Amir Amini

Drift-Like Terms Minimization in the Responses of a Generic Tin Oxide Gas Sensor

The responses of a tin oxide gas sensor vary with humidity level in the surrounding atmosphere. Such response variations can cause error in the estimation of the concentration level of the target gas and need compensation measures. Different methods have been used to compensate the effect of humidity, which usually require utilization of other parallel environmental sensors and costly data fusion methodology. Particularly, such a drift alters the response patterns obtained from a real or virtual sensor array and hinders gas recognition. Here, we report response patterns recorded from a virtual array made by operating temperature modulation at different ambient humidity levels for three different target gases. Humidity level varied from 30-70% and response patterns were recorded for methanol, ethanol and 1-propanol at a wide concentration range. It is shown that by utilizing the thermal shock-induction method for the temperature modulation of the sensor, the drift levels are low, and with a single set of training data collected at RH=50%, responses obtained in the whole humidity range can be discriminated from each other. The clusters volumes in the feature space grow with the span of the ambient humidity variations, but they remain separate allowing gas recognition.

Differentiating among Gas Mixtures Using a Single Tin Oxide Gas Sensor

Despite all their positive features oxide-based resistive gas sensors are nonselective and respond similarly for different gas and gas mixtures. The authors have recently demonstrated that the response patterns generated by a generic tin oxide gas sensor induced by thermal shocks contain considerable amounts of information regarding the nature of the present gas. Here, the results of using a similar technique on different two-component gas mixtures are reported. The gas mixtures are (1-butanol)x (2-butanol)1-x, (1-propanol)x (2-butanol)1-x, (1-butanol)x (1-propanol)1-x, and (1-butanol)0.33 (2-butanol)0.33, (1-propanol)0.33, each at various total concentrations. The diagnostic features of the response patterns were extracted, by applying wavelet transform, and used for their discrimination in a three dimensional feature space. The positions of the clusters related to different gases are consistent with their composition and facilitate estimating the individual concentrations of the components.

Obtaining Highly Selective Responses from a Bulk Tin Oxide Gas Sensor

Generic gas sensors are commonly used for the detection of different airborne contaminants due to their high sensitivity, long life and low cost, but they generally suffer from the variety of drifts and the lack of selectivity. Different techniques have been developed for selectivity enhancement in metal oxide gas sensors, among which operating temperature modulation is well known. It has been observed that sharp pallet temperature changes provide more analyte-related information. Due to the high thermal capacitance of the device, applying step voltage pulses to a bulk tin oxide gas sensor fails to provide step pallet temperature variations. On the other hand, the low thermal capacity of the custom made microheater gas sensors renders them vulnerable to all kinds of thermal noise and agitations. A novel technique is reported for temperature modulation, which facilitates sharp temperature rises of the gas sensitive pallets in generic gas sensors [. In this technique, a sharp heating voltage spike, considerably surpassing the nominal heating voltage, is applied prior to each heating voltage step. The thermal impact of these spikes is adjusted by controlling v2dt for obtaining the closest variations to the ideal temperature profile. Here, the advantages and effectiveness of the technique are demonstrated by differentiating among iso-butanol, tert-butanol, 1-butanol and 2-butanol contaminations in a wide concentration range in air using only a single generic tin oxide gas sensor.

Selective Hydrogen Detection in a Highly Contaminated Background Using only a Single Generic Metal Oxide Gas Sensor

Detection of highly ppm range hydrogen concentration in atmospheres contaminated with various volatile organic compounds is in demand for numerous applications. Different devices and techniques have been applied for the problems which are mostly based on utilization of hydrogen permeable membranes. Here, we have used a single generic metal oxide gas sensor for this task. No filter or membrane is utilized. The operating temperature of the sensor is modulated with a voltage waveform specifically designed for producing step-like temperature changes on the oxide pallet. By applying four different step-like temperature jumps, each of 1s duration, the sensor produces response patterns which are processed with common pattern recognition techniques. The technique was examined by its practical use for the ~10 ppm (volume) hydrogen measurement in a background containing ~1000 ppm ethanol. The analysis takes only 4s, and the obtained patterns are reproducible.

Fast Diagnosis of Volatile Organic Compounds with a Temperature-Modulated Chemoresistor

Virtual arrays formed by operating temperature modulation of a commercial non selective chemoresistor have been utilized for gas identification. Here, we are reporting the details of a refined system which distinctly classifies methanol, ethanol, 1-butanol, acetone and hydrogen contaminations in a wide concentration range. A staircase voltage waveform of 5 plateaus is applied to the sensor’s microheater and gas recognition is achieved in 25 s. Sensor’s output is modeled by an “autoregressive moving average with exogenous variables” (ARMAX) model. The modeling parameters obtained for an unknown analyte are utilized as the components of its feature vectors which afford its classification in a feature space. Cross-validation in the 5 to 100 ppm concentration range for H2, and 200 to 2000 ppm for the other analytes examined, resulted in an overall classification success rate of 100%.