🔍 Key Concepts

• Lower Explosive Limit (LEL) – the minimum concentration of a vapor in air that can ignite • Upper Explosive Limit (UEL) – the maximum concentration of a vapor in air that can ignite • What it means for a mixture to be too lean or too rich to burn

💭 Think About

• Think about what happens if the vapor concentration is below the LEL. Can it burn, or is it too lean? • Think about what happens if the vapor concentration is above the UEL. Can it burn, or is it too rich? • If the UEL is 100%, what does that say about whether a mixture can ever be too rich to burn?

✅ Before You Answer

• Be clear on the definitions of LEL (too lean) and UEL (too rich) and what ranges can actually ignite • Check which choices misunderstand what a combustible gas indicator actually measures and how its scale is usually expressed (often as % of LEL, not % by volume) • Eliminate any answer that suggests ignition is guaranteed whenever the substance is present, rather than only within the flammable range