How to Use Toxic‑Harmful Gas Detectors in Industrial Settings
Flammable gases are the most common hazardous gases encountered in industrial sites such as petrochemical plants. They mainly include organic gases like alkanes and certain inorganic gases such as carbon monoxide.
For flammable‑gas explosion to occur, specific conditions must be met: flammable gas at a certain concentration, a sufficient amount of oxygen, and an ignition source that provides adequate heat. These are the three elements of explosion (the explosion triangle shown in the left‑hand figure above). None of the three can be missing; in other words, fire or explosion will not take place if any one condition is absent.
When flammable gases (vapors, dusts) mix with oxygen and reach a certain concentration range, explosion will be triggered upon contact with an ignition source of sufficient temperature. The concentration range within which flammable gases explode when exposed to an ignition source is defined as the explosive concentration limit, or explosion limit for short, generally expressed in percentage (%). In fact, explosion does not occur at every mixing ratio but only within a specific concentration range, as indicated by the shaded area in the right‑hand figure above.
No explosion will happen when the flammable‑gas concentration is below the LEL (Lower Explosive Limit, insufficient flammable gas) or above the UEL (Upper Explosive Limit, insufficient oxygen). LEL and UEL values vary for different flammable gases (refer to Issue 8 for details), which requires special attention during instrument calibration. For safety purposes, alarms are normally triggered at 10 % LEL and 20 % LEL of flammable‑gas concentration. 10 % LEL corresponds to a warning alarm, and 20 % LEL corresponds to a danger alarm. This explains why flammable‑gas detectors are also known as LEL detectors.
It should be noted that 100 % reading on an LEL detector does not mean the flammable‑gas concentration reaches 100 % by volume; instead, it indicates 100 % of the Lower Explosive Limit. For methane, 100 % LEL equals 4 % VOL (volume concentration). In practical applications, catalytic‑combustion detectors are widely used for LEL‑based measurement of these gases. Their core is a dual‑bridge detection unit, commonly referred to as a Wheatstone bridge. One platinum‑wire bridge is coated with catalytic‑combustion material. When any combustible gas is ignited on the electrode, temperature variation changes the resistance of the platinum‑wire bridge. This resistance change is proportional to flammable‑gas concentration, and the gas concentration value is calculated by the instrument's circuit and microprocessor.
Thermal‑conductivity VOL‑type detectors, which directly measure the volume concentration of flammable gases, are also commercially available. Integrated detectors supporting both LEL and VOL measurement can be found on the market as well. VOL‑type flammable‑gas detectors are especially suitable for measuring flammable‑gas volume concentration (VOL) in oxygen‑deficient environments.
