What is the primary purpose of conducting a purge?

• A. Remove moisture from the gas.
• B. Purge oxygen from the system.
• C. Eliminate nitrogen from the lungs and the UBA.
• D. Purge CO2 from the system.

Answer: (C)

Purging a closed-circuit breathing system is about flushing inert gas, mainly nitrogen, from both the lungs and the UBA loop. In a reset or purge, you’re replacing nitrogen-rich residual gas with the intended gas mix so the diver breathes a safer, oxygen-enriched composition. This lowers the overall nitrogen load the diver carries, reducing nitrogen narcosis risk and helping manage decompression considerations on ascent. It’s not about drying the gas or removing CO2 (that’s handled by the scrubber), and purging oxygen would defeat the purpose of maintaining the correct breathable mix. So the primary aim is to eliminate nitrogen from the lungs and the UBA.