How to Plan a Decompression Dive: Stops, Gases, Gradient Factors

What makes a dive a decompression dive?

A decompression obligation starts the moment a direct ascent would break the surfacing limit of the model you are diving. Before that point the ascent is yours to shape. After it, the model owes you time at shallow depth, and that time is not optional.

Our deco planner draws the line the same way: no stop is required if an ascent at 9 m/min on the bottom gas reaches the surface without passing GF High. At 40 m (130 ft) on air that allows 9 minutes of bottom time at GF High 85%, 7 minutes at 70% and 12 minutes at 100%, descent included. One setting decides when this dive turns into a deco dive. No-decompression limits covers where those limits come from.

Agencies draw their own boundaries around the same idea. PADI's Tec 40 certifies "limited decompression dives to 40 meters/130 feet" with one deco cylinder of up to 50% oxygen and "up to 10 minutes of decompression". TDI trains divers "to plan and conduct a standard staged decompression dive not exceeding a maximum depth of 45 Metres/150 Feet". GUE Tech 1 allows 51 m and no more than "30 minutes of unadjusted decompression time". Those courses exist for the gear, the gas and the drills, not because the arithmetic is hard, so read this page as the plan and a tech course as the way to dive one.

The inputs a schedule is built from

Almost everything a schedule depends on is set before you get wet. Take the ascent rate: DAN notes that the US Navy and NOAA use 30 ft/min (9 m/min) while recreational agency recommendations sit anywhere between 30 and 60 ft/min, and that divers in the faster group of one trial had higher bubble grades. Shearwater's Perdix manual states that its decompression calculations assume 10 m/min, a little quicker than our planner's 9 m/min default.

The rest of the list is short:

• Maximum depth and bottom time, which our planner holds at the deepest point with the descent counted inside it.
• Bottom gas, entered as analysed: air, nitrox or trimix.
• Deco gases, up to two. EAN50 and oxygen are the usual pair.
• Descent and ascent rate, 18 m/min down and 9 m/min up by default.
• Stop spacing and last stop: 3 m steps, last stop at 3 m or 6 m.
• Gradient factors, the conservatism dial.
• A bottom SAC and a deco SAC, which turn the schedule into litres.

Gradient factors: which number does what to the schedule

Both gradient factors trim Bühlmann's M-values, and dive computer algorithms explains that mechanism. A finished plan shows the division of labour between them. Doolette and Mitchell describe it in one sentence: "lowering the first gradient factor forces deeper stops to limit supersaturation in the fast tissues early in the ascent, and lowering the second will produce longer shallower stops".

Numbers make the split obvious. Take 40 m for 30 minutes on air, no deco gas. Hold GF High at 85% and lower GF Low from 40% to 30%: the ceiling on leaving the bottom deepens to 18.8 m and the stop time changes by one minute, 43 against 42. Hold GF Low at 40% and lower GF High to 70% instead: the first stop stays at 18 m while the stops grow to 62 minutes.

Because stops sit on a 3 m grid, a GF Low change often shows up as nothing until the ceiling crosses a grid depth. GF Low 30% and 40% both give an 18 m first stop here; 50% moves it to 15 m and the untrimmed model at 100/100 moves it to 9 m.

Runtime is the clock from leaving the surface, so a low GF High is paid for in water time: 30/70 keeps you under 22 minutes longer than 40/85. The 100/100 row is Bühlmann with no margin, a reference point rather than a setting to dive.

Deep stops, and why the advice on GF Low splits

The strongest data on deep stops comes from a US Navy trial that kept the total stop time fixed and only moved it around. Divers spent 30 minutes at 170 fsw (about 52 m) on air while working 130 watts on a cycle ergometer, then ascended at 30 fsw/min (9 m/min) on one of two schedules, each with 174 minutes of stop time: first stop at 40 fsw (12 m), or first stop at 70 fsw (21 m). The deep stops schedule gave 11 cases of decompression sickness in 198 dives against 3 in 192, and the trial was stopped at its interim analysis. One deep stops case was later excluded; the difference held.

Doolette and Mitchell explain the outcome without any argument about good or bad bubbles: deeper stops lower supersaturation in the fast compartments and pay for it with continued gas uptake in the slow ones. Their review adds a caveat worth carrying into your planning. Had those Navy schedules included a switch to 50% oxygen at 70 fsw (21 m), slow compartment supersaturation would have dropped on both profiles, the risk with it, and the gap between them probably too small to measure. A dive with oxygen decompression is not the dive that was tested.

That is why printed GF advice is still spread out. Neal Pollock, writing for Shearwater, maps the choices instead of naming one: GF Low values of 20 or less are "for those who believe in deep stops", while 30 or more "get divers off the bottom". David Doolette, one of the Navy trial's authors, published his own setting: "I choose my GF low to be about 83% of the GF high, for instance GF 70/85." Shearwater ships computers on 30/70. Published advice therefore covers GF Low values anywhere between 20 and 70, so dive what your instructor teaches and know what it does to your stops.

