Each percentage volume listed corresponds to 100% of the Lower Explosive Limit (LEL) for a specific gas or vapor, indicating the point at which ignition becomes possible. Therefore, flammable gas detectors must be calibrated to respond optimally to the specific gas hazard and should measure from zero (no gas) up to the flammable threshold, which is defined as 100% LEL. All flammable gas detectors are designed to display readings from 0 to 100% LEL. This means that individuals assessing flammable gas hazards do not need to know the exact percentage at which a particular gas forms a flammable mixture; instead, they can quickly gauge the risk level based on the % LEL displayed. For example, a reading of 10% LEL indicates that the gas is 10% of the way to becoming flammable, while 50% LEL indicates that it is halfway to ignition. At this point, the specific gas involved is less critical for risk assessment.

Typically, gas detection systems are programmed with alarm thresholds, usually set at 10-20% LEL for the first warning alarm and 20-40% LEL for a second-stage alarm, which prompts evacuation or further action.

It's also important to note that some flammable gases and vapors may pose a more significant toxic hazard than a flammable one. For example, carbon monoxide has a flammable threshold of 12.5% by volume in air but has a Short Term Exposure Level (STEL) of only 300 ppm (0.03% by volume) over 15 minutes. In this case, a toxic gas detector that measures in ppm is more pertinent since it can indicate danger before the gas reaches flammable concentrations.

Additionally, each flammable gas or vapor has an Upper Explosive Limit (UEL), which represents the concentration at which there is too much gas or vapor for combustion to occur due to insufficient air. In gas detection, we typically focus on identifying leaks before reaching the LEL, rather than monitoring the UEL.