Safety Stop Depth

[sotgecho]

What is the reasoning for the standard practice depth of 15 ft for a safety stop? Are their physiological reasons i.e. better gas exchange for nitrogen out of your system, less nitrogen load, etc.?

Is it better to do gradual stops? For example doing a dive and your max depth was 90 ft and then do a 1 minute stop at 45 ft then move up every 15 ft for 1 minute at each depth until you reach 15 ft.

Thanks for the insight.

 

[TSandM]

This is actually somewhat controversial. NAUI is now teaching, I believe, a two minute stop at half of your maximal depth, and then three minutes in the shallows. The Marroni studies (available on the DAN website) suggested that an ascent with stops was a better way to prevent bubbles than a generally slow ascent, but their slow ascents began at the bottom. Decompression software generally creates a profile where the diver goes up relatively rapidly when deep, and then progressively slows as he approaches the surface (which shape is a consequence of the assumed exponential nature of loading and unloading of inert gas).

The bottom line is that the decompression model you are using has built into it some assumptions about ascent rate. With the RDP, for example, the no-deco times generated assume a steady ascent at 60 fpm, or somewhat less. So long as you do not exceed that speed, you don't actually REQUIRE any stops, but they are recommended as an extra safety net, and to slow the diver in the shallows, where many people don't maintain control of their ascent rate. The necessity for deeper stops on recreational dives is more debatable. DCS is rare in properly executed recreational dives, so any change which caused a reduction in DCS would have to be studied over a VERY large number of dives to be proven effective.

 

[sotgecho]

Thank you for the insight.

Is there any research in doing a safety stop for longer then 3 minutes say like 6 minutes? Or is it not prudent because there are diminishing returns on the amount of nitrogen exchanged beyond 3 minutes?

I am trying to develop my knowledge as I read more about dive theory.

 

[DMDavid]

I am no decompression expert but my understanding is that the 15ft stop places you within 1/2 of an atmosphere from the surface giving a final oportunity to off gas at a lower rate than being on the surface.

While this would be considered a deco stop, it is not required but rather highly recommended as a added safety precaution.

Kind of like the old soda bottle example, if you don't open it completely just shy of fully opening it then you will achieve a higher chance of the soda not bubbling over.



FB-Florida Scuba Diver

 

[DA Aquamaster]

Deco theory is nice, but there is also a practical side. In large swells you will get pressure change at a deeper safety stop than you would at a 10' stop due to the mechanics of the swell and wave motion. In rough seas 15 or 20 ft makes more sense.

It is also potentially a safety issue. I have seen numerous divers get hit or nearly get hit by the props or rudders of large boats in rough seas. Again, being a bit deeper at 15-20 ft can reduce that risk.

 

[dmoore19]

Thank you for the insight.

Is there any research in doing a safety stop for longer then 3 minutes say like 6 minutes? Or is it not prudent because there are diminishing returns on the amount of nitrogen exchanged beyond 3 minutes?

I am trying to develop my knowledge as I read more about dive theory.

The book Deco for Divers by Mark Powell helped me understand a lot of the theory. I will say it is not a "fun" read but it is informative and helped me understand a lot of the theory behind decompression theory, bubble theory etc. You can find it on amazon.com

 

[DocVikingo]

Is there any research in doing a safety stop for longer then 3 minutes say like 6 minutes? Or is it not prudent because there are diminishing returns on the amount of nitrogen exchanged beyond 3 minutes?

Hi sotgecho,

If you decide to perform them, it wouldn't be a good idea to extend deep stops beyond NAUI's recommendation due to concerns about actually taking on additional nitrogen.

As for safety stops per say, provided you are able to remain steadily and safely at ~15' an extention may add a very tiny margin of safety.

Regards,

DocVikingo

 

[TSandM]

Again, even if you OMIT a safety stop altogether (except, perhaps, in the highest pressure groups, where a safety stop is "mandatory" -- and we have argued about what that means here before!) you should be fine. DCS is rare. Whether a 3 minute safety stop or a 6 minute stop is "better" would require being able to show a statistically and practically significant difference in DCS rate, and doing so would almost certainly require more dives than any entity today can or is willing to fund for research, when we already have perfectly good tools.

My general attitude is that if there is something to see in water shallower than 30 feet, and I'm warm and comfortable and have plenty of gas, I stay as long as I want up there (or at least to the boat's time limit). I've done 30 minutes of additional "deco" at 20 feet, on a staged decompression dive, just because we were somewhere beautiful, and there was no real reason not to swim back to the boat.

 

[sotgecho]

I purchased the Deco for Divers seeing it recommended here and elsewhere.

Currently as part of my DMC internship I have been diving a lot more recently. Last week I did about 8 dives (within 3 days) with profiles in the 90 to 85 fsw for on average of 52 minutes total dive time on 34% to 36% O2; surface intervals of 45 to 50 mins between dives . I have been making slow ascents and making a stop for 1 minute or so at half my max depth mostly to adjust my buoyancy because I have become really aware of how much my buoyancy shifts with an AL80 and to shoot a bag.

My questions and concern really comes from the nitrogen load I take on doing a lot of deep long dives over a short period of time. I have talked to some of the other DMs and instructors some practice the 1 minute at half the depth and to proceed to 15ft for 3 minutes as the final stop. The reason for the 1 minute stop is to gauge how well the group of divers they are leading is ascending to the final stop since all the diving we do it deep drift diving.

 

[Mr Carcharodon]

Why 15 fsw? Well much deeper than 30 feet you will not be off gassing efficiently, and may be on gassing nitrogen in your slower compartments that never got up close to ambient during the dive. That the stop depth is not shallower that 15 fsw likely stems from the physiologists hedging about where they placed the limits. They hedge because you can always think of a situation that does not fit very well with the work they did. Your situation, many deep dives in a row, is one of those. So someone could ask is the limits still apply. For repetitive dives a safety stop gives you some margin against residual nitrogen in slow compartments. Maintaining your ascent rate and doing a brief pause at 15 fsw should work for fast compartments. A few feet on either side if 15 fsw will not make much difference.