A Capacitance Humidity-Sensing Transducer

This paper presents a design of a smart humidity sensor. First we begin by the modeling of a Capacitive MEMS-based humidity sensor. Using neuronal networks and Matlab environment to accurately express the non-linearity, the hysteresis effect and the cross sensitivity of the output humidity sensor used. We have done the training to create an analytical model CHS “Capacitive Humidity Sensor”. Because our sensor is a capacitive type, the obtained model on PSPICE reflects the humidity variation by a capacity variation, which is a passive magnitude; it requires a conversion to an active magnitude, why we realize a conversion capacity/voltage using a switched capacitor circuit SCC. In a second step a linearization, by Matlab program, is applied to CHS response whose goal is to create a database for an element of correction “CORRECTOR”. After that we use the bias matrix and the weights matrix obtained by training to establish the CHS model and the CORRECTOR model on PSPICE simulator, where the output of the first is identical to the output of the CHS and the last correct its nonlinear response, and eliminate its hysteresis effect and cross sensitivity. The three blocks; CHS model, CORRECTOR model and the capacity/voltage converter, represent the smart sensor.

The increasing need of environmental and physiological monitoring propels the development of humidity sensors, especially for applications aimed at personal health detection. Among the various types of humidity sensors, capacitive humidity sensors are particularly noted for their rapid response, low power consumption, and high sensitivity. However, conventional capacitive humidity sensors, including widely used planar interdigitated electrodes (IDE) and parallel plate (PP) structures, face challenges with achieving high sensitivity and producing stable output signals. Thus, this work introduces a novel 3D electrode configuration called the “Go‐frame (GF),” designed to surmount these limitations by enhancing linearity, sensitivity, and stability. The GF humidity sensor consists of a pin structure on the bottom electrode, which is coated with a sensing material, and capped with a grid‐shaped top electrode. The experimental findings demonstrate that the GF humidity sensor outperforms both IDE and PP structures, achieving a sensitivity of 65.27 fF/%RH, low hysteresis of 4.06%, and enhanced signal stability. Additionally, the 3D‐structured humidity sensor exhibits high repeatability and long‐term stability over a week. This advancement in electrode design represents a significant innovation in capacitive humidity sensing technology, promising widespread adoption in critical applications, including real‐time human respiration monitoring.

Humidity sensors have been extensively used in various fields, and numerous problems are encountered when using humidity sensors, including low sensitivity, long response and recovery times, and narrow humidity detection ranges. Using graphene oxide (G-O) films as humidity sensing materials, we fabricate here a microscale capacitive humidity sensor. Compared with conventional capacitive humidity sensors, the G-O based humidity sensor has a sensitivity of up to 37800% which is more than 10 times higher than that of the best one among conventional sensors at 15%–95% relative humidity. Moreover, our humidity sensor shows a fast response time (less than 1/4 of that of the conventional one) and recovery time (less than 1/2 of that of the conventional one). Therefore, G-O appears to be an ideal material for constructing humidity sensors with ultrahigh sensitivity for widespread applications.

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.

Flexible humidity sensors have attracted substantial attention due to the promising application in medical health monitoring. Up to now, the realization of such devices with ultrafast response/recovery and controllable morphology via a facile and cost-effective approach still remains a severe challenge. Herein, a high-performance flexible capacitive humidity sensor by creating the arc-shaped hollow structure with the poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] nanocone arrays is developed. Particularly, a facile method based on combining the hot-pressing method and the anodized aluminum oxide template transfer method is proposed to realize the large-scale preparation of the nanostructure arrays with controllable morphology. Benefiting from the open arc-shaped hollow structure and the large-area P(VDF-TrFE) nanocone arrays, the proposed humidity sensor shows several conspicuous features, such as ultrafast response/recovery time (3.693/3.430 s), long-term stability (25 days), excellent bending stability (10,000 cycles), unaffected capacitance response to humidity in a certain temperature range (20−50℃) and high selectivity toward water vapor. The above salient superiorities enable the proposed humidity sensor to be successfully utilized in extraction of a variety of physiological signals including breathing detection, skin noncontact sensing, and real-time monitoring of diaper wetting process and skin humidity, which reveals great potential value in prevention and diagnosis of many diseases.

