Why 15 ft for a safety stop?

[MikeFerrara]

*Edited* - or maybe it isn't true, now that I think about it. I see that you could continue to load up on nitrogen, but then the cause of DCI wouldn't be the loading of nitrogen, it would instead be the expansion of that nitrogen as you ascended. Which is certainly the cause for the bends.

Expanding gas effects the size of the bubbles but if all the gas is disolved, there are no bubles (as far as a disolved has model is concerned). The disolved gas models (Haldanian) don't directly address the behavior of bubbles as the "bubble models" do. Disolved gas models basically deal with the difference between the disolved gas pressure withinin multiple theoretical tissue compartments and ambient pressure...in other words, how fast the gas is trying to come out of solution...too fast and you get bent.

The disolved gas model isn't driven by Boyle's Law...it's more of Henry's law sort of thing (gas transport laws).

 

[Doppler]

I really want to stay away from this thread but it's like a bloody car wreck... disturbing and fascinating at the same time.

Heaps of stuff here that's total bull**** and a fair amount of stuff that is completely and utterly off on a terrible tangent... has no place in a forum obviously aimed at folks new to diving. In any event, let's try to address one issue:

So lets say that we try an experiment to see how to inflict the least amount of stress on the balloon while bringing it up.

That's fine to model the behavior of a balloon... but a diver's body and in particular gas within that body has several other factors influencing it other than ascent in the water column. Modern deco theory suggests that bubbles will grow even when a diver maintains his depth... one reason being that dissolved gas wants to return to its free phase form and will take the path of least resistance to do it... look up Graham's Laws of Diffusion and Effusion. There are a couple of other issues as well... Mike brought one up in his post... but start off with the assumption (non-Haldanian) that bubbles form even on the most benign sport dives and Grahams is something to consider when thinking about staying "safe."

A 15-foot stop is kind of arbitrary but it works and is above the off-gassing ceiling for any dive... so use it... you'll feel better.

 

[MikeFerrara]

and a fair amount of stuff that is totally off on a terrible tangent... has no place in a forum entitled New to Diving.

If we're thinking of new divers as 12 year olds who haven't been to school yet, that's probably true.

A 15-foot stop is kind of arbitrary but it works and is above the off-gassing ceiling for any dive... so use it... you'll feel better.

But why? LOL, the op already got that answer in their OW text and on the tables that were handed out in class...cuz I said so.

 

[murdrcycle]

That's fine to model the behavior of a balloon... but a diver's body and in particular gas within that body has several other factors influencing it other than ascent in the water column. Modern deco theory suggests that bubbles will grow even when a diver maintains his depth... one reason being that dissolved gas wants to return to its free phase form and will take the path of least resistance to do it... look up Graham's Laws of Diffusion and Effusion. There are a couple of other issues as well... Mike brought one up in his post... but start off with the assumption (non-Haldanian) that bubbles form even on the most benign sport dives and Grahams is something to consider when thinking about staying "safe."

Not to split hairs, but does modern deco theory suggest that bubbles will grow even when a diver maintains depth or does it suggest that bubbles will form even when a diver maintains depth? And yes, I understand that the balloon comparison is an over-simplification, but once again this is being made overly-complicated and the base theory that repeated expansion and contraction causes more stress than keeping the size constant.

A 15-foot stop is kind of arbitrary but it works and is above the off-gassing ceiling for any dive... so use it... you'll feel better.

I will feel better if I use a 15-foot stop. In the Middle Ages people suffering from headaches felt better having holes drilled in their heads because it let the evil spirits out. But I will not just feel better but, more importantly, will know better if, once again, I understand what to do in case I can't make a stop at 15 ft.

The retort "because I said so" or "because that is what the experts agree upon" does not instill any faith in me, especially when it seems that the experts agreed upon a generalization that has a one-size-fits-all appeal.

 

[TSandM]

murdrcycle, I think it's fantastic that you're thinking about this stuff, and you're coming very close in a lot of ways to building a model in your mind that makes sense. You're oversimplifying it in places, and you're a little off base in others, but at least you're trying to come up with a rational picture of WHY you are doing what you are doing, and when you are a situation that doesn't fall within the protocol you have been taught, and how to cope with that.

Dr. David Sawatzky, who writes a column for Diver Magazine, said something that stuck with me. He said that the biggest reduction in DCS incidence occurred with Haldane's insight into the inert gas ratios, and the slowing of ascents rates. Everything that's been done since is really tweaking the very small remaining incidence of DCS. Today, DCS is rare in recreational diving, especially if people have reasonable skills and respect what they've been taught.

