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Pellistors employ catalytic combustion to measure combustible gases or vapours in air up to the Lower Explosive Limit (LEL)* of the gas.
The standard sensor consists of a matched pair of elements, typically referred to as a detector and compensator (reference element). The detector comprises a platinum wire coil embedded within a bead of catalytic material. The compensator is similar except that the bead does not contain catalytic material and as a consequence is inert.
Figure 1 - Pellistors
Both elements are normally operated in a Wheatstone bridge circuit, that will produce an output only if the resistance of the detector differs from that of the compensator.
The bridge is supplied with a constant dc voltage that heats the elements to 500-550°C. Combustible gases are oxidised only on the detector element, where the heat generated increases its resistance, producing a signal proportional to the concentration of combustible gas. The compensator helps to compensate for changes in ambient temperature, pressure, and humidity, which affect both elements equally.
Most pellistors have the pairs of elements housed in separate metal cans. In a complete gas detector (to be used in a potentially explosive atmosphere) the cans will normally be mounted inside a flameproof enclosure consisting of a metal sinter and housing. This enclosure allows gas to reach the sensor whilst ensuring that the hot sensor elements cannot ignite an explosive gas mixture.
Since the design here is critical, it is usual for the enclosure to be certified to National Standards by a recognised test house. This can be a lengthy and costly exercise especially if certification is sought in different countries. As an alternative complete detectors are available with both elements mounted inside a flameproof enclosure approved to the latest European (ATEX) and North American (CSA & UL) standards.
Detection of explosive atmospheres relies on the accurate measurement of combustible gases below the LEL concentration. Safety applications, therefore, are not generally concerned with measuring the volume concentration of gas. Measurements are more usually expressed as a percentage of the LEL concentration of the gas (%LEL).
Most combustible gas detection techniques are designed to detect a wide range of gases. Ideally the output of a sensor will be independent of the gas being measured. In reality, however, the variation in physical properties affect the output. Catalytic oxidation sensors are no exception, so the response a pellistor gives to the same volume concentration of different gases will vary. However when exposed to the same %LEL concentration of different gases, the variation in output is fairly small compared to other detection techniques. As safety applications are interested only in %LEL measurements this is a major advantage.
The variation in output for the same %LEL concentration of different gases is termed 'relative sensitivity'. Tests have been carried out to determine the experimental values of relative sensitivity for CiTipeLs to a range of combustible gases.
Certain substances are known to have a detrimental effect on catalytic sensors. There are two mechanisms by which this can occur:
Some compounds will decompose on the catalyst and form a solid barrier over the catalyst surface. This action is cumulative and prolonged exposure will result in an irreversible decrease in sensitivity. Typical poisons are organic lead and silicon compounds.
Certain other compounds, especially H2S and halogenated hydrocarbons, are absorbed or form compounds that are absorbed by the catalyst. This absorption is so strong that reaction sites in the catalyst can become blocked and normal reactions are inhibited. The resultant loss of sensitivity is temporary and in most cases a sensor will recover after a period of operation in clean air.
Most compounds fall into one of these two categories, although some will exhibit both mechanisms to greater or lesser extent. In applications where either poisoning or inhibition are likely to be present, CiTipeLs should be protected from exposure to any compounds to which they do not specifically exhibit resistance.
*The LEL of a gas is the minimum concentration of that gas in air at which an ignition source will cause an explosion.
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