Safety stop on Oxygen or Nitrox

[BlackDog8]

In light of the recent Rick Allen case (exploding Tank), I am wondering if anybody has compared the benefits of doing a safety stop (5 min at 15-20 ft) on 100% Oxygen or a 50% Nitrox mixture? If there isn't much of a difference, handling/filling a Nitrox tank would be preferable. I am looking at a 100-120ft dive on 30-32% Nitrox with a deep safety stop. Usually I go 2-3 minutes under my max bottom time.

Thanks in advance for any responses.:dontknow:

[Blackwood]

I doubt there is any data whatsoever about doing unnecessary (per the algorithm) stops on accelerated deco gas versus backgas.

[robertarak]

I doubt there is any data whatsoever about doing unnecessary (per the algorithm) stops on accelerated deco gas versus backgas.

What he said.

[kanonfodr]

One time a guy I was diving with finished his recreational-limits dive with 50% because he had to fly the next morning, then did the second, shallow dive on that gas as well.

Peace,
Greg

[rjack321]

I doubt there is any data whatsoever about doing unnecessary (per the algorithm) stops on accelerated deco gas versus backgas.

Not only that but 50% is actually more dangerous to deal with than just transfilling 100% O2 (because you have to top it off with gas that is even more likely to carry contaminants).

Just do 6+mins instead of 3min on a different gas. Maybe even spread that time across of few stops something like:
40-1
30-2
20-2
10-3

Would be as good or better than a short 3min stop on 100% where you blood as barely made one trip around your tissues before you're ascending. Also reduces the risk of inadvertently breathing a deco gas at the wrong depth and toxing to zero (since you never brought it).

[DevonDiver]

Stock answers:

If you want to accelerate decompression, do the appropriate technical diving course.

If you haven't done any technical training, then you shouldn't be doing mixed/multi-gas diving or using O2 >40%.

IMHO:

Using a nitox pony for ascent/safety stop is going to increase your nitrogen off-gassing. This could be done on a 2-gas computer or software dive planner/custom tables. The 'con' is that you'd still need the appropriate training to ensure the gas switch, MOD etc were done properly, or else it would be an unsafe proposition.

If it were for a safety stop, then you are below your NDL anyway, so the level of risk/benefit offered by (untrained) gas switching is not justified.

[String]

The chances of recreational DCS are so slim already there's unlikely to be any statistically measurable benefit to using a rich mix for 3 minutes.

And with 100% O2 id say the risk of someone screwing up their buoyancy and getting hurt (or switching at depth) is great than any potential benefit.

Risk/Benefit analysis for me comes out as "don't do it."

[fdog]

The way this is phrased

doing a safety stop (5 min at 15-20 ft) on 100% Oxygen

tells me you don't have the skills yet.

Not that using 100% after a recreational dive is a bad thing - we clean up with O2 routinely after recreational NDL dives, before driving from sea level over a 7000' mountain pass - but that's divers that understand the procedures and risks of what they are doing.

So, like everyone else said:


Please don't do it
Please take the appropriate classes



All the best, James

[Crowley]

In the context of recreational diving, the whole concept is meaningless. The safety stop allows for a reduced pressure gradient between gas in solution and the ambient pressure and therefore makes the "ascent" less risky in terms of susceptibility to decompression sickness.

There are clear benefits to decompressing on oxygen rich gas mixtures, and therefore there is a theoretical benefit to making a safety stop on a rich gas, but the benefit is a few minutes here and there on a surface interval. If somebody is pushing the limits of recreational diving so close to the bone that they require an O2 rich safety stop, they are no longer diving recreationally and should invest in tec courses.

Cheers

C.

[h90]

My guess would be:
If would like to have more safety in the safety stop: do it a couple minutes longer or add 2 min at 3 meter.
If want to reduce N2 in the body: Take an O2 emergency mask and sleep with it half an hour.
(there are some dive operators in Thailand which staff (non diver) is doing that if they have a terrible hangover.) (I always ask myself if someone needs the O2 is there real any left??)