If you are doing the dives you should be doing as someone with 7 dives (which means relatively shallow and not pushing NDL limits), you should be fine doing a controlled ascent without a safety stop, or doing your safety stop anywhere above 30 feet. I could go into the mathematics of why this is true, but it has to do with the fact that the typical recreational dive is usually in the range of one hour long, which means that you're really worried about nitrogen going in and out of fairly fast compartments (because the slow ones don't have time to get much load in them). The good news is that nitrogen loads into fast compartments quickly, but offloads quickly, too. (Remember, this is all theory.)

Remember that the concept of NDL is that you can do a direct ascent to the surface, at a controlled rate, from any point in an NDL dive. NDL limits are different in different tables, or on different computers, because of the different assumptions that are made in constructing the underlying model. But all of the tables and computer models have been subjected to some amount of empiric validation -- meaning that they had people dive within those limits, and monitored their symptoms. So what this means is that, if you are forced into a situation where you can't do a safety stop from one of your relatively shallow and short dives, as long as you control your ascent rate, you are probably okay. I say probably, because some DCS occurs where people have done nothing wrong, and those cases remind us that our understanding of all the factors that go into nitrogen dynamics in the body is incomplete at best.

The same kind of analysis tells you that you are doing yourself good by doing a safety stop pretty much anywhere above 30 feet (again, given that you have been doing relatively short recreational level dives). The shallower you do the stop, the stronger the gradient driving nitrogen out of your body, but the more difficult it is to maintain a steady depth, especially if conditions are not ideal. The deeper the stop, the more slowly you are eliminating nitrogen. But for all intents and purposes, in recreational dives, above 30 feet you are offgassing all the compartments that you are worried about.

If you really want to know the how and why behind those statements, Erik Baker's paper cited above is a good place to start. Another very good read is the pair of threads by Dopper (who posted above, and is a very erudite man) on TDS, which I mentioned before. If you read those and want to know more, PM me and I'll send you some more URLs. There is a lot of good reading material on the web about decompression. There are some good books, and GUE has an excellent and very interesting DVD with interviews from people who are actively doing research in this area.

 

[shadragon]

Let us assume you have not been diving for a week. Before the first dive, your body tissues have gasses diffused in them with a value equal to the partial pressure of 1 atmosphere. Lets say sea level to keep things simple. For the purposes of this example, we will call that value Y. As you descend, the pressure of the water will force the gas you breathe into your tissues to try to equalize to the partial pressure of the depth you are at. The longer you remain at any given depth, the more your body will load up on those gasses until they reach equilibrium with the surrounding pressure. This pressure is highest at the deepest part of your dive. Lets call that value of gasses in your blood at any given point X. X will not necessarily be at equilibrium with the surrounding water, but it will certainly be higher than Y.

OK, as you ascend and water pressure decreases, your body now has gasses at a higher partial pressure than the water. So your body will naturally begin to off gas to match the pressure around you. As everyone should know, if you go up too quickly your body forms bubbles in your bloodstream due to rapid off gassing and this may cause DCI. So the longer you take going from X to Y the slower (and safer) the transition will be.

If you do a continuous ascent your body is exposed to less and less water pressure causing more of a difference between the pressure of the gasses in your tissues and the effects water depth. Depending on your physiology (hydration, amount of rest, physical condition, etc) and rate of ascent this may be more than your system can metabolize and you get bubbles.

So instead of a continuous ascent, you do a safety stop. The depth of that stop does not matter. Value X is still higher than Y, but the water pressure is now constant as you are neither ascending nor descending. As the partial pressure in your blood gasses come closer to the saturation point dictated by the depth you are at the difference gets smaller and smaller. Lets choose an arbitrary value for example purposes and say the amount of N2 in your blood is 32. As I understand tissue half lives the blood gas value will reduce to 50% of its remaining amount with each time increment. So at the end of one minute the new value is 16, it will be 8 after 2 minutes, 4 after 3, 2 after 4 and 1 after 5 minutes, etc. Each time unit reducing by half the value. As you stay at a constant depth the rate of off gassing slows down over time as there is less and less pressure difference with each passing minute. It will take a long time to equalize totally, but that does not matter. However, as you can see the most amount is lost in the first three minutes going from 32 to 4 or roughly 12.5% of its original value or an 88% reduction. As you are not ascending the rate of off gassing slows down over time and the overall average is lower. This is a safer situation than a continuous ascent where your body may be maxed out.

So the key factor does not seem to be the depth of your safety stop as the OP asked, but the time you spend at them. If you do a deep dive then several stops on your way back up the line combined with a slow rate of ascent between them will give your body time to equalize.

