Pellistors Circuit

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. This is a figure of the block diagram of the 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.