Safety stop on Oxygen or Nitrox

[String]

Hmmm... I do not quite get it... Yes, statistics say that this "certain percentage" you mention is actually 85%...

Are you trying to say that almost every single diver will sonner or later get bends without any exceptions? The only ones who avoid it are just very very very lucky?

No. You have to be very unlucky to get a bend especially within tables/computer limits. But every year some people are unlucky.

The chances are small, theres a massive individual difference in tolerance as well. The number of dives most people do in their lives then statistically very very few will get bent. However, some do.

Practically speaking, for recreational diving, 18m(60ft) per minute is fine

However there is a lot of research into ascent rates that found those rates produced a lot of bubbling and therefore potential DCS (or sub-clinical DCS). It seems to be currently that rates of 6-10m/min produce least bubble. Drop below that you get more. Go above it and you get significantly more.

Decompression theory is far from an exact science. Its all just a statistical model where a pre-defined acceptable risk level.

 

[Den7]

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[Den7]

I don't think that is what he was saying at all. If you are looking for a guarantee that you won't get bent, then don't dive. The chances of getting bent on any given dive range from unlikely to certainly, but it is never impossible. The way to keep it in the unlikely range is to maintain a conservative dive profile. Do that and you have an excellent chance of never being bent.

Ohh, I see. Thanks for clarifying. You will never get food-poisoned if you never eat. (joke). However, in my first post I had some exact ascent rate figures and depths in mind and thought it was somehow possible to calculate if they would be in "the unlikely range"... But if there are chances of getting bent of absolutely any dive range then I guess my question was perhaps pointless...

 

[Den7]

No. You have to be very unlucky to get a bend especially within tables/computer limits. But every year some people are unlucky...

Quote:
Practically speaking, for recreational diving, 18m(60ft) per minute is fine...

Decompression theory is far from an exact science. Its all just a statistical model where a pre-defined acceptable risk level.

I see your point. Thanks for sharing.

However, my computer's manual says the following, "At depths greater than 60 feet (18 meters), ascent rates should not exceed 60 feet per minute (18 meters per minute). At depths of 60 feet (18 meters) and shallower, ascent rates should not exceed 30 feet per minute (9 meters per minute). " That how and why I started wondering if you may get bent ascending at 18m/60ft speed from 8m/27ft depth...

 

[String]

I doubt you'd find any diver these days prepared to risk an 18m/min ascent. Its very VERY fast. Scarily so. You can get away with it a little more at depth due to the pressure changes over a larger area. In the shallows 10m/min or ideally a lot slower is far saner.

The most critical part in any ascent is the last 10m. This should be slow. Really slow. The shallower you are the slower you want to be going.

 

[LenC]

Ohh, I see. Thanks for clarifying. You will never get food-poisoned if you never eat. (joke). However, in my first post I had some exact ascent rate figures and depths in mind and thought it was somehow possible to calculate if they would be in "the unlikely range"... But if there are chances of getting bent of absolutely any dive range then I guess my question was perhaps pointless...

A joke, but quite correct. I think the emphasis on ascent rate is really addressing an issue in the wrong direction but for the right reason. It is just easier to establish a rule which will make one conservative, thus putting them in the "unlikely to get bent" range. It really is not how fast you come up, but how quickly the pressure changes. The pressure change from 90' to 30' is exactly the same as the change from 30' to 0'. So, theoretically, you should travel those two distances in the same amount of time. The problem, of course, is that the pressure change increases as you come up. So it is not a question of doing 60' per minute until you hit 30' and then slowing to 30' per minute. It is simpler to just apply the most conservative ascent rate to the entire dive rather than try to determine what ascent rate you should be at for any particular depth.

 

[Puffer Fish]

Hmmm... I do not quite get it... Yes, statistics say that this "certain percentage" you mention is actually 85%...

Are you trying to say that almost every single diver will sonner or later get bends without any exceptions? The only ones who avoid it are just very very very lucky?

Interesting point. From personal experience, during the decade of the 70's I knew precisely one diver that got a DCS hit...and he deserved it, and went on to kill himself 3 or 4 months later.

During the time frame that he did that, I was in charge of the two dive shops in the Panama Canal Zone. In the three years I was doing that, we averaged just over 100 fills per week (across two shops), for something between 15,000 and 20,000 dives. They were all air, and represented everything from black coral diving to fairly shallow. We had two cases of DCS, one deserved and one was most likely a PFO (had hit on very first dive over 30 ft.

Today, for the last decade, of people I know, it has been averaging around 2 per year...A DCS rate perhaps 50 times what it was 30 years earlier.

What happened?

First off, there are very, very few undeserved hits...I would consider a PFO an undeserved hit, but rapid accents, dehydration and going from cold to hot are not undeserved hits.

But the issue is actually computers... they allow us to go right to the edge. If you think about it, why did slow accents and safety stops become so important.

If they will share the information, I go over every dive profile where someone got a hit, and just don't see that much of a surprise.

In the 70's (or 80's or even part of the 90's), diving tables had some many conservative aspects built in, that getting a hit was actually pretty hard to do. Today, just dive to the last minute of the computer's no deco limit, do a just too fast accent, and then a pop to the surface that last 15 feet, and do it enough times and people will eventually have issues.

So yea, 15% sounds about right.

 

[Den7]

I doubt you'd find any diver these days prepared to risk an 18m/min ascent. Its very VERY fast. Scarily so. You can get away with it a little more at depth due to the pressure changes over a larger area. In the shallows 10m/min or ideally a lot slower is far saner.

The most critical part in any ascent is the last 10m. This should be slow. Really slow. The shallower you are the slower you want to be going.

... But in your first post just half an hour ago you said, quote "Practically speaking, for recreational diving, 18m(60ft) per minute is fine... " unquote. Now you are saying quote, "I doubt you'd find any diver these days prepared to risk an 18m/min ascent." unquote. If I asked you let's say tomorrow how fast would it be?

 

[String]

... But in your first post just half an hour ago you said, quote "Practically speaking, for recreational diving, 18m(60ft) per minute is fine... " unquote. Now you are saying quote, "I doubt you'd find any diver these days prepared to risk an 18m/min ascent." unquote. If I asked you let's say tomorrow how fast would it be?

18m/min is a higher risk (certainly of bubbling) than 10m/min. But still a low risk relatively speaking.

Again it depends where your personal risk level is and where you want to draw that line.

Personally i wont ascend > 10m/min at all and no more than 3m/min for the last 10m of depth.

Even 10m/min "feels" very fast and takes some getting used to.

 

[DevonDiver]

I think 18m/min (max) is fine, if used with tables and their inherent conservatism.

When a diver loses that conservatism by switching from tables to a dive computer, then the need for slower ascents and safety stops becomes much more critical.

That said, I can't think of any reason (excepting emergencies) where a diver would be in such a big rush to surface that they would feel the need push the maximum ascent rate.