Deep stops after the NEDU trial
For years, stopping deeper was thought to be safer. A 2011 US Navy trial challenged that, and decompression practice has been shifting since. What the evidence says, and what it doesn't.
Read this first Information, not instruction. Nothing here replaces training with a qualified instructor.
Every decompression schedule is a model, not a measurement. No algorithm knows what's happening in your tissues. Each one is a mathematical bet about how inert gas moves and when bubbles start to cause harm. The argument about deep stops is an argument about which bet is better.
Two families of model
Dissolved-gas models descend from Haldane's work in 1908. They track inert gas loading in a set of theoretical tissue compartments, each with a different half-time. You can ascend as long as no compartment exceeds its limit, called an M-value. Bühlmann's ZH-L16 is the most widely used.
Bubble models, such as VPM-B and RGBM, try to limit the growth of bubbles that form from tiny nuclei in the body. Because bubbles grow fastest early in the ascent, these models typically place the first stops deeper than a dissolved-gas model would.
Gradient factors
Erik Baker's gradient factors, published in the late 1990s, let divers add conservatism to Bühlmann in two places. The first number, GF low, sets how close to the M-value you get at your first stop. A low value like 20 or 30 forces a deeper first stop. The second number, GF high, sets how close you get when you surface.
For years, low GF-low values were popular because they mimicked the deep stops of bubble models. Settings like 20/80 or 30/70 were common.
The NEDU trial
In 2011 the US Navy Experimental Diving Unit published a trial comparing two schedules for the same air dive, to 170 feet for 30 minutes. Both had the same total decompression time. One spent more of it deep; the other spent more of it shallow. The deep-stop schedule produced significantly more decompression sickness.
The explanation offered was that time spent deep lets slower tissues keep loading with gas, which then has to come off later. The shallower schedule gave slower tissues more time to offload.
What the trial did and didn't show
- It tested one air dive profile, not the long trimix dives many tech divers do.
- It compared two schedules with equal total time. It didn't test whether adding deep stops on top of a normal schedule causes harm.
- It measured decompression sickness, not bubbles or subtler effects.
Those limits matter, and the result is still debated. But it was a controlled human trial, which is rare in this field, and it pointed away from what much of the technical community believed at the time.
Decompression
How should the first stop be set?
Two reasonable positions after 2011.
Shallower first stops
Use Bühlmann with a higher GF low, often 40 to 55, and put conservatism into GF high and the shallow stops.
Why divers choose itCloser to what the NEDU evidence supports. Simple to set on most computers.
Watch out forSome divers report feeling worse after dives with very shallow first stops. The evidence base is still one main trial.
Keep moderate deep stops
Stay with a bubble model or a lower GF low, arguing that long trimix dives differ from the air dive tested.
Why divers choose itA long record of expedition dives without problems. The physiological case for controlling early bubble growth still has supporters.
Watch out forThe best controlled evidence we have points the other way.
Questions to ask of any plan
- What model and settings is my computer using, and do my team's computers agree?
- Where does my plan put its extra time, deep or shallow?
- Have I planned for lost deco gas, and does that plan still work?
- Am I changing settings because of evidence, or because of what my last group did?