Residual Nitrogen

Excess nitrogen still dissolved in your tissues after a dive, which shortens the no-decompression limits of any repetitive dive that follows.

Residual nitrogen is the extra nitrogen still dissolved in your body after you surface from a dive. Off-gassing does not finish at the ladder: fast tissues unload within minutes, but slower tissues keep releasing nitrogen for many hours, so for the rest of the day you are walking around with more nitrogen on board than someone who has not been diving. If you enter the water again during that window, you begin the new dive partly loaded, which is the defining feature of a repetitive dive.

The practical consequence is shorter no-decompression limits on every dive after the first. In the era of dive tables this was managed explicitly, you carried a pressure group letter through the surface interval and converted it to residual nitrogen time that had to be subtracted from the next dive's allowance. Dive computers now do the same accounting continuously and invisibly, tracking each theoretical tissue compartment between dives, which is why your computer shows less available bottom time on dive two even at the same depth.

You manage residual nitrogen with structure: longer surface intervals shed more of it, doing the deepest dive first keeps repetitive profiles conservative, and the last dive of a trip sets the clock for flying after diving. It is also the reason a multi-day liveaboard schedule slowly tightens your limits, your slow tissues never fully reset overnight, which is exactly why many liveaboard schedules build a dive-free afternoon into the middle of the week.