Classification and Operating Mechanisms of Toxic Gas Detectors
Based on hazard characteristics, hazardous gases are divided into two categories: combustible gases and toxic gases. Due to their different physical properties and hazard mechanisms, targeted detection methods are adopted respectively. Toxic gas detectors are professional specialized instruments for such gas monitoring scenarios.
Combustible gases are widely encountered hazardous substances in petrochemical and other industrial sites, mainly including organic gases such as alkanes and inorganic gases represented by carbon monoxide. Three essential conditions are required to trigger a combustible gas explosion: a certain concentration of combustible gas, sufficient oxygen, and an ignition source with adequate heat. These three elements are indispensable, and no fire or explosion will occur if any condition is missing.
When combustible gas vapor or dust mixes with oxygen and reaches a specific concentration range, explosion will be induced once exposed to a high-temperature ignition source. The explosive concentration range of combustible gases is defined as the explosion limit, which is generally expressed as a percentage (%). Explosions only occur within a specific concentration interval. No explosion risk exists when the gas concentration is below the Lower Explosive Limit (LEL) due to insufficient combustible gas, or above the Upper Explosive Limit (UEL) due to insufficient oxygen supply.
Different combustible gases have unique LEL and UEL values, which must be fully considered during instrument calibration. For safety supervision, most detectors set alarm thresholds at 10% LEL and 20% LEL. The 10% LEL threshold serves as a warning alarm, while 20% LEL is set as a danger alarm. For this reason, combustible gas detectors are also commonly known as LEL detectors.
Toxic gas detectors are classified into three types according to different sensor operating principles:
A) Physicochemical property sensors
Including semiconductor sensors (surface-controlled, volume-controlled and surface potential types), catalytic combustion sensors, and solid thermal conductivity sensors.
B) Physical property sensors
Including thermal conductivity sensors, light interference sensors, and infrared absorption sensors.
C) Electrochemical property sensors
Including controlled potential electrolysis sensors, galvanic cell sensors, membrane ion electrode sensors, and solid electrolyte sensors.