The parallel plate capacitive humidity sensor based on the grid upper electrode is considered to be a promising one in some fields which require a humidity sensor with better dynamic characteristics. To strengthen the structure and balance the electric charge of the grid upper electrode, a strip is needed. However, it is the strip that keeps the dynamic characteristics of the sensor from being further improved. The numerical method is time- and cost-saving, but the numerical study on the response time of the sensor is just of bits and pieces. The numerical models presented by these studies did not consider the porosity effect of the polymer film on the dynamic characteristics. To overcome the defect of the grid upper electrode, a new structure of the upper electrode is provided by this paper first, and then a model considering the porosity effects of the polymer film on the dynamic characteristics is presented and validated. Finally, with the help of software FLUENT, parameter effects on the response time of the humidity sensor based on the microhole upper electrode are studied by the numerical method. The numerical results show that the response time of the microhole upper electrode sensor is 86% better than that of the grid upper electrode sensor, the response time of humidity sensor can be improved by reducing the hole spacing, increasing the aperture, reducing film thickness, and reasonably enlarging the porosity of the film.

This study demonstrates the vertically integrated environment sensor with a capacitive relative humidity sensor (RH-sensor) and a resistive temperature detector (RTD) using the TSMC 0.18μm 1P6M CMOS process. Features of this study are: (1) multiple sensing units could be vertically implemented and integrated in one chip; (2) fast response humidity sensor is realized based on fence-shaped capacitor (supported by oxide pillars) with polyimide (PI) filler; (3) simple post-CMOS processes include metal wet etching and pneumatic dispensing of PI. Measurement results indicate the RH sensor with sensitivity of 0.051%/%RH and response time of 16s; and temperature coefficient of resistance (TCR) of RTD is 0.28%/°C.

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

Layers of graphene oxide (GO) deposited on various substrates have been investigated for their capacitive and resistive features. It has been shown that such layers demonstrate large specific capacitances. Here we report the fabrication of a first GO-on-textile capacitive humidity sensor. GO layers are deposited on aluminum silicate textile, and the impedances of the deposits are measured at different atmospheric conditions using various voltage sweeping frequencies. The results demonstrate the dependence of the layer impedance on the humidity level in the surrounding atmosphere. Further, it is clarified that the layer capacitance is humidity dependent. A capacitive humidity sensor is designed and fabricated. The capacitance of a typical device varies from 130 pF, measured in dry synthetic air, to 5000 pF measured in air with 95% relative humidity (RH). Interestingly, these high sensitivities are measured at low frequencies (f=0.01 Hz); sensitivity to humidity decreases as the voltage sweeping frequency increase. For the first time, it is shown that the sensitivity of these capacitive humidity sensors considerably increase when very low frequencies are used for capacitance measurement. The ratio of the capacitance at RH= 95% to the capacitance at RH <; 0.1% both measured at 0.01 Hz is 32 times larger than the same calculated at 100 Hz. The device is suitable for wearable electronic applications.

The need for humidity sensors in various fields have led to the development and fabrication of sensors for use in industries such as the medical, textile, and laboratories. This chapter reviewed humidity sensors, major types and applications with emphasis on the optical fiber, nanobricks, capacitive, resistive, piezoresistive and magnetoelastic humidity sensors. While optical fiber sensors are best for use in harsh weather conditions, the nanobricks sensors have excellent qualities in humidity sensing. Capacitive sensors make use of impedance and are more durable than the equivalent resistive sensors fabricated with ceramic or organic polymer materials and have short response and recovery times which attest to their efficiency. Piezoresistive sensors have fast response time, highly sensitive and can detect target material up to one pictogram range. Magnetoelastic sensors are very good and can measure moisture, temperature and humidity between 5% and 95% relative humidity range. It was concluded that sensors have peculiar applications.

