--- title: "Decompression Procedures" description: "How staged decompression diving works in practice: ascent control, deco gas switches, MOD limits, the 6 m oxygen stop, and contingency planning." canonical: "https://donarundas.com/depth-notes/decompression/decompression-procedures" author: "Donarun Das — TDI Trimix Instructor, KISS Sidewinder CCR Instructor" category: "decompression" published: "2025-06-15" tags: ["decompression", "gas-switch", "accelerated-deco", "gradient-factors"] 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." --- # Decompression Procedures ## What the procedure is actually for A decompression schedule is a plan for letting dissolved inert gas leave your tissues without forming bubbles that hurt you. The theory behind that plan lives in other articles: how tissues load and unload, what an [M-value](https://donarundas.com/depth-notes/decompression/how-decompression-models-work) is, how [gradient factors](https://donarundas.com/depth-notes/decompression/gradient-factors) tune the whole thing. This article is about the hands-and-feet part, the things you do in the water to make the plan come true. There are really only four jobs. Control the ascent rate. Hit the stops the computer asks for. Switch to a richer gas at the right moments. And carry a backup for when something breaks. Get those four right and decompression is mostly patience. Get the gas switch wrong and you can hurt yourself faster than any missed stop would. So a deco gas like EAN50 or pure oxygen isn't there to give you oxygen for its own sake. It's there to *remove the inert gas competing for space in your lungs*. More oxygen in, more nitrogen and helium out. The catch is that the same oxygen that accelerates your deco becomes toxic if you breathe it too deep. Which brings us to the only piece of arithmetic you can't skip. ## The number that keeps the gas safe: MOD Every gas has a depth past which its oxygen becomes dangerous, its **Maximum Operating Depth**. You find it by deciding the highest oxygen partial pressure (**PO₂**) you'll accept, then asking how deep that mix reaches before it crosses that line: > **MOD (m) = ( PO₂ limit ÷ oxygen fraction − 1 ) × 10** The PO₂ limits are conventions, not hard physiological edges (the same caution applies as in [CNS oxygen toxicity](https://donarundas.com/depth-notes/physiology/cns-oxygen-toxicity)). There are two of them, and the split is really about *dose*, not just caution. That bottom row is why the **classic oxygen stop sits at 6 m**. It isn't tradition. It's the deepest pure oxygen can go at a 1.6 ceiling, and the shallowest useful stop on most schedules. Most of your decompression gets earned right there, breathing oxygen at 6 m, because that's where the oxygen window is widest and the inert gas pours out fastest. The rule that ties it together is short: **switch to the richest mix whose cap your current depth respects.** Richest, because rich gas unloads inert gas fastest. At or shallower than its cap depth, because switching shallower than a gas allows is safe. Switching *deeper* than it allows is how people poison themselves. ## A switch, in order Here's the *shape* of an ordinary accelerated-deco ascent from a 38 m air dive carrying EAN50, to show how the pieces stack up. The stop times below are schematic, just enough to show where the time concentrates; your real schedule comes from planning software and your training, not from this table: | Depth | Time | Gas | |---|---|---| | 38 → 18 m | ascend 9 m/min | back gas (air) | | 18 m | switch | → EAN50 | | 9 m | 1 min | EAN50 | | 6 m | 5 min | EAN50 | | 3 m | 3 min | EAN50 | The bulk of the obligation clears at 6 m on the richest gas you're carrying. If you also had pure oxygen, you'd switch to it at 6 m and finish the 6 m and 3 m stops breathing that. Same shape, one more switch, even more off-gassing push at the end. ## Timing the switch: deep enough to count, shallow enough to be safe Two forces decide when a deco gas goes in your mouth, and they pull against each other. The oxygen window pulls you to switch *early*. The whole value of a richer gas is the wide off-gassing gradient it opens, so the sooner you're breathing it, the more decompression you bank. That argues for going onto each gas the moment its cap allows: EAN50 at 18 m, oxygen at 6 m, not a metre shallower than you have to. Two things pull the other way. Oxygen toxicity caps how deep each gas can go, which is the MOD arithmetic above. And isobaric counterdiffusion caps how deep