The Pellistor Catalytic Gas Detector

Pellistors are miniature calorimeters used to measure the energy liberated by the burning of a combustible (flammable) gas or vapour. A pellistor consists of a coil of small- diameter platinum wire supported in a refractory bead on which is deposited a layer of catalytic material, on which the gas is burnt. The coil serves two purposes. Firstly, it is used to heat the bead electrically to its operating temperature, about 500°C, and secondly it is used to detect changes in temperature produced by the oxidation of the flammable gas. The earliest forms of catalytic gas sensors consisted solely of bare coils of platinum wire, operating at 800-1000°C. At such temperatures, platinum wire evaporates extremely quickly causing signal drifts resulting from a reduction in the wire diameter. The specification for such a sensor, which is still produced commercially, requires that the sensor has a life of 100 hours. The majority of present day devices, as stated earlier, have the coil cloaked in a porous ceramic onto which is deposited the precious metal catalyst. The enhanced catalytic activity resulting from the much larger surface area of catalyst available permits much lower operating temperatures of around 500°C, resulting in lower power drain and longer device lifetime.


The concept of the pellistor is based on the fact that the most foolproof way to determine whether a flammable gas is present in air is to test a sample by trying to burn it! A pellistor consists of a very fine coil of wire suspended between two posts. The coil is embedded in a pellet of a ceramic material, and on the surface of the pellet (or 'bead') there is a special catalyst layer.

In operation, a current is passed through the coil, which heats up the bead to a high temperature. When a flammable gas molecule comes into contact with the catalyst layer, the gas 'burns'. The reaction occurs without a flame since the level is below the Lower Explosive Limit (or LEL) of the gas. However, just as in a burning reaction, heat is released which increases the temperature of the bead. This rise in temperature causes the electrical resistance of the coil to rise. There is another bead in the circuit which is identical to the detector bead, but does not contain any catalyst

This bead will react to changes in humidity, ambient temperature etc, but will not react to flammable gas. All that is required is a comparison of the resistance of one bead against another in a Wheatstone bridge type circuit in order to obtain a meaningful signal.