In order to correct the solar radiation error of relative humidity, the mainstream capacitive sounding humidity sensor HC103M2 is selected and investigated by simulation analysis and experimental verification. First, the basic theories for solar radiation error and sensor error itself are elaborated, and simulational and experimental platforms are introduced. The computational fluid dynamics (CFD) method is utilized to theoretically investigate the dry error of the humidity sensor caused by solar radiation heating, which is related to radiation intensity, altitude, and solar elevation angle as well as reflectivity, thickness, and shape of the shield. Then, in order to verify the accuracy of the simulation, an experimental platform including a humidity sensor and two temperature sensors to measure the solar radiation heating is built to analyze the relative error of humidity obtained by the CFD simulation and experiment. It is found that their maximum deviation is 3.30% and the average error is 1.94%, which indicates that the calculation using the CFD method is accurate and feasible. In order to easily and operationally predict the solar radiation heating of the humidity sensor, a back propagation (BP) neural network fusion algorithm based on three inputs of radiation intensity, air pressure, and solar elevation angle is proposed. Compared with the solar radiation heating obtained by CFD simulation, the maximum absolute error is about 0.2K, and the relative error of humidity is about ±1.30%. Finally, a case of vertical humidity profile correction considering the temperature-sensitive error of HC103M2 is analyzed. The response time of sensor measurement and the airflow into the shield are discussed as well. The corrected results after taking solar radiation heating into account are more similar to those measured by RS92 and cryogenic frost point hygrometer (CFH). This result shows that the prediction model is accurate, which may be applied to correct the dry error and further improve the accuracy of humidity measurement.

In this study, we fabricated a highly flexible fiber-based capacitive humidity sensor using a scalable convergence fiber drawing approach. The sensor’s sensing layer is made of porous polyetherimide (PEI) with its porosity produced in situ during fiber drawing, whereas its electrodes are made of copper wires. The porosity induces capillary condensation starting at a low relative humidity (RH) level (here, 70%), resulting in a significant increase in the response of the sensor at RH levels ranging from 70% to 80%. The proposed humidity sensor shows a good sensitivity of 0.39 pF/% RH in the range of 70%–80% RH, a maximum hysteresis of 9.08% RH at 70% RH, a small temperature dependence, and a good stability over a 48 h period. This work demonstrates the first fiber-based humidity sensor fabricated using convergence fiber drawing.

We propose a high-performance porous metal–insulator–metal-type capacitive humidity sensor based on a functional polymer mixed with TiO2 microparticles subjected to inductively coupled plasma (ICP) and reactive-ion etching (RIE) treatments. The humidity sensor is composed of a porous top electrode, a TiO2-containing functional polymer humidity-sensitive layer subjected to two types of oxygen plasma treatment, a bottom electrode, and a glass substrate. The initial O2 ICP dry-etching utilizes higher intensity plasma for deep etching in the inlet holes on the top electrode to increase the contact area and shorten the vapor absorption path, thereby yielding high sensitivity and low hysteresis. Further, the RIE treatment leads to roughening of the polymer etching surface and further improving the performance of the humidity sensor. The functional polymer mixed with TiO2 microparticles exhibits excellent hysteresis over a wide humidity sensing range. The fabricated sensors are tested at various relative humidity (RH) values, achieving an ultra-low hysteresis of 0.64% RH at 60% RH, a high sensitivity of 1.24 pF/% RH, a fast response time of less than 25 s, good temperature dependence, and a stable capacitance value with a maximum error rate of 0.15% over 120 h of continuous testing.

Open porous multi-walled carbon nanotube (MWCNT) film networks were used as an upper electrode of polyimide (PI)-based capacitive humidity sensor for highly fast response time. MWCNT films were deposited by spray-dry method and patterned by O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma. PI capacitive humidity sensors with MWCNT film electrodes showed a very short response time of less than 3 s and a good linearity of 0.9995 compared to the humidity sensors with porous Cr electrodes. The large improvement in response time was explained on the base of a percolated pore network that is more accessible for water molecules.