Why should the decompression gas be richer than the back gas?

When using decompression gas during a dive, it is important for it to be richer than the back gas in order to effectively facilitate the release of excess nitrogen from the body's tissues. This is crucial because as divers ascend from depth, nitrogen, which has been absorbed into the body's tissues under the pressure of water, needs to be released in a controlled manner to prevent decompression sickness.

A richer mixture, typically with a higher percentage of oxygen or a lower percentage of nitrogen, helps expedite the off-gassing of nitrogen. As a diver ascends, the surrounding pressure decreases, allowing dissolved gases to come out of solution; the enriched decompression gas allows for a more efficient exchange, thereby promoting safe and rapid elimination of nitrogen from the tissues.

This focus on the gas composition directly impacts the body's ability to handle the nitrogen load, making it a critical component of dive planning and execution, particularly in deeper or longer dives where decompression stops are necessary.