Detection of gases is a critical task in many chemical process industries (CPI) facilities to avoid hazards to personnel and to the environment. Gases may present risks of explosion, flammability, toxicity, environmental pollution and displacement of breathable air. This one-page reference provides information on common classes of gas detectors and on commonly monitored gases in industry.
Gas sensor classes
Gas-detection technologies can be classified according to the characteristics of the gases they detect: either toxic gases or combustible gases, and most gas-detection technologies fall into one of four broad categories based on their mode of operation: electrochemical sensors and metal-oxide semiconductor sensors are generally used to detect toxic gases; and infrared and catalytic sensors are used for detecting combustible and explosive gases.
Electrochemical. Electrochemical sensors are based on an electrochemical cell whose current increases when the molecule of interest makes contact with the sensing electrode. The target gas may be oxidized or reduced at the working electrode, and a small, but detectable flow of electrons is produced from the reaction there. A measuring electrode and a counter electrode are connected to the cell as well.
Metal-oxide semiconductor (MOS). Metal-oxide semiconductors are based on the principle that gas adsorption onto, and desorption from, the surface of a metal oxide changes the conductivity of the material. When target molecules contact a thin film of high-surface-area sensor material, the concentration of charge carriers (electrons or holes) changes, and the conductivity or resistivity is altered in a measurable way.
Catalytic. Most sensors of this type work by catalytic oxidation, where the combustible gas of interest comes into contact with a catalytic surface (often platinum-treated wire coil) and is oxidized. This releases heat of reaction, and the wiring resistance is changed by the temperature rise. Typically, a bridge circuit is used to indicate the resistance change. The increased resistance — compared to the resistance in clean air — is used to indicate the gas concentration.
Infrared. Infrared sensors work via a system of light transmitters and receivers. When combustible gases of interest come within the field of view of the receiver, a portion of the radiation is absorbed, changing the power of the light between the transmitter and the receiver.
Common detectable gases
The table  contains information on possible gas detection targets.
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