Oxygen on a Multi-Day Trip

Two clocks, and only one resets overnight

Almost everything you're taught about oxygen toxicity is about a single dive: keep your oxygen pressure under 1.4 bar, watch the CNS clock (the central-nervous-system exposure percentage your computer shows), surface. Spend a week on a liveaboard or an expedition and a different question quietly takes over. Not "is this dive safe?" but "what does all this oxygen add up to, day after day?"

The answer turns on a quirk worth understanding. You're running two oxygen clocks at once, and they behave nothing alike over a trip. One of them resets while you sleep. The other doesn't.

The handle that makes this stick: the CNS clock is a stopwatch you reset to zero every night; your lungs are an odometer that only ever counts up. Over a week, the odometer is what governs you.

Why the CNS clock doesn't stack

Your dive computer's CNS percentage isn't just a bottom-time counter. On the surface it decays, with a half-time of about 90 minutes, so it roughly halves every hour and a half. Run six of those half-times and you're at essentially zero, which takes about nine hours. In other words, a normal night's surface interval wipes the CNS clock clean. You wake up at 0% no matter how hard you pushed it yesterday.

That's why CNS exposure, for all its drama, almost never becomes the thing that limits a multi-day trip. It's a within-the-day problem. (Worth flagging: that 90-minute recovery figure is a modelling convention rather than a hard, measured number, but the practical point holds.)

Why pulmonary load does stack

Your lungs don't get the same overnight reset. The inflammation from a long, oxygen-rich day is only part-healed by morning, so each day starts with a little carried over from the last. Track it as OTUs (the pulmonary oxygen dose unit from the pulmonary article) and the old REPEX schedule even bakes the carry-over in: it lowers your allowed daily dose as a trip goes on.

The crossover: where the 2025 rule actually lands

In 2025 a NOAA panel relaxed the CNS limit at a 1.3 bar setpoint (the oxygen pressure a rebreather holds you at) to as much as four hours of working dive plus four hours of resting deco in a day. That's a real change for long rebreather dives, but it's a CNS rule, and a per-day rule. It says nothing about your lungs, and nothing about tomorrow.

Here's what that means once you string days together. A day that maxes the new allowance, roughly eight hours at a 1.3 setpoint, works out to about 700 OTU, which is most of your single-day lung budget of ~850. Do that two days running and you've blown past any sensible multi-day lung budget, even though your CNS clock reset to zero overnight both times.

A week, roughly budgeted

Picture three long rebreather days in a row, each an eight-hour dive near a 1.3 setpoint, about 700 OTU apiece. Day 1's budget is 850, so 700 is comfortable. Day 2's budget has dropped to 700, so you're sitting right on it. Day 3's budget is about 620, and another 700-OTU day puts you over, even though your CNS clock read zero every morning. The cure isn't a slightly shorter dive on day 3; it's a recovery day earlier in the week that lets the odometer wind back.

Build your own week below: set the trip length, dives a day, setpoint, and bottom time, and watch the dose run against the shrinking daily budget.

How the people who study this budget it

OTUs are the legacy default, and they're rough (the pulmonary article covers why). The newer models are built for exactly this multi-day problem: Ran Arieli's index scales risk with time-squared and includes recovery between dives, and Barbara Shykoff's model predicts symptom probability across a series of 1.3-bar dives. Risberg and colleagues turned Arieli's work into practical multi-day limits with recovery days built in. None of these are in your computer yet, but they all point the same way: plan a whole trip's oxygen, not just a dive's.

So how do you total your own dose? Your computer mostly won't, so run the planned dives through desktop software like Subsurface or MultiDeco, which tallies OTUs for you. Or carry one rule of thumb: a maxed eight-hour day at 1.3 is about 700 OTU, so two big days back to back is roughly your ceiling, and three needs a rest day in between.

What this looks like on the boat

• Budget oxygen across the week, descending. Treat the early days as your big-dose days and ease off as the trip goes on, the way the REPEX taper does.
• Build in recovery days. A genuine low-oxygen or no-dive day resets more of the lung load than any short surface interval will.
• Drop the setpoint on long dives toward 0.7 to 1.2 bar. At 1.3 versus 1.2 the deco you save is trivial; the lung dose you save over a week is not.
• Take air breaks on long, high-oxygen dives. Even a five-minute break meaningfully lowers CNS risk on the day.
• Stop at the first lung symptom. A new dry cough or chest tightness means a rest day, full stop, and it will resolve if you let it.
• Arrive fit, rested, and hydrated, and stay that way. None of it shows on the computer, but all of it widens your margin. The chemistry article explains why.

Keep reading

• CNS Oxygen Toxicity: the clock that resets, and the 2025 1.3 rule in full
• Pulmonary Oxygen Toxicity: the clock that carries over, and why OTUs are shaky
• Oxygen Toxicity & ROS: the chemistry under both

References

1. Hoyt J, Murphy FG, Pollock NW, Mitchell SJ, et al. Revised guideline for central nervous system oxygen toxicity exposure limits when using an inspired PO₂ of 1.3 atmospheres. Diving and Hyperbaric Medicine. 2025;55(3):262–270.
2. Hamilton RW, et al. REPEX: development of repetitive excursions, surfacing techniques and oxygen procedures for habitat diving. NOAA Office of Undersea Research, 1989.
3. Arieli R. Calculated risk of pulmonary and CNS oxygen toxicity: a toxicity index derived from the power equation. Diving and Hyperbaric Medicine. 2019;49(3):154–160.
4. Arieli R. The pulmonary oxygen toxicity index. Respiratory Physiology & Neurobiology. 2023;315:104114.
5. Shykoff BE. Residual oxygen time model for oxygen partial pressure near 1.3 atm. Undersea and Hyperbaric Medicine. 2015;42(6):547–564.
6. Shearwater Research. Shearwater and the CNS Oxygen Clock (the ~90-minute surface recovery half-time used in dive computers).

Multi-day oxygen budgeting is the part of CCR planning students most often skip. I build it into every rebreather and expedition course I run. Ask me about training.