Sunday, April 22, 2007

Gas Sensors

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 Vapour.

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 a 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.

Copper Oxide (CuO) thin films were deposited using a reactive DC sputtering method for gas sensor applications. The structure of the films determined by means of an X-Ray diffraction method indicates that the phase of Copper Oxide can be synthesized in the total pressure and temperature ranges of 6-8.5 mbar and 151-192 °C, respectively. The resistivity of the film synthesized at a substrate temperature of 192 °C increases from 0.104 to 0.51 Ohm-m after absorbing Carbon Dioxide gas at 135 °C. The gas sensitivity of the film synthesized at the substrate temperature of 192 °C increases up to 5.1 in the presence of Carbon Dioxide gas at 160 °C. The gas sensitivity in the presence of Nitrogen gas reaches only 1.43 even at 200 °C.

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