--- title: "Oxygen on a Multi-Day Trip" description: "Managing oxygen toxicity across a liveaboard or expedition: why the CNS clock resets overnight but pulmonary OTU load carries from day to day." canonical: "https://donarundas.com/depth-notes/physiology/oxygen-multi-day-diving" author: "Donarun Das — TDI Trimix Instructor, KISS Sidewinder CCR Instructor" category: "physiology" published: "2025-06-04" tags: ["oxygen-toxicity", "multi-day", "ccr", "ppo2"] disclaimer: "Dive theory published by a working instructor for education. It is not medical advice and not a substitute for training. Decompression, gas, and depth decisions belong with your own training, your dive computer, and a dive medical officer." license: "© Donarun Das. Quote with attribution and a link to the canonical URL." --- # 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. ## How the people who study this budget it OTUs are the legacy default, and they're rough (the [pulmonary article](https://donarundas.com/depth-notes/physiology/pulmonary-oxygen-toxicity) 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](https://donarundas.com/depth-notes/physiology/oxygen-toxicity-ros) explains why. ### Keep reading - [CNS Oxygen Toxicity](https://donarundas.com/depth-notes/physiology/cns-oxygen-toxicity): the clock that resets, and the 2025 1.3 rule in full - [Pulmonary Oxygen Toxicity](https://donarundas.com/depth-notes/physiology/pulmonary-oxygen-toxicity): the clock that carries over, and why OTUs are shaky - [Oxygen Toxicity & ROS](https://donarundas.com/depth-notes/physiology/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.](https://donarundas.com/contact)* ## Common questions ### Does oxygen toxicity reset between dive days? Partly. The CNS oxygen clock decays with a roughly 90-minute half-time, so a night's sleep clears it. The pulmonary (OTU) load only part-heals overnight and carries from day to day, which is why it, not CNS, governs a long trip. ### How many OTUs can you do in a day? The REPEX scheme uses about 850 OTU on a single day, tapering across consecutive days (roughly 700, then 620, then 525, and lower on long trips). A maxed eight-hour day at a 1.3 bar setpoint is already around 700 OTU. ### How do you manage oxygen on a liveaboard? Budget oxygen across the whole week, treat early days as the big-dose days, build in recovery days, and on long rebreather dives drop the setpoint toward 0.7 to 1.2 bar. Plan the trip's oxygen, not just the dive's. --- **Source:** [https://donarundas.com/depth-notes/physiology/oxygen-multi-day-diving](https://donarundas.com/depth-notes/physiology/oxygen-multi-day-diving) **About the author:** Donarun Das is a TDI Trimix and KISS Sidewinder CCR instructor teaching technical and rebreather diving in the Maldives, Sri Lanka, and India. He holds the deepest verified civilian dive in India (103 m, open-circuit trimix, Lakshadweep, March 2026). Courses: https://donarundas.com/courses — enquiries: https://donarundas.com/contact > Dive theory published by a working instructor for education. It is not medical advice and not a substitute for training. Decompression, gas, and depth decisions belong with your own training, your dive computer, and a dive medical officer. ## Sitemap See the full [sitemap](https://donarundas.com/sitemap.md) for all pages available as Markdown.