you dare trade helium for nitrogen, because swapping too deep lets nitrogen flood in faster than helium leaves (the helium section below has the detail). So the gas you want is the richest one your current depth can safely take, and the helium-to-nitrogen change in particular waits until you're shallow. Your **GF Lo** (the gradient-factor setting that governs your deepest stop) quietly sets the deep end of all this. It fixes your first and deepest stop, and you can't open the oxygen window above a gas's cap until your ascent actually reaches that depth. A deep-biased GF Lo keeps you on lean back gas longer before you ever get onto deco gas, which is one more reason the deep-stop instinct costs more than it looks. A flatter, shallower-biased schedule gets you into the oxygen window sooner. The [gradient factors](https://donarundas.com/depth-notes/decompression/gradient-factors) article digs into why. Then watch the dive actually happen. **GF99**, the live supersaturation readout on a technical computer, should be *falling* once you switch and settle onto a stop. That's the richer gas doing its job, the window pulling gas out of your tissues. If GF99 stalls or climbs while you hang, the dive and the plan have drifted apart, and the number tells you before your body does: hold the stop longer, or drop back to the previous stop, until it starts falling again. Reading GF99 and Surface GF in the water is a skill of its own, covered in [safe ascents](https://donarundas.com/depth-notes/decompression/safe-ascents). ## Helium, and the rule that surprises people When you've been deep on trimix, there's a tempting idea: helium leaves the body faster than nitrogen, so switching off helium on the way up should speed up your deco. For typical dives, it mostly doesn't. The careful review by **Doolette and Mitchell (2013)** looked hard at this and found that switching off helium does **not** meaningfully accelerate decompression from typical bounce dives, the deep dives most technical divers actually do [1]. The intuition is real for very long saturation-style exposures, but it doesn't pay off on a single deep dive. Two further findings from the same work shape how careful teams handle the helium-to-nitrogen transition: > **Handling the He → N₂ switch** > > - **Heliox carries a higher rate of *neurological* DCS** than equivalent air or nitrox decompression. Helium is not a free pass. > - **Delay the helium-to-nitrogen switch until relatively shallow**, and use the highest *safe* FiO₂ (oxygen fraction) your stop depth allows. Swapping helium out for nitrogen too deep can drive **isobaric counterdiffusion**, helium leaving a tissue while nitrogen floods in, which has been linked to **inner-ear DCS**. A high-oxygen, low-inert-gas deco gas limits how much nitrogen rushes in to replace the departing helium. The short version: don't switch off helium expecting faster deco, and when you do change inert gas, do it shallow and on the richest gas your depth allows. ## The misconceptions worth unlearning A couple of beliefs do real harm in the water. **"Switching off helium speeds up my deco."** For the single deep dives most technical divers do, it basically doesn't. Helium does leave the body faster than nitrogen, but dropping it on the way up doesn't meaningfully shorten your stops [1]. Carry helium to manage narcosis and gas density, not as a shortcut through decompression. **"More oxygen is always better, so any switch is a good switch."** Only at the right depth. Breathe a deco gas deeper than its cap and the oxygen itself becomes the hazard: EAN50 at 30 m sits at 2.0 bar PO₂, far over the limit and a real seizure risk. Trade helium for nitrogen too deep and you invite isobaric counterdiffusion and inner-ear DCS. A gas is only ever as good as the depth you breathe it at. The thread through both: confirm the gas, then confirm your depth, before anything goes in your mouth. ## Job four: when the plan breaks The four-jobs list ended with "carry a backup," and that job is a discipline of its own. Real dives lose a deco gas, find a bottle that won't breathe, or blow a stop, and handling those is exactly the part you cannot learn from an article. What's worth knowing here is the shape of the risk, so you understand why the training exists: - **Lost or unbreathable deco gas.** Your decompression suddenly has to run on back gas, which is slower and may not even reach your planned stops. This is why divers carry redundant gas and plan a "what if I lose this bottle" version of every dive. - **A failed oxygen switch at 6 m.