During powered degassing, what is the target maximum gas concentration to ensure when closing the degassing box?

• A. <2.0%
• B. <1.25%
• C. <0.5%
• D. <5%

Answer: (B)

During powered degassing, the aim is to seal the degassing box only when methane levels are low enough to be safely contained and to maintain a safe environment if ventilation changes later. The target is a methane concentration of less than 1.25% by volume. This gives a solid margin below the 5% lower explosive limit, while also allowing for instrument accuracy and small rebound effects when the system is shut or sealed. Closing at a higher level would increase the risk of forming or leaving behind an explosive mixture, and closing at an even stricter level (like 0.5%) isn’t the standard requirement, though it would be safer.