[BlackDog8]

Thanks for all the responses. I was looking for a simple safe way to improve my profiles which are always well within the NDl for whatever mixture I am diving with.
As we all get older it is sometimes very obvious the tables do not account for older divers.

[1_T_Submariner]

I have not heard about the tank explosion that you are referencing. I'll look for it.

Why not just hang at your safety stop a while longer? If you are within NDLS and have enough gas. I have made what could be considered 30 min safety stops really just because there was some really cool stuff at 15'.

[VooDooGasMan]

Blackdog, you can purchase a diveplanner of doing a 75% deco, on diverite express, it is less than what you want for your safety reasons, and I have used it for some time now.

I think that the tank that blew was ashame, and you must be careful of all high pressure tanks.

As for your age the 75% is a great formula and takes a few minutes of deco off, with a small deco bottle.

Happy Diving

[DevonDiver]

Thanks for all the responses. I was looking for a simple safe way to improve my profiles which are always well within the NDl for whatever mixture I am diving with.
As we all get older it is sometimes very obvious the tables do not account for older divers.

Which tables? And why don't they?

What is the issue? Post-dive fatigue?

(sorry for all the questions... just trying to get my head round what the problem is) :)

It's pretty easy to manually add conservatism to any table or computer that you use. I'd look at adding deepstops and extended safety stops as an initial solution. See how those improve things.

Otherwise, you could add some 32% into a pony cylinder. It'd still be a useable redundant gas source for most recreational dive profiles (confirm your MOD)...and there'd be no harm sucking from it for a few minutes on your safety stop.

Stay well clear of >40% for in-water use, unless specificially trained to use it. Even then, it's a serious proposition to use 100% at a safety stop.

If you want some benefit from 100% O2... then save it for on the boat... lie back, sip an iced fruit juice (or warm coffee..depending on location!) and sniff some pure stuff to your hearts' content. :D

[String]

Thanks for all the responses. I was looking for a simple safe way to improve my profiles which are always well within the NDl for whatever mixture I am diving with.
As we all get older it is sometimes very obvious the tables do not account for older divers.

The most effective and proven way is just to slow your ascent rate, don't dive sawtooth profiles and extend a safety stop. Those and backing off from the limits of the deco model you're using will combine to be far more effective in increasing the safety margin than 3 mins breathing a different gas and far less risky.

[TSandM]

It's all about risk-benefit. If you dive comfortably within the limits of whatever decompression model you are using, your DCS risk is extremely low (even as we age!). On the other hand, the survival rate for divers who experience oxygen toxicity seizures underwater is very, very poor. To avoid the dangerous, type II DCS or gas embolism issues, control in the water column is probably more important than total nitrogen load (assuming, again, you stay within your limits). That control is also precisely what you need to be able to use accelerated decompression safely.

I am technically trained and have the equipment and supplies to do O2 decompression whenever I want. But I have almost never used it for NDL dives, because the risk simply outweighs the benefit. And the one time I HAVE done it, I had an uneasy moment when I discovered that high current in tight quarters with a scooter exceeded the bandwidth I had for precise buoyancy control. Everything came out fine, but it reminded me of why one doesn't take rich mixes lightly.

[Blackwood]

FWIW, I imagine that with >1000 dives, the OP would probably be able to learn gas switching without too much trouble. When doing repetitive NDL dives, I frequently bring O2 on the last dive for "cleanup."

But in the context of the thread. I think quantifying its benefits would be difficult at best. Anecdotally I "feel better," but it's a) impossible to truly compare one set of dives to another, and b) can't rule out placebo,

[DevonDiver]

FWIW, I imagine that with >1000 dives, the OP would probably be able to learn gas switching without too much trouble.

I am positive that an Advanced Nitrox course would be an easy transition for him... and would answer many of his questions :D


When doing repetitive NDL dives, I frequently bring O2 on the last dive for "cleanup."


I don't see the need to do this at the safety stop. For a no-deco dive, the benefit is so intangible that O2 might as well just be breathed on the boat.

When I lived in the UK, I had an O2 cylinder that was often used as a
'pick me up'. The best hangover cure I ever used was 1hr of O2, along with 2 bottles of Lucozade Hydroactive and a McDonald's breakfast... :rofl3:

[Blackwood]

Ya, it doubles as practice I guess.

