Electrochemical
These work by allowing gases to diffuse through a membrane that has a porous composition. It then passes an electrode which will be chemically oxidized or sometimes reduced. The identification and amount of gas is configured by how much gas is oxidized by the electrode. Simple but effective.
Catalytic bead (pellistor)
This is the type of sensor most commonly used to detect combustible gases that present the potential hazard of exploding. Beads are situated at opposite arms of the Wheatstone bridge circuit. As they are heated up the catalysts, containing combustible compounds, will oxidize. The difference between the active and passive beads with identify how much gas is present.
Photoionization
Photoionization detectors (PIDs) are able to detect gas and it’s concentration levels by making use of a high-photon-energy UV lamp.
Infrared point
Infrared (IR) point sensors and depending on how the concentration and type of gas, the sensor beam with being absorbed. By determining how it is absorbed, the sensor determines which gas is present and how concentrated the gas is.
Semiconductor
Semiconductor sensors the chemical reaction that occurs when gas comes into contact with the sensor, often tin, to identify and measure the gas.