film gas sensor has been much of interest because of microelectronic batch-fabricated compatibility, reproducibility, and ability to form multilayer device structures. In this work, thin film based gas sensing circuit is designed for immediate applications of CO detection for environmental monitoring. Ion assisted deposition (IAD) process offers several advantages for gas sensor fabrication, including reactive deposition for gas-sensitive metal-oxide material optimization and improved thin film adhesion for better device reliability. The metal oxide layer was deposited on alumina or glass substrates. The sensors were tested with reducing gases, in the temperature range between 200oC and 350 oC and the electrical change in gas sensor is detected.
Gas sensors interact with a gas to initiate the measurement of its concentration. The gas sensor then provides output to a gas instrument to display the measurements. Common gases measured by gas sensors include ammonia, aerosols, arsine, bromine, carbon dioxide, carbon monoxide, chlorine, chlorine dioxide, Diborane, dust, fluorine, germane, halocarbons or refrigerants, hydrocarbons, hydrogen, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrogen selenide, hydrogen sulfide, mercury vapor, nitrogen dioxide, nitrogen oxides, nitric oxide, organic solvents, oxygen, ozone, phosphine, silane, sulfur dioxide, and water vapor. Important measurement specifications to consider when looking for gas sensors include the response time, the distance, and the flow rate.
The response time is the amount of time required from the initial contact with the gas to the sensors processing of the signal. Distance is the maximum distance from the leak or gas source that the sensor can detect gases. The flow rate is the necessary flow rate of air or gas across the gas sensor to produce signal. Gas sensors can output a measurement of the gases detected in a number of ways. These include percent LEL, percent volume, trace, leakage, consumption, density, and signature or spectra. The lower explosive limit (LEL) or lower flammable limit (LFL) of a combustible gas is defined as the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and ignited. In gas-detection systems, the amount of gas present is specified in terms of % LEL: 0% LEL
being a combustible gas-free atmosphere and 100% LEL being an atmosphere in which the gas is at its lower flammable limit. The relationship between % LEL and % by volume differs from gas to gas. Also called volume percent or percent by volume, percent volume is typically only used for mixtures of liquids. Percent by volume is simply the volume of the solute divided by the sum of the volumes of the other components multiplied by 100%. Trace gas sensors given in units of concentration: ppm. Leakage is given as a flow rate like ml/min. Consumption may also be called respiration given in units of ml/L/hr. Density measurements are given in units of density: mg/m^3. A signature or spectra measurement is a spectral signature of the gases present; the output is often a chromatogram. Common outputs from gas sensors include analog voltage, pulse signals, analog currents and switch or relays. Operating parameters to consider for gas sensors include operating temperature and operating humidity.
1.1.1.
APPLICATIONS OF GAS SENSORS
(i) Gas Leak Protection: - These are areas of industry where the possibility of flammable gas build-up is small, but the consequences of a gas escape could be catastrophic. These tend to be industries which by their nature have large volumes of gases piped around the works:
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Engineering companies
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Metal working plants
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Research laboratories
(ii) Confined Space Entry
The most prominent application for portable gas detection instruments. The instrument is used to check the atmosphere of sewers, tanks and other vessels prior to entry for maintenance purposes. These instruments invariably are 'multi-gas'. They have 3 or even 4 sensors included in the package. Large volumes of these instruments are purchased by:
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Public utilities - especially water and telecoms
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Chemical and petrochemical - for entry into vessels
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Cabling contractors
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Piling contractors
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Tunnelling contractors
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Civil engineers
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Landfill operators
(iii) Hazardous Area: Working Areas of industry where the build-up of flammable gas or vapour is an ever present danger. These instruments are very often the same multi-gas instruments used for confined space entry, but there are areas where single gas monitors ('explosimeters') are used. Typical industrial sectors here are:
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Chemical and petrochemical industries
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Oil/gas exploration
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Mining