[DevonDiver]

People often see the Advanced Nitrox course purely as the first step into tech. But it can be taken in a single cylinder... and does present a higher level of understanding about the risks and benefits of O2 use about the recreational limits (>40%). :)

[String]

Advanced Nitrox for me isn't really a "tech" course. Its the first part of the severe dive skills and competency workout you need before starting tech training. From that, id say its very useful even for someone who has no interest in technical diving as it WILL massively improve their comfort level and skills regardless of their entry level.

[Rhone Man]

It's all about risk-benefit. If you dive comfortably within the limits of whatever decompression model you are using, your DCS risk is extremely low (even as we age!).

Yes and no. When I was in Truk, we were all doing 4+ dives a day below 100 feet, and although we were all staying within the NDLs on the computers, toward the end of the week people were experiencing minor joint pains in the evenings which had disappeared by the following morning. Maybe nothing to do with DCS, but I have my suspicions. The models just get shakier and shakier the more heavy repetitive diving you do (which we all knew anyway, right?).

It crossed my mind more than once that carrying a bottle of 80% for the safety stops (had it not been so exhorbitant in that part of the world) might have been a very prudent precaution.

Just for the record, we were doing 6 minute safety stops after each dive.

[ramasule]

I am positive that an Advanced Nitrox course would be an easy transition for him... and would answer many of his questions :D



I don't see the need to do this at the safety stop. For a no-deco dive, the benefit is so intangible that O2 might as well just be breathed on the boat.

When I lived in the UK, I had an O2 cylinder that was often used as a
'pick me up'. The best hangover cure I ever used was 1hr of O2, along with 2 bottles of Lucozade Hydroactive and a McDonald's breakfast... :rofl3:

I second this. My buddies and wife are lifeguards at a pool. There has been a couple times after some very heavy drinking that I have ended up there sucking back some O2 with mcdonalds and poweraid. I can confirm the above statement by Devon as 100% true :)

[rjack321]

Yes and no. When I was in Truk, we were all doing 4+ dives a day below 100 feet, and although we were all staying within the NDLs on the computers, toward the end of the week people were experiencing minor joint pains in the evenings which had disappeared by the following morning. Maybe nothing to do with DCS, but I have my suspicions. The models just get shakier and shakier the more heavy repetitive diving you do (which we all knew anyway, right?).

It crossed my mind more than once that carrying a bottle of 80% for the safety stops (had it not been so exhorbitant in that part of the world) might have been a very prudent precaution.

Just for the record, we were doing 6 minute safety stops after each dive.

Ok I get there are no boosters in Truk.

But if you were just going to do 6mins on a different gas... why on earth would you use 80%? 2000psi of O2 even in a cylinder as small as an al19 is enough for 6mins at 20ft. In an al40 2000psi of O2 is good for ~30mins at 20ft.

And since you were diving air/nitrox and nitrogen is the only inert you need to eliminate, having 20% less of it is significant.

[DevonDiver]

But if you were just going to do 6mins on a different gas... why on earth would you use 80%? 2000psi of O2 even in a cylinder as small as an al19 is enough for 6mins at 20ft. In an al40 2000psi of O2 is good for ~30mins at 20ft.


OMG.... did you just make a case for............a Spare Air cylinder!! :shocked2::shocked2::shocked2:

:rofl3::rofl3::rofl3:

[rjack321]

OMG.... did you just make a case for............a Spare Air cylinder!! :shocked2::shocked2::shocked2:

:rofl3::rofl3::rofl3:

Nitrox spare air :cool2:

[DevonDiver]

You know they actually sell that, right? sigh

I was thinking of one for 80%... complete with MOD decal. That'd turn some heads on the dive boat...

[Rhone Man]

But if you were just going to do 6mins on a different gas... why on earth would you use 80%? 2000psi of O2 even in a cylinder as small as an al19 is enough for 6mins at 20ft. In an al40 2000psi of O2 is good for ~30mins at 20ft.

