4 Gas Detection
The Pellistor is Dead? Long Live the Pellistor!
Since the early 1960s, millions have people have trusted pellistor (catalytic bead) sensors to protect them from the hazards of fl ammable gases in their workplace. From underground applications such as mining or tunnelling, through a wide range of water treatment, oil, gas and petrochemical works, pellistor technology has remained the most frequently deployed to warn of rising gas levels and through their incorporation in sophisticated instruments and detectors, provided alarms to ensure the correct safety measures can be taken or evacuation can be initiated. The technology has gained ubiquity, not only from its wide range of measureable gases but also as a result of its inherent simplicity which requires simple driver electronics, but also the relatively small size of sensors and their affordable prices. However, more recently the technology has seen stiff competition from the advent of infra-red and other optical sensors, which deliver some real advantages and has led some to sound the death knell for pellistor technology. In this article, we discuss the historical development of the pellistor and by reviewing its future developmental roadmap, explain why there is still plenty of life left in the technology, and plenty more lives to be saved.
A fundamental essence of the technology is that it senses fl ammability – if the gas can burn, it will burn on a pellistor and there are some gases such as hydrogen which cannot be detected using infra-red sensors.
What is a Pellistor?
As a result of greater worker safety concern and tightening safety regulations in the British coal industry, there was considerable effort invested in the development of fl ammable gas technology which resulted in the invention of the “pellet resistor” or pellistor. In simplest terms, a pellistor is a coil of fi nely wound wire, typically platinum, around which a mixture of ceramic material and catalyst is deposited to form a small bead (hence the common alternative name of catalytic bead sensor.)
present. Pellistors are typically used in pairs, one ‘detector’ bead contains the active catalyst and is designed to respond to gas, the other ‘compensator’ bead is manufactured without catalyst and only responds to changes in environmental conditions, thus providing inherent temperature compensation.
Since a pellistor is a heat source, it is also a potential ignition point for fl ammable gas if the concentration increases beyond the lower explosive limit (LEL), and so the beads are typically encased in stainless steel housing with a metal sinter fi tted. The purpose of the sinter is to ensure that any fl ame generated around the bead is unable to propagate outside of the sensor and initiate an explosion.
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Author/Contact Details: Rob White
Sales and Marketing Director SGX Sensortech Ltd 2 Hanbury Road, Chelmsford,Essex, CM1 3AE,
United Kingdom
Tel: +44 (0) 1245 809110 Mob: +44 (0) 7527 420078
Email:
rob.white@sgxsensortech.com Web:
www.sgxsensortech.com
IET September / October 2014
www.envirotech-online.com Figure 1: Structure of a pellistor
By application of an electrical current, the bead is heated to between 300-400°C and at this temperature when fl ammable gas is encountered it will burn on the surface of the sensor raising its temperature further. Since the resistance of the platinum coil inside the bead is highly dependent on its temperature, the presence of gas can be measured by maintaining the sensor in an electrical bridge circuit, the output is linearly proportional to the concentration of fl ammable gas
Over the years, signifi cant enhancements have been made to the technology to increase its usefulness. One inherent disadvantage of pellistors is that the catalyst materials used in the beads are susceptible to a reduction in sensitivity when exposed to gases such as sulphides, silicones and halides. In some cases, the reduction is reversible (inhibition), in many cases the reduction is permanent (poisoning). The problem has been signifi cantly reduced by the introduction of advanced fi lter materials inside the sensor designed to protect the sensitive catalyst from such gases and prolong their working lifetimes. In addition, some manufacturers have adopted an alternative bead structure; historically pellistor beads have been manufactured with a hard ceramic core onto which catalyst is doped in a hard shell, but it is also possible to build the bead as a mixture of ceramic and catalyst as a slurry. The latter approach provides a much greater availability of catalyst surface area and a direct increase in poison resistance.
A further disadvantage of pellistor technology is the relatively high power requirement used to maintain the bead at the correct temperature. For fi xed gas detection systems this is not typically problematic, however for portable battery
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