Pellistor

A pellistor is a type of catalytic bead sensor used in gas detectors to sense combustible hydrocarbon gases. It consists of two small beads mounted on a ceramic support within a protective housing. The active bead is coated with a platinum catalyst to promote oxidation of hydrocarbons, while the reference bead is inert or less active to provide temperature compensation.

Typically the beads form part of a Wheatstone bridge. An electrical current heats both beads; in clean air their resistances are balanced. When flammable gas is present, oxidation on the catalytic bead releases heat, raising its temperature and resistance, which unbalances the bridge and generates a detectable signal. The reference bead compensates for ambient temperature changes, improving stability and repeatability.

Performance and maintenance: pellistors respond rapidly to a range of hydrocarbons, with sensitivity that can vary by gas type. The output is often used to estimate flammable gas concentration in terms of the lower explosive limit. Calibration with known gas mixtures is required for accuracy. Pellistors can be poisoned or degraded by certain contaminants, such as sulfur compounds, silicones, and halogenated hydrocarbons, and may drift with temperature and humidity, reducing life and reliability.

Applications: they are widely used in portable and fixed industrial gas detectors to monitor combustible vapors in oil and gas, petrochemical, mining, and related environments.

Limitations: they are non-specific to particular hydrocarbons, can exhibit cross-sensitivity to other combustibles, and require periodic maintenance, calibration, and occasional replacement. They are typically used alongside other sensors to provide comprehensive gas detection.