And since you were diving air/nitrox and nitrogen is the only inert you need to eliminate, having 20% less of it is significant.

You could use pure O2; I just threw 80% out there because out of habit I don't like to go richer than that.

I did find it interesting though: after each dive I made it a habit to check my "no-fly" time on my Suunto, and normally it would be around 25 hours, except for after the one planned decompression dive I did, where after spending a chunk of time plus extended safety stop on a fairly rich mix, I came out with a calculated "no fly" time of a meagre 15 hours.

[rjack321]

You could use pure O2; I just threw 80% out there because out of habit I don't like to go richer than that.

Because? You were trained in the differences between 80% and 100% no?



I did find it interesting though: after each dive I made it a habit to check my "no-fly" time on my Suunto, and normally it would be around 25 hours, except for after the one planned decompression dive I did, where after spending a chunk of time plus extended safety stop on a fairly rich mix, I came out with a calculated "no fly" time of a meagre 15 hours.

That's because repetitive NDL diving (esp. with the relative rapid ascents recreational divers typically do) is frequently more aggressive than planned decompression dives. At least mathematically.

[String]

Multiple dives over a short period of time combined with short surface intervals are still where most tables and deco models struggle.

[Rhone Man]

Because?

Personal preference.


You were trained in the differences between 80% and 100% no?

Yes; thanks for trying not to sound too patronising.

[Den7]

Hello there!

I have one question for experts on ascent rates.

Let me start form a little foreword.

Common regular dive computer manuals mention two different ascent rates as follows: 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). I went through a number of various PADI class books on purpose but did not find a single word about the second ascent rate (30 feet / 9 meters) mentioned anywhere. -that would be another story-

The Oceanic computer I own (VT3) has a VARI (Variable Ascent Rate Indicator) that consists of 5 segments. The computer automatically switches its calculations in between two different basic ascent speeds of 30ft / 9m and 60ft / 18m per minute. Each segment represents your speed increase pretty much the same way like a speedometer does (it's also quite easy to calculate each segments' exact speed knowing the maximum allowed rate or simply by refering to your dive computer manuals' VARI charts).

Now my question is if the shallower ascent rate (30ft/9m) is violated (PADI classbooks mention only 60ft/18m) then I assume it can be disregarded and do not cause any decompression issues provided we do not go into deco and do not dive deeper than 30ft/9m? Does it not work as a conservative factor which in this case reduces the risk as twice as much (60:30=2 or 18:9=2) does it?

Would appreciate any advices. Thanks.

[DevonDiver]

The PADI ascent rates are based upon using their tables. The textbook does mention that if using a computer, you should follow the manufacturers recommendations (inc ascent rates) in all respects.

[Den7]

OK. Thanks. I always follow the manufacturers' recommendations but I was also wondering what others may think as I thought there was some logic in my reasonings... If I do not get exceeded nitrogen accumulations in my tissues then I could not possibly get any decompression problems is that not right?

[Crowley]

OK. Thanks. I always follow the manufacturers' recommendations but I was also wondering what others may think as I thought there was some logic in my reasonings... If I do not get exceeded nitrogen accumulations in my tissues then I could not possibly get any decompression problems is that not right?

No it's not right...
firstly - how do you know what the saturation limits of your tissues are? The models are purely theoretical and are accurate insofar as they have been tested. Maybe one person is a bit of an exception to the norm. There are all sorts of variables that can contribute to decompression problems - physical conditioning, body fat, illness, scar tissue, PFO, vascular/circulatory problems, heat, cold, level of activity underwater... That's why conservatism is essential.

As for the ascent rate - different studies arrive at different conclusions - in some cases an 18m per minute ascent seems to be beneficial over 9m per minute (I lost this paper, will try to dig it out again). The actual limit is somewhat of a compromise between older models and what navy divers were doing at the time - basically "we want 30ft/min - no, we want 100ft/min, okay, you can have 60ft/min" is what it boils down to - have a look at a brief history at this article:
DAN Divers Alert Network (http://www.diversalertnetwork.org/news/Article.aspx?newsid=514)

Different computers adopted different ascent rates based on the mathematical model they were using.

