Principles of Electrochemical Gas Detectors
Quite a number of combustible, toxic and hazardous gases exhibit electrochemical activity and can be electrochemically oxidized or reduced. These reactions are used to identify gas components and measure gas concentration. Electrochemical gas sensors fall into several sub‑categories:
⑴ Galvanic‑cell gas sensors (also known as fuel‑cell sensors or self‑powered battery‑type sensors). Their working principle is similar to that of ordinary dry‑cell batteries, except that the carbon‑manganese electrodes are replaced by gas‑sensing electrodes. Taking an oxygen sensor as an example, oxygen is reduced at the cathode. Electrons flow through an ammeter to the anode, where lead metal is oxidized. The magnitude of the current is directly proportional to oxygen concentration. This type of sensor can effectively detect oxygen, sulfur dioxide, chlorine and other gases.
⑵ Controlled‑potential electrolytic‑cell gas sensors. These sensors work very well for detecting reducing gases. Unlike galvanic‑cell sensors, their electrochemical reactions take place under applied electric current, making them genuine coulometric‑analysis sensors. They have been widely applied for detecting carbon monoxide, hydrogen sulfide, hydrogen, ammonia, hydrazine and other gases, and represent the mainstream sensor for toxic‑hazardous gas measurement.
⑶ Concentration‑cell gas sensors. Electrochemically active gases on two sides of an electrochemical cell spontaneously generate a concentration‑dependent electromotive force. The magnitude of this electromotive force correlates with gas concentration. Typical practical examples include automotive oxygen sensors and solid‑electrolyte carbon‑dioxide sensors.
⑷ Limiting‑current gas sensors. Certain oxygen‑concentration sensors are built on the principle that the limiting current within an electrochemical cell relates to carrier concentration. They are used for oxygen measurement in automobiles and for detecting oxygen content in molten steel.
