The surface interval is off-gassing time, not just lunch

Posted 12 July 2026 · Field Note #30

A surface interval is simply the time you spend out of the water between two dives — and it does real physiological work, even while you're eating a sandwich. Understanding what's happening turns it from dead time into a tool you can plan with.

When you surface, your tissues are still supersaturated: they hold more dissolved nitrogen than the nitrogen pressure you're now breathing can hold in balance. At the surface you're inhaling air at 1 bar, in which nitrogen's partial pressure is far lower than in your loaded tissues. That difference — the gradient between tissue and lung — is what drives gas back out through your breath, without forming problematic bubbles, as long as you ascended within your limits. The longer the interval, the more your slow compartments unload.

But tissues clear at very different speeds, and this is the part that catches people out. Fast compartments are nearly done off-gassing within an hour. Slow ones can still be releasing nitrogen many hours later. So a short interval mostly resets the fast tissues while leaving the slow load almost untouched — your next dive's no-stop limit improves a little, but not as much as you might hope, because the slow tissues are the ones quietly governing repetitive limits.

That's also why the benefit of a longer interval isn't linear. The first hour buys you a lot back from the fast tissues; the third and fourth hours work mainly on the slow ones — exactly what you want before a deep repetitive dive or a flight.

The practical takeaway: don't treat the interval as something to minimise so you can squeeze in more dives. Guidance generally favours at least an hour between dives, and meaningfully longer after deeper or harder ones. Hydrate, stay warm, move gently, and let your computer's reducing residual-nitrogen credit confirm that your tissues have actually had time to unload.

All Field Notes · daily on @DecoLogFieldNotes

⚠ Educational only — not a substitute for training or a dive computer. Verify every number independently and dive within your certification.