Practically speaking, for recreational diving, 18m(60ft) per minute is fine - if it wasn't, lots and lots and lots of people would get bent, and they don't. It can't be a matter of pure luck that millions of divers worldwide abide by this and fail to get bent. For the same reason, back to the original post - the concept of doing a safety stop on an enriched air mixture or pure O2 is, for the vast majority of recreational dives, not relevant.

Your dive computer will calculate gas loss based on what you do. If it is programmed for different ascent rates at different depths and you violate that, it will maybe have you make a short "deco" stop or give you a "ceiling" at which you must maintain yourself for a period of time. This is purely down to the mathematics and the research behind the computer. Practically speaking, you're unwise to violate anything, even if one computer tells you different from another.

Cheers

C.

[LenC]

If you are not doing decompression diving, then the risks of using high levels of o2 outweigh any minimal benefits you might gain.

[String]

If I do not get exceeded nitrogen accumulations in my tissues then I could not possibly get any decompression problems is that not right?

Exactly wrong. Every year a certain percentage of bends are from people diving conservatively within the recommendations of their tables and/or computer.
The only way to not get decompression problems is to not dive.

[Den7]

Exactly wrong. Every year a certain percentage of bends are from people diving conservatively within the recommendations of their tables and/or computer.
The only way to not get decompression problems is to not dive.

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?

[Den7]

Thank you so much for your insights! I really do appreciate! My comments are highlighted below in red. Yes, my question was in pure mathematics where I gave you the counting out / reading point and hoped you could advise from the practical point of view. It is of course unwise to violate anything at all but doing pure mathematics and theory I thought that my simple calculations had some logic... (take a look at my first initial post)...


No it's not right...
firstly - how do you know what the saturation limits of your tissues are? I will use my computer's readings and will base on its implemented algorithm (Pelagic DSAT, Pelagic Z +, etc.). I might as well reduce the readings (conservative factor) to be on a safe side. The models are purely theoretical and are accurate insofar as they have been tested. Maybe one person is a bit of an exception to the norm. There are all sorts of variables that can contribute to decompression problems - physical conditioning, body fat, illness, scar tissue, PFO, vascular/circulatory problems, heat, cold, level of activity underwater... Agreed, it makes a big difference and adds to deco issues. That's why conservatism is essential.

As for the ascent rate - different studies arrive at different conclusions - in some cases an 18m per minute ascent seems to be beneficial over 9m per minute (I lost this paper, will try to dig it out again). The actual limit is somewhat of a compromise between older models and what navy divers were doing at the time - basically "we want 30ft/min - no, we want 100ft/min, okay, you can have 60ft/min" is what it boils down to Here you go. Despite various schools, systems, algorithms, tables, etc. it looks like everybody agreed to have one general basic rate - have a look at a brief history at this article:
DAN Divers Alert Network (http://www.diversalertnetwork.org/news/Article.aspx?newsid=514)

Different computers adopted different ascent rates based on the mathematical model they were using.

Practically speaking, for recreational diving, 18m(60ft) per minute is fine - if it wasn't, lots and lots and lots of people would get bent, and they don't. It can't be a matter of pure luck that millions of divers worldwide abide by this and fail to get bent. For the same reason, back to the original post - the concept of doing a safety stop on an enriched air mixture or pure O2 is, for the vast majority of recreational dives, not relevant.

Your dive computer will calculate gas loss based on what you do. If it is programmed for different ascent rates at different depths and you violate that, it will maybe have you make a short "deco" stop or give you a "ceiling" at which you must maintain yourself for a period of time. This is purely down to the mathematics and the research behind the computer. Practically speaking, you're unwise to violate anything, even if one computer tells you different from another.

Cheers

C.

[LenC]

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?

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.

[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]

deleted

[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. :D (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. :D (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? :D

[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? :D

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.

[Crowley]

... 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? :D

In String's defence, it was me that said that 18m/min is fine - not him

Do I ascend a this rate? No - I do what my computer tells me to, and the Suunto RGBM model suggests that microbubble formation on ascent limits the ascent rate to 10 metres per minute. I stick to that.

