Advanced 8 minutes
Repetitive diving: when the previous dive still counts
Residual nitrogen, surface intervals, and why the last day of a trip is the most delicate
A diver does four dives a day for six days on a trip. On the sixth day they dive exactly the same profile as on the first, within all their computer's limits, yet their physiological state has nothing to do with day one.
In this article you'll see what residual nitrogen is, how the surface interval works, why dive order matters, what happens on multi-day trips, and why you need to wait before flying.
Real recreational diving is almost never a single isolated dive: it's a trip with several dives a day over several days. This is the part of planning that gets neglected the most.
Residual nitrogen
When you get out of the water, nitrogen off-gassing doesn't stop: it continues for hours. As you saw in the article on dissolved gases, slow tissues take much longer than fast ones to return to equilibrium with atmospheric pressure.
That means if you get back in before you've eliminated all that load, you don't start from zero. That remainder is residual nitrogen, and it's why your second dive of the day has less available time than the first at the same profile.
Computers track it continuously and automatically, which makes the process invisible to the diver. The only visible sign is that the available time comes out shorter than expected.
It's worth understanding this for one concrete reason: if the planner gives you significantly less time than you assumed for the second dive, that's not a malfunction. It's the load you're still carrying.
Keep reading with full access
This article is part of the full Encyclopedia. A donation from USD 10 unlocks every article, every species photo and the downloadable app, and helps sustain the Scuba Sign Paperless Diving Community project.