🔍 Key Concepts

• Combustible gas indicator (explosimeter) operation – it actually burns a small sample on a heated filament • Definitions of lower explosive limit (LEL) and upper explosive limit (UEL) for flammable gases • Effect of oxygen content and inert gas (like CO2 or nitrogen) on whether a mixture will burn

💭 Think About

• Ask yourself: Under which condition would the instrument no longer be able to burn the sample gas on its sensor? • Which option describes a situation where the gas mixture is outside the flammable range, so combustion on the sensor can’t occur normally? • Look at each choice and decide: does this condition make it easier or harder for the gas sample to burn on the sensor element?

✅ Before You Answer

• Verify which explosive-limit term (LEL vs UEL) applies to a mixture that is too rich to burn. • Check that a combustible gas indicator requires sufficient oxygen to work correctly – so would ‘sufficient oxygen’ tend to help or hinder operation? • Confirm whether being “free of CO2” would by itself stop the sensor from operating, compared to a condition where the fuel concentration is beyond the flammable range.