Classification of Gas Sensors Based on Operating Principles
① Gas sensors utilizing physicochemical properties: semiconductor type (surface-controlled, bulk-controlled, surface potential type), catalytic combustion type, solid-state thermal conductivity type, etc.
② Gas sensors utilizing physical properties: thermal conductivity type, optical interference type, infrared absorption type, etc.
③ Gas sensors utilizing electrochemical properties: potentiostatic electrolysis type, galvanic cell type, membrane ion electrode type, solid electrolyte type, etc.
Toxic and harmful gases are classified into two major categories by hazard level: combustible gases and toxic gases. Different properties and hazards correspond to different detection methods.
Combustible gases are the most common hazardous gases in petrochemical and other industrial sites, mainly including organic gases such as alkanes and certain inorganic gases like carbon monoxide.
Three essential conditions must be met simultaneously for a combustible gas explosion to occur: a certain concentration of combustible gas, sufficient oxygen, and an ignition source with adequate energy. These are the three elements of explosion; the absence of any one factor will prevent fire and explosion. When combustible gases (vapors, dust) mix with oxygen and reach a specific concentration range, an explosion will be triggered once exposed to an ignition source of sufficient temperature. The critical concentration range for combustible gas explosion is defined as the explosive concentration limit, abbreviated as explosive limit, generally expressed in volume percentage (%). In fact, the mixture only explodes within a specific concentration window rather than at all mixing ratios.
No explosion will take place when the gas concentration is below the LEL (Lower Explosive Limit, insufficient fuel) or above the UEL (Upper
Explosive Limit, insufficient oxygen). Each combustible gas has unique LEL and UEL values, which must be strictly observed during instrument calibration. For safety purposes, alarms are typically set at 10% LEL (warning alarm) and 20% LEL (danger alarm). This is why combustible gas detectors are also referred to as LEL detectors.
