How do catalytic gas sensors (pellistors) work?

Catalytic combustion has been the most widely used method of detecting flammable gases in Industry since the invention of the catalytic pelletized resistor (or "Pellistor") in the mid 1960's.

A Pellistor consists of a very fine coil of platinum wire, embedded within a ceramic pellet. On the surface of the pellet is a layer of a high surface area noble metal, which, when hot, acts as a catalyst to promote exothermic oxidation of flammable gases. In operation, the pellet and so the catalyst layer is heated by passing a current through the underlying coil. In the presence of a flammable gas or vapour, the hot catalyst allows oxidation to occur in a similar chemical reaction to combustion. Just as in combustion, the reaction releases heat, which causes the temperature of the catalyst together with it's underlying pellet and coil to rise. This rise in temperature results in a change in the electrical resistance of the coil, and it is this change in electrical resistance which constitutes the signal from the sensor.  

Pellistors are always manufactured in pairs, the active catalysed element being supplied with an electrically matched element which contains no catalyst and is treated to ensure no flammable gas will oxidise on it's surface. This "compensator" element is used as a reference resistance to which the sensor's signal is compared, to remove the effects of environmental factors other than the presence of a flammable gas.

Pellistor Drive/Measurement Circuit: A simple Wheatstone Bridge
to compare the resistance of two hot elements

The advantage of using this technique when detecting flammable gases for safety purposes is that it measures flammability directly.

Nemoto provides matched pair Pellistors conveniently mounted in TO4 size headers, or as complete flameproof Gas Detection Heads for use as field devices within fixed Gas Detection Systems.