The working principle of catalytic sensors is based on flammable gas oxidation: when a combustible gas comes in contact with the catalyst surface it is oxidised. The reaction releases heat, which causes the resistance of the wire to change.
A catalytic palletised resistor (or “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 catalyst layer is heated by passing a current throughout the underlying coil. In the presence of flammable gas 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 its underlying pellet and coil to rise. This rise in temperature results in a change in the electrical resistance, which constitutes the signal from the sensor.
Pellistor sensors 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 that no gas will oxidise on its 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.