Gas sensors can be divided into three major categories in terms of principle.
A) Gas sensors utilizing physical and chemical properties, such as semiconductor based (surface controlled, volume controlled, surface potential based), catalytic combustion based, solid thermal conductivity based, etc.
B) Gas sensors utilizing physical properties such as thermal conductivity, optical interference, infrared absorption, etc.
C) Gas sensors utilizing electrochemical properties, such as constant potential electrolysis, galvanic cell, diaphragm ion electrode, fixed electrolyte, etc.
According to the hazards, we classify toxic and harmful gases into two categories: combustible gases and toxic gases.
Due to their different properties and hazards, their detection methods also vary.
Combustible gases are the most common hazardous gases encountered in industrial settings such as petrochemicals. They are mainly organic gases such as alkanes and certain inorganic gases such as carbon monoxide. The explosion of combustible gases must meet certain conditions, which are: a certain concentration of combustible gas, a certain amount of oxygen, and a source of fire with sufficient heat to ignite them. These are the three elements of explosion (as shown in the explosion triangle in the left figure above), all of which are indispensable. In other words, the absence of any of these conditions will not cause a fire or explosion. When combustible gases (steam, dust) and oxygen are mixed and reach a certain concentration, they will explode when exposed to a fire source with a certain temperature. We refer to the concentration at which combustible gases explode when exposed to a source of fire as the explosive concentration limit, abbreviated as the explosive limit, which is generally expressed in%. In fact, this mixture does not necessarily explode at any mixing ratio and requires a concentration range.