** Without the oxygen stop, your final off-gassing slows right down and the obligation grows. You need a fallback gas and a schedule that matches it. - **A missed or blown stop.** Surfacing early, or floating up off a stop, can leave you supersaturated past the plan, and the response is trained, not improvised. The takeaway isn't a procedure. It's that decompression diving demands redundancy and a worked-out contingency for every gas, and you build those with an instructor before you ever need them. ## What's settled and what's still argued Some of this is solid ground. The MOD arithmetic, the 1.4 / 1.6 conventions, the oxygen stop at 6 m, the ascent-rate discipline, and using the highest *safe* FiO₂ on deco are well established and not seriously contested. Other parts are genuinely open. The **optimal depth to switch from helium to nitrogen** is still argued, and the **best gradient factors for helium-heavy trimix** are genuinely unsettled. When you read a confident, one-size-fits-all answer to either, read it skeptically. > **Running the dive, in order** > > 1. Topside: plan the dive, choose your gases, and analyse and mark every cylinder yourself. > 2. Leave the bottom on time and ascend at about 9 to 10 m/min, slowing as you get shallow. > 3. Hold the deepest stop your schedule asks for (set by your GF Lo). > 4. At each gas's cap depth, switch to the richest gas your depth allows: confirm the gas, confirm the depth, then breathe it, hand on the valve. > 5. Breathe oxygen at the 6 m stop, where most of the off-gassing happens. > 6. Watch GF99 fall on every stop; if it stalls, hold longer. > 7. Crawl the final ascent to the surface. ## The honest summary Decompression procedure is four disciplines stacked together: a controlled ascent, the planned stops, the right gas at the right depth, and a backup for when the plan breaks. The math you must own is small, MOD and the 1.4 / 1.6 PO₂ limits, but it's the math that stops a routine ascent from becoming an emergency. Most of your decompression gets earned breathing oxygen at 6 m, the helium-savings shortcut mostly isn't real, and your habits around the gas switch matter more than any clever schedule. Confirm every gas before you breathe it, switch as deep as each gas's cap allows so the oxygen window opens early, and let the model do the heavy lifting while you watch GF99 tick down. ### Keep reading - [M-Values, Half-Times, and Tissue Compartments](https://donarundas.com/depth-notes/decompression/how-decompression-models-work) - [Gradient Factors](https://donarundas.com/depth-notes/decompression/gradient-factors) - [Safe Ascents](https://donarundas.com/depth-notes/decompression/safe-ascents) - [CNS Oxygen Toxicity](https://donarundas.com/depth-notes/physiology/cns-oxygen-toxicity) ## References 1. Doolette DJ, Mitchell SJ. *Recreational technical diving part 2: decompression from deep technical dives.* Diving and Hyperbaric Medicine. 2013;43(2):96–104. 2. NOAA. *NOAA Diving Manual*; and US Navy. *US Navy Diving Manual*, sources of the PO₂ limits and the MOD conventions. 3. Baker EC. *Understanding M-values* and *Clearing Up the Confusion About "Deep Stops".* Immersed, 1998. *Planning a decompression dive or building your gas-switch discipline? [Get in touch.](https://donarundas.com/contact) I teach this for real, in the water.* ## Common questions ### Why is the oxygen decompression stop at 6 metres? Because 6 m is the deepest pure oxygen can go at a 1.6 bar oxygen partial pressure, and it is the shallowest useful stop. Most of your decompression is earned there, where the oxygen window is widest and inert gas leaves fastest. ### How do you switch decompression gases safely? Switch to the richest mix whose oxygen limit your current depth respects, and do it mechanically every time: analyse the gas, confirm its switch depth, confirm your actual depth, then breathe it. Switching to the wrong gas too deep can trigger an oxygen seizure. ### Does switching off helium speed up decompression? For the single deep dives most technical divers do, not meaningfully. Helium leaves the body faster than nitrogen, but dropping it on the way up does not shorten typical stops, and swapping it for nitrogen too deep can cause inner-ear problems. --- **Source:** [https://donarundas.com/depth-notes/decompression/decompression-procedures](https://donarundas.com/depth-notes/decompression/decompression-procedures) **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.