In the area I work, the diving we do is almost ridiculously conservative - 1.4ata, no deco, etc. - and this is for our own safety as much as it is for others. We keep it conservative because we are almost permanently fizzy, and this is what our customers expect of us, because they want safe, enjoyable dives floating about with the fish.

If people want to push limits - whether diving or racing motorcycles, this is fine, but it's not for the average diver, nor the average biker. 150mph through heavy traffic is indeed possible, (I know, because I've done it) but just because it's possible doesn't mean it should be advocated. You adapt your riding to the road conditions just in the same way you adapt your diving to what you are doing at the time.

Different computers will tell you different things - but for all I might know, or whatever people on this forum tell me, I'm not going to argue with a company that's invested millions of dollars in research over 30 years, however conservative their computers are.

Stay safe

C.

[String]

In the area I work, the diving we do is almost ridiculously conservative - 1.4ata, no deco, etc. - and this is for our own safety as much as it is for others. We keep it conservative because we are almost permanently fizzy, and this is what our customers expect of us, because they want safe, enjoyable dives floating about with the fish.
.

That is a big thing. The current tables and models fall down in particular when dealing with short surface intervals and long periods of repeat/multiple diving such as you get when working.

2-3 dives a day, 7 days a week for sometimes weeks on end is going to put you far more on the edge of the model than someone doing 2 dives a day for 3 days then going home.
Its (relatively) unusual to see an instructor get bent even on these schedules. That said i know of 3 instructors with perforated ear drums in the last 4 months now!

[Den7]

In String's defence, it was me that said that 18m/min is fine - not him

Do I ascend a this rate? No - I do what my computer tells me to, and the Suunto RGBM model suggests that microbubble formation on ascent limits the ascent rate to 10 metres per minute. I stick to that.

In the area I work, the diving we do is almost ridiculously conservative - 1.4ata, no deco, etc. - and this is for our own safety as much as it is for others. We keep it conservative because we are almost permanently fizzy, and this is what our customers expect of us, because they want safe, enjoyable dives floating about with the fish.

If people want to push limits - whether diving or racing motorcycles, this is fine, but it's not for the average diver, nor the average biker. 150mph through heavy traffic is indeed possible, (I know, because I've done it) but just because it's possible doesn't mean it should be advocated. You adapt your riding to the road conditions just in the same way you adapt your diving to what you are doing at the time.

Different computers will tell you different things - but for all I might know, or whatever people on this forum tell me, I'm not going to argue with a company that's invested millions of dollars in research over 30 years, however conservative their computers are.

Stay safe

C.

Thank you for your comments, Crowley. I appreciate.

What surprises me the most was that none of my instructions told me that the safest maximum ascend rate would be about 10 meters / 33 feet per minute. In fact, my computer's manual says it is even 9m/30ft per minute. They all for some reason kept on insisting that 18 meters / 60 feet per minute was the maximum safe limit. The same data (18/60) is also written in my PADI textbook. I never push the limits though but to find out that the recommnded limit is actually twice as less... came out as a big surprise to me (when I bought my new computer) which is also contrary to what the PADI OW textbook says. Thanks goodness I always sticked to much slower ascend rates.....

That's why I asked the initial question (just an idea that crossed my mind) if twice as less or twice as much the other way around would not jeapordize my safety limit which in both cases would get all down to 18m/60ft per minute only... It looks like I was wrong and my idea did not prove to be right...

[Blackwood]

The same data (18/60) is also written in my PADI textbook.

That's because their model is based on a 60fpm ascent rate. Slower is considered 'safe' (longer time decompressing than the model predicts is necessary), faster is considered 'unsafe' (shorter time decompressing than the model predicts is necessary).

No ascent rate per dissolved gas models is 'unsafe' in and of itself. But when you are diving to limits computed using a specific ascent rate, that rate becomes important. One could just as easily construct a 120fpm table as a 60fpm table.

Regarding the PADI/DSAT model, it may 'work better' at 60fpm than 30fpm (higher offgasing gradient). In either case, you should be both theoretically and statistically okay if you abide by all the limits in the model (including ascent rate).