Gas sensors fall into three major categories in terms of working principles.
Gas sensors are classified into three main categories according to their operating principles.
The first category refers to gas sensors utilizing physicochemical properties, including semiconductor sensors (surface-controlled type, volume-controlled type and surface potential type), catalytic combustion sensors and solid-state thermal conductivity sensors. The second category covers gas sensors adopting physical properties, such as thermal conduction sensors, optical interference sensors and infrared absorption sensors. The third category is gas sensors based on electrochemical properties, including constant-potential electrolytic sensors, galvanic cell sensors, membrane ion electrode sensors and solid electrolyte sensors.
In accordance with hazard characteristics, toxic and harmful gases are divided into two types: combustible gases and toxic gases. Their distinct properties and hazard mechanisms lead to differences in detection methods.
Combustible gases are common hazardous gases in petrochemical and other industrial scenarios, mainly including organic gases such as alkanes and certain inorganic gases represented by carbon monoxide. The explosion of combustible gases requires three essential conditions: combustible gas at a specific concentration, oxygen with a certain content, and an ignition source with sufficient heat. These are the three elements of explosion (explosion triangle), all of which are indispensable. The absence of any single condition will prevent fire and explosion accidents.
An explosion occurs when combustible gases (vapors or dusts) mix with oxygen and reach a specific concentration range upon contact with an ignition source of a certain temperature. The critical concentration range at which combustible gases explode when exposed to an ignition source is defined as the explosion concentration limit, commonly referred to as the explosion limit and measured in percentage (%).
In fact, gas mixtures can only explode within a specific concentration range, as shown in the shaded area of the corresponding figure. No explosion will occur if the combustible gas concentration is lower than the Lower Explosive Limit (LEL) due to insufficient gas content, or higher than the Upper Explosive Limit (UEL) due to insufficient oxygen content. Different combustible gases have unique LEL and UEL values, which must be fully considered during instrument calibration.
For safety purposes, alarm thresholds are generally set at 10% LEL and 20% LEL. Specifically, 10% LEL is set as the warning alarm point, and 20% LEL as the danger alarm point. This explains why combustible gas detectors are also known as LEL detectors. It is worth noting that the 100% reading on an LEL detector does not represent 100% gas volume concentration, but 100% of the lower explosive limit of the corresponding gas. Taking methane as an example, 100% LEL is equivalent to a volume concentration (VOL) of 4%. Catalytic combustion detectors based on the LEL detection principle are the most widely used instruments for combustible gas detection in industrial operations.
The core detection unit of the detector is a dual-circuit bridge, namely the Wheatstone bridge. One platinum wire bridge is coated with a catalytic combustion material. When any flammable gas is ignited on the electrode, the temperature change will alter the resistance of the platinum wire bridge. The resistance change is proportional to the concentration of the combustible gas, and the gas concentration value can be calculated accurately through the instrument's circuit system and microprocessor.
