Catalyticbead

Catalyticbead, commonly referred to as a catalytic bead, is a small ceramic bead used as the catalytic element in pellistor-based combustible gas sensors. It is coated with a platinum-group catalyst that promotes the oxidation of combustible gases such as methane, propane, and hydrogen when they reach the bead surface. The exothermic reaction raises the bead's temperature, producing a measurable change in its electrical resistance.

In typical sensors, the catalytic bead is paired with a second inert bead to form a differential, or Pellistor, element. The beads are integrated into a Wheatstone bridge circuit; when gas oxidizes on the catalytic bead, the temperature rise changes its resistance, which in turn changes the bridge output. The signal is correlated with gas concentration in the detectable range. Some designs operate the beads at elevated temperatures to improve response speed and sensitivity.

Construction often uses a ceramic core (alumina) coated with dispersed platinum-group metal catalysts on a support such as alumina. The beads are housed in a protective cartridge that allows gas diffusion while preventing mechanical damage.

Applications include portable and fixed gas detectors for industrial safety, mining, refinery, and environmental monitoring, particularly for hydrocarbons and other flammable gases. Advantages include simplicity, ruggedness, and fast response. Limitations include dependence on oxygen presence, sensitivity to catalyst poisoning by sulfur compounds or silicones, cross-sensitivity to humidity and temperature, and the need for periodic calibration and replacement due to ageing of the catalyst.

See also: Pellistor, Gas detector, Catalysis.