🔍 Key Concepts

• Combustible gas indicator (CGI) purpose and what it actually compares your reading to • Difference between flash point, fire point, and explosive limits • Meaning of LEL (lower explosive limit) and UEL (upper explosive limit) in gas measurements

💭 Think About

• Ask yourself: when a CGI says "20%" on the display, is that 20% of a temperature value, or 20% of some concentration limit? • Which limit is used as the reference point for safety—do we normally stop hot work at some percentage of the lower limit or the upper limit? • Think about which value represents the minimum concentration where an explosion can occur—how would a safety instrument logically be scaled?

✅ Before You Answer

• Verify which term (flash point, fire point, LEL, UEL) refers to a gas concentration in air, not a temperature • Confirm which limit (lower or upper) is the starting point where a mixture first becomes capable of burning or exploding • Recall standard shipboard practice: hot work is usually prohibited above a certain percentage of the critical explosive limit shown on the CGI