Introduction to Gas Detection - Oliver IGD - #9

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Infra-RedIn recent years infra red transducers have been employed for flammable gas detection.The technique is based on the fact that certain types of gases absorb certain wavelengthsof infra red energy. Gases which can be detected and measured by this technique are those which are termed heteroatomic gases. This means a gas which has a chemical bondsuch as CO (Carbon Monoxide), CO2 (Carbon Dioxide), CH4 (Methane). Gases that donot have such a chemical bond i.e N2 (Nitrogen), He (Helium) cannot be measured withthis technique.In practice a filament is used to generate the infra red energy. The Infra red light (energy)is focused into a beam usually using a convex mirror. The light beam passes through thegas to be measured and falls onto two detectors. The detectors are made selective to aparticular gas by being used in conjunction with an optical light filter that only passes anarrow range of light frequencies. One detector is made selective to the target gas. Theother detector is chosen to be in an area of the infra red spectrum that is not likely to haveany of its energy absorbed. The difference between these two signals is related to the measured gas concentration. If the compensating channel filter is correctly chosen itshould correct for any degradation in the light source or soiling of the optics.The benefit of this technique is that it is so selective. This is a bonus for monitoring gasessuch as carbon dioxide, which is difficult to measure by any other technique. Its selectivity can be a handicap for flammable gas detection as the bandwidth of the detector must be acompromise to be narrow enough to exclude the detection of non flammable gases. Thiscan inevitably mean that some flammable gases depending where they occur will beexcluded. The device does not therefore act as detector for all flammable gases unlessmultiple detectors are employed which increases cost. Other problems can arise due to interference’s from water vapour which is a good infra red absorber and has multiplespectra. Toxic Gases Electrochemical CellsAn electrochemical cell is a type of air battery developed from fuel cell research. Suchdevices are self powered micro fuel cells. They use a capillary barrier to limit the amount of toxic gas to be detected reaching an assembly made up of electrodes immersed in anelectrolyte solution. The electrodes are usually platinum or gold. There are usually threeelectrodes termed ‘reference’, ‘counter’ and ‘measuring’. An electric potential is appliedbetween the measuring and reference electrode. An electro-chemical reaction takes placewhen the target gas meets the measuring electode. Electrons are liberated and migratethrough the electrolyte to the counter electrode. The current that is generated by thisaction is directly proportional to the gas concentration. The cells are made selective to a gas by the choice of electrode material, electrolyte and sometimes by employing absorbingchemical layers in the diffusion membrane to remove interfering gases.This technique works very well for a number of gases and has the benefit that it requireslittle power making it ideal for portable equipment. In some cases a sensor for a particular gas may also respond to other gases so care should be taken to understand thecharacteristics of the sensor to be employed. The Oliver IGD Tocsin 101 range of toxic gassensors are based on electrochemical sensors with a typical in service life of 2-3 years.The sensors can be easily replaced in field.