Present work deals with the development of a low cost moisture sensor for measuring moisture in the range of 2.5–25 ppm using nanoporous thin film of alumina (γ-Al 2 O 3 ). Thin film of γ-Al 2 O 3 prepared by low cost and simple sol–gel method offers very high surface to volume ratio with distribution of micro pores for moisture detection. Pore morphologies of the film have been studied by FESEM, AFM, XRD and BET methods. To form a capacitive sensor, the porous film of 3–6 μm thickness was dip-coated in between two parallel gold electrodes. The characteristic of the sensors have been examined by Agilent 4294 A impedance analyzer for the frequency range of 1–20 kHz. The characteristics of the sensor can be used for making trace moisture sensor for commercial applications. The capacitance change with moisture is almost linear with 1.05% nonlinearity and 0.33% hysteresis.

A novel sol–gel thin film porous alumina based capacitive sensor for measuring trace moisture in the range of 2.5–25 ppm

A microcontroller-based digital hygrometer system using a low-cost moisture sensor in the range of 3.7–100 ppm has been developed. The sensor is of capacitive type and consists of nanoporous thin film of alumina $(\gamma - \hbox{Al}_{2}\hbox{O}_{3})$ dip coated in between two parallel gold electrodes onto an alumina substrate. The behavior of the sensor has been modeled with an object to develop a signal processing circuit to convert capacitance change into frequency. The detection electronics circuit is based on a relaxation oscillator whose output is suitable for interfacing with a digital device. The sensor has been characterized with the circuit and then output frequency is calibrated in terms of ppm. The accuracy of the digital hygrometer when compared with the commercial dew point meter is found to be $\pm\!$ 1 PPM.

A Digital Hygrometer for Trace Moisture Measurement

In this paper, we propose novel fully analog uncalibrated readout circuits for wide-range capacitance estimation. The working principle is based on a modified De-Sauty ac bridge configuration where two capacitances and two resistances are employed. Through the use of a voltage controlled resistor and of a suitable feedback loop, an accurate estimation over a wide range of the sensor capacitance variation is possible, also without the knowledge of the other bridge component values. The first two proposed solutions are able to estimate, experimentally, ~1.5 decades (optimized range: [25-840 nF]) and more than 3 decades [900 pF-1.1 μF] of capacitive variations, respectively, ensuring, continuously, the bridge equilibrium condition; the third interface allows the evaluation of the sensor capacitance also outside this condition, increasing the capacitive variation decades to more than 4 [81 pF-1.1 μF]. The proposed topologies have been also modified to detect the capacitive sensor parasitic resistance. In this case, experimental results on PCB have demonstrated the capability to simultaneously estimate ~ 1.7 capacitive [2-100 nF] and 1.5 resistive variation decades [33 kΩ-1.2 MΩ]. Finally, a suitable optimization of the operating range of the first proposed solution, obtained through a proper passive components sizing, has been performed with the aim to employ the commercial humidity sensor HCH-1000 in a high accuracy (11.4 bits) RH% detection.

Novel Modified De-Sauty Autobalancing Bridge-Based Analog Interfaces for Wide-Range Capacitive Sensor Applications

Efforts have been made by the researchers to enhance the sensitivity of a humidity sensor by selecting and fabricating different nanostructures thin films and optimizing their pore morphology. However, other characteristics, such as nonlinearity, hysteresis, reproducibility, response and recovery times, and selectivity, are equally important for real time measurement, monitoring, and controlling of humidity. It is reported that a low signal frequency is essential for enhanced sensitivity, but the effect of the signal frequency on other characteristics is rarely investigated. We have investigated the role of the signal frequency on the response characteristics of a nanostructure ceramic thin-film interdigitated electrode humidity sensor for the first time. Our studies show that although the sensitivity is high at low frequency, the performance of the sensor is highly nonlinear and frequency dependent. At high frequency above 100 kHz, the sensitivity is reduced, but other characteristics of the sensors are improved significantly. Improved characteristics are due to the reduced effect of double layer capacitance that plays a significant role for extreme sensitivity at low frequency. The effect of double layer capacitance is rarely considered for evaluating the response characteristics in the literatures. Results show that at 200-kHz frequency, nonlinearity and hysteresis are 9.39% and 16.77%, while at 100 Hz, these values are 5588% and 70.8%, respectively. Improve response characteristics will reduce the complexity of signal conditioning, particularly nonlinearity and hysteresis, the compensation of which is essential for almost all types of humidity sensors.

Investigation of the Electrical Characteristics on Measurement Frequency of a Thin-Film Ceramic Humidity Sensor

This paper presents a novel environmental sensor for humidity detection, based on an air-filled substrate integrated cavity resonator (SIW). The sensor, made of a multilayer printed circuit board, exhibits high sensitivity, repeatability stability and fast response/recovery time. This is of particular interest for environmental sensor applications that generally require low-cost and excellent sensing performance and properties. The detection principle is based on a frequency shift due to the permittivity variation of the humid air. A sandwich of a middle substrate containing a large air-filled region accentuates the sensitivity facing the humidity. The possible use of top and bottom layers of the low-cost standard substrate, such as FR-4, allows for a very low-cost and self-packaged integrated millimetre-wave system. The operation frequency of the proposed structures operates in X-Band at around 7.63 GHz and exhibits a frequency shift of 79.05 MHz for relative humidity in the range of 20–85% RH with an operating temperature at 30 °C. The air-filled SIW sensor presents a new solution that improves the sensor’s sensitivity for environmental sensing and has the advantage of providing a low-cost manufacturing process.

Air-filled substrate integrated cavity resonator for humidity sensing

A simple signal conditioning circuit for converting capacitance change into frequency for capacitive humidity sensors is presented. It is based on a relaxation oscillator in which output frequency is linearly related to the capacitive unbalance of an active bridge. The design, analysis, and experimental results of the circuit and its application to a sol gel thin film porous γ-Al 2 O 3 based humidity sensor are reported. Experimental results confirm the theoretical value predicted. The circuit covering wide capacitance measurement range has the potential for remotely monitoring measurement parameters accurately.

An oscillator based active bridge circuit for converting capacitance change into frequency for capacitive humidity sensor

The present work is aimed to develop porous silicon (PS)-based sensor to measure very low humidity concentration with possible industrial applications This work is in continuation of our previous results reported (Islam et al, Sens Lett 6, 746 (2008)) that the PS layer with pore morphology above 10 nm shows good response above 50 ppmV To obtain improved sensitivity in the lower range below 50 ppmv, the porous structure with pore dimensions below 10 nm is fabricated by controlling the anodization parameters using precision Solatron Electrochemical Cell (1280C) The sensor based on capacitive principle has porous top metal electrode having pores above 1 µm formed on the porous layer by screen printing technique The mesh structure increases the area of the electrodes and allows the moisture molecules penetrating through the mesh thus improving both sensitivity and ohmic nature of the contacts The electrical properties were evaluated by impedance spectroscopy for different contents of water vapour in N2 gas with Agilent 4294A Impedance Analyzer

A. Gangopadhyay

Papers

An oscillator based active bridge circuit for converting capacitance change into frequency for capacitive humidity sensor

Porous silicon in low moisture content dry gas by impedance spectroscopy