🔍 Key Concepts

• lower explosive limit (LEL) vs upper explosive limit (UEL) and what a combustible gas indicator actually displays • difference between flammable (explosive) limits and autoignition • what it means when a meter shows "10%, 20%, 50%" on its scale

💭 Think About

• Ask yourself: when a combustible gas meter says "20%" on the display, 20% of what is it referring to—room air, gas concentration, or some defined safety limit? • Think about which of the choices represents a reference point used in safety practice to warn you before the atmosphere becomes ignitable. • Consider how firefighters and mariners use these meters: they often say "do not enter if it reads more than 10% of ..."—what is the "of" here?

✅ Before You Answer

• Look up the definition of lower explosive limit (LEL) and how it is used with gas meters. • Confirm whether an autoignition temperature or concentration is something a portable field meter is calibrated to, or a property found in tables. • Clarify whether flammable limit concentration is a single value or a range between lower and upper limits.