Deco gases: where the switch happens, and what it saves

A deco gas goes on at the deepest stop where its oxygen partial pressure is inside your limit. The arithmetic is MOD work: depth = (ppO₂ limit ÷ FO₂ − surface pressure) × 10, rounded down to the stop grid. Our planner allows 1.62 bar for deco gases with 1.01325 bar at the surface, so EAN50 comes out at 22.3 m and waits for the 21 m stop (1.56 bar there), EAN80 at 10.1 m waits for 9 m (1.53 bar) and oxygen at 6.1 m goes on at 6 m (1.61 bar). One stop deeper, EAN50 at 24 m would be 1.71 bar.

That is a resting limit, not a working one. GUE caps planned ppO₂ at 1.4 bar "during the working phase of the dive" and allows up to 1.6 bar "during the resting phase", which is what a stop is. TDI makes it a graded skill to "confirm gas switch(es) at depth with buddy/team members", and our planner holds at least a minute at the switch depth for that.

More oxygen means a steeper gradient for inert gas to leave, which Doolette and Mitchell describe as accelerating decompression "by faster washout of inert gas from all compartments". On the 40 m dive one EAN50 stage nearly halves the stop time, and oxygen at 6 m takes off another 6 minutes.

EAN80 shortens the stops more than EAN50 here because it is richer where most of the time is spent, but it cannot be breathed at 21 m or 15 m, so those minutes come out of the back gas. The same review advises switching "at points during the decompression that avoid peak supersaturation of the inner ear and the shallower the better".

Reading the schedule: stops, TTS and runtime

Here is the plan our planner writes for 40 m for 30 minutes on air with an EAN50 stage at GF 40/85. Read the third column as a clock, since that is how the dive is run.

The 30 minutes of bottom time include the 2.2 minutes of descent at 18 m/min. The 21 m line is the gas switch, held one minute so you can change regulators and check the gas; the model's own first stop here is 15 m, since the GF Low ceiling on leaving the bottom was 16.7 m. The stops add up to 24 minutes and the travel between them to about 5, which together are the time to surface, TTS 29 minutes. Runtime 59 minutes is when you surface.

TTS is the number to plan gas against, because a problem at the end of the bottom time leaves all of it ahead of you. It grows while you are still down there: five more minutes at 40 m make it 36.

None of it is a promise. The schedule answers a square dive at sea level, and real dives are neither square nor at model temperature. Divers get decompression sickness on schedules that stay inside the model: in one closely monitored technical project using ZH-L16, two of 122 trimix dives ended in DCS needing treatment. Shearwater's manual states the wider position in a line: "There is significant disagreement in the decompression research community about the nature and practice of decompression." Dive the computer, and carry the plan as the backup.

The schedule decides how much gas you carry

Stop times set the volume. At 20 L/min on the bottom and 15 L/min on the stops, the air-only version burns 4,119 litres, which is 172 bar out of a twin 12 L. From a 200 bar fill that leaves 28 bar and no allowance for trouble. With the EAN50 stage the back gas drops to 3,048 L (127 bar) and the stage takes 667 L, about 61 bar from an AL80.

Those are consumption figures with nothing held back. Rock bottom works through the reserve arithmetic for a no-stop dive; here the same logic runs against the whole TTS, for two divers, and covers the deco gas too. GUE writes the rule into its standards: a dive ends when any team member reaches minimum gas, and each diver must carry "a supply that allows two divers sharing gas to reach the surface or another breathable gas supply". TDI grades loss of decompression gas and deco gas sharing as course skills.

What breaks a plan in the water

The changes that rewrite a schedule underwater are ordinary, and the table below prices five of them.

If stops get missed and a diver surfaces early, the problem stops being arithmetic. DAN's guidance for suspected decompression illness is 100% oxygen "until the diver arrives at a medical facility", fluids by mouth for an alert diver, EMS to the nearest facility and a call to its emergency hotline; skin symptoms can appear "within minutes to several hours following a dive". In the water, these are the changes and the responses agencies teach:

• A slow ascent costs travel time, not stop time: at 6 m/min the stops stay at 24 minutes and the surface arrives 2 minutes later.
• Deeper or longer than planned means a new schedule before you leave the bottom. TDI trains "appropriate modifications to deco schedule in decompression emergency (over time, over depth)", and Shearwater lets you lower GF High during the dive, which its manual suggests "if you worked much harder on the bottom segment than expected".
• A lost deco cylinder sends you back to the back gas schedule, 42 minutes of stops instead of 24. TDI trains loss of decompression gas, and GUE asks Tech 1 divers to manage "a failed decompression cylinder using available team resources".
• A missed up-line or a lift bag that will not deploy is on TDI's contingency list too, which is why a reel and an SMB are required kit on the course.
• Cold and workload change the dive the model priced. The Navy divers in the deep stops trial worked on the bottom and were at rest and cold on the stops, the combination a temperate water wreck dive produces.