Catalytic bead sensors sense the presence of gas at around 20% LEL. Two platinum coils are embedded in a bed of alumina and the voltage difference across them is measured using a Wheatstone bridge circuit configuration. One wire contains a catalyst that makes it an oxidative environment while the other wire inhibits oxidation. When an oxidizable gas is
present in the sensor, oxidation occurs at one wire and not at the other. This induces a voltage difference across the wire and an alarm is triggered to indicate the presence of a gas at around 20% LEL.