitive VAISALA sensor. The setup is capable of controlling RH steadily and continuously with a deviation of less than 0.2% RH. The technique was adopted to determine wetting soil-water retention curves (SWRC) of statically compacted London Clay, under both free-swelling and constant volume conditions. The RH within the chamber was increased in a step-wise fashion, with each step maintained until vapour equilibrium between the chamber atmosphere and the soil samples was established. Independent filter paper measurements further validate the method, while the obtained retention curves complement those available in the literature for lower ranges of suction.

Passive Wireless Multi-Parameter Sensor System for Hermetic Environment Monitoring

This paper presents a passive wireless multi-parameter sensor system, which is suitable for monitoring the environment in hermetic space like industrial painting room, coal mine, and so on. The system consists of two subsystems: the primary system and the secondary system. The primary system transmits power and receives modulated data through a coupled transformer composed of primary and secondary inductors printed in their subsystems, respectively. The secondary system comprises micro capacitive temperature, pressure and relative humidity sensors and uses a relaxation oscillator to convert capacitance to frequency. Using time division multiplexing unit, it can simultaneously measure multiple sensors. The converted frequency signal is used to vary the amplitude of the waveforms on the coupling inductors. The modulating signal is recovered from the carrier using the envelope detection unit. The measurement results indicate that the pressure, temperature, and relative humidity sensitivities of the passive wireless multi-parameter sensor system are 5.6 fF/hPa, 250fF/°C and 71.4 fF/%RH, respectively. Three sensors linearly response to the change of the environment and the system has good consistency.

Inkjet and microcontact printing of functional materials on foil for the fabrication of pixel-like capacitive vapor microsensors

The work presented demonstrates the utilization of micro-contact printing of self-assembled monolayers (SAMs) of gold nanoparticles (NPs) to pattern the porous thin metallic film composing the top electrode of an ultra-fast capacitive relative humidity sensor based on miniaturized parallel-plates electrodes. The rest of the device, which occupies an area of only 0.0314mm2, is fabricated by inkjet printing stacked individual drops of functional materials, namely gold NPs for the bottom electrode and a polymeric humidity sensing layer, on a polymeric foil. Compared to other printing methods, the use of microcontact printing to pattern the top electrode enables the additive transfer of a solvent-free metallic layer that does not interact chemically with the sensing layer, permitting the thinning of the latter without risk of short-circuits between electrodes, and broadening the range of usable sensing materials for detection of other gases. Thinning the sensing layer yields to ultra-fast response devices with high values of capacitance and sensitivity per surface area. The fabrication process is compatible with low heat-resistant polymeric substrates and scalable to large-area and large-scale fabrication, foreseeing the development of low-cost vapor sensing sheets with high space–time resolution, where every sensor would correspond to a pixel of a large array.

Random networked multi-walled carbon nanotube film as an upper electrode for high-speed capacitive humidity sensors

This paper reports the fabrication of a high-speed polyimide capacitive relative humidity sensor with a multi-walled carbon nanotube (MWCNT) film network electrode, which is coated by the spray-deposition process. A spray-coated MWCNT film top electrode has high surface area, low resistivity, and interconnected open pore structure, which makes water molecules accessible to all polyimide surfaces. Capacitive humidity sensors with MWCNT film electrodes have three times faster adsorption and desorption times than porous metal electrode sensors. The much faster response times of MWCNT electrode sensors is attributed to the percolated pore network, which allows more water molecules to be accessible to polyimide surfaces.

Capacitive humidity sensors based on the dielectrophoretically manipulated ZnO nanorods

In this paper, the capacitive relative humidity sensors based on dielectrophoretically manipulated ZnO nanostructure were constructed and studied. The humidity-sensitive ZnO nanorods were first synthesized by the thermal decomposition technique and deposited among the micromachined electrodes pairs via dielectrophoretic manipulation. The manipulated samples were detected as the capacitive sensors in which the nanostructured ZnO acted as the sensing elements. The capacitive humidity sensors were constructed by detecting the variations of the ZnO dielectric constant changing with the humidity environment. The results showed that the sensor had high humidity sensitivity, good stability, fast response/recovery time and well reproducibility. The dielectrophoresis manipulation can also improve the sensitivity of the sensor efficiently.