This is over-simplified for example purposes, but should demonstrate the value of safety stops. Until real time blood gas analysis technology becomes available that is...

 

[MikeFerrara]

I will feel better if I use a 15-foot stop. In the Middle Ages people suffering from headaches felt better having holes drilled in their heads because it let the evil spirits out. But I will not just feel better but, more importantly, will know better if, once again, I understand what to do in case I can't make a stop at 15 ft.

The retort "because I said so" or "because that is what the experts agree upon" does not instill any faith in me, especially when it seems that the experts agreed upon a generalization that has a one-size-fits-all appeal.

The decompression models give us a benchmark and provide some general information we can use to make decisions but, beyond that, we're pretty much winging it. Nobody is measuring what's actually going on in our individual body and there are a ton of variables that the models just don't take into account. Hence the often heard recommendation to dive conservatively.

A safety stop is just a bit of conservatism added in the form of a small amount of in-water decompression when the model suggests that we can ascend directly to the surface. Doing shorter, shallower, fewer dives and more "friendly" profiles all add conservatism.

As far as "why" or "how" a safety stop helps, several of us have tried to explain and have pointed you to sources where you can get a more detail.

 

[Doppler]

Not to split hairs, but does modern deco theory suggest that bubbles will grow even when a diver maintains depth or does it suggest that bubbles will form even when a diver maintains depth?

Rather than posting here, read any of the several references already given and you will find the answer. I even offered to email you an article which would possibly help explain some of this stuff... but you seem to need to reinvent the wheel.

And yes, I understand that the balloon comparison is an over-simplification, but once again this is being made overly-complicated and the base theory that repeated expansion and contraction causes more stress than keeping the size constant.

I have no idea what you mean... absolutely none. Please explain it... I am not trying to be funny. I just do not get it. If you are suggesting that bubbles can be kept a constant size, please cite the science. Everything I have read on the subject and the tiny bit of research I've been involved with seems to say otherwise. What worries me here is that loose and inappropriate comparisons with children's toys are being used in a public forum and some poor schmuck may read it and think it's worth remembering.

The retort "because I said so" or "because that is what the experts agree upon" does not instill any faith in me, especially when it seems that the experts agreed upon a generalization that has a one-size-fits-all appeal.

I don't think anyone is saying do it because the experts said so... there are no experts in decompression theory... we are all part of the experiment. (And some of us are keeping score is all.)

Your original question was: I was just wondering - it seems a little arbitrary. Does that mean that a safety stop at 20 feet is less safe? Or one that begins at 20 ft and ends at 10 ft 3 minutes later won't allow proper off-gasing? Or, what if there is a very convenient place to stop at 25 or 10 feet, but not at 15? Should I try to hover at 15 anyway?"

You have been given several suggestions of where to go and what to read to learn more, but in the final analysis it comes down to doing what your body tells you works best for you. Since you are an inexperienced diver, it seems correct to suggest you follow what has become best practice for sport divers rather than experiment with other options. The reasons for a safety stop have already been explained to you and will become much clearer if you read some of the material suggested to you.

I apologize if I sound short with you. Not my intention. Please go do some reading. You will find it valuable.

By the way, when sport diving agencies adopted the practice of teaching a safety stop after a dive within the NDL, it was more in response to awfully fast ascents -- almost runaway ascents -- being the norm... Even now, very few divers actually ascent at a sensible speed. Most exceed it by a huge margin. Since staged decompression dives at the time Safety Stops where adopted as a norm, did not favor deep stops, the majority of time spent off-gassing was done at 6 metres and 3 metres... hence the choice of depth... there is no magic to it. As mentioned before, it's more or less arbitrary... but it works because above all else, it slows the overall ascent.

One might also add, dive nitrox. But to explain why, we'd have to deliver a nitrox course... not going to happen here.

 

[Doppler]

As everyone should know, if you go up too quickly your body forms bubbles in your bloodstream due to rapid off gassing and this may cause DCI.

Not quite... there is good evidence that bubbles form in a diver's body on even the most benign sport dive... modern ascent behavior is informed by an assumption that managing these pre-existing bubbles is critical since they are influenced not just by a drop in ambient pressure, but also by the behavior of dissolved gas returning to its natural state via the easiest route... making existing bubbles larger.

The depth of that stop does not matter.

Actually, it does. It matters a lot. Any stop during ascent must be made above the off-gassing ceiling and on an NDL sport dive, probably at least one bar above that point. This may be a point of debate but there is a strong set of data that suggest this practice will actually help drive dissolved gas out of solution. The trick is knowing not just where to stop, but for how long. To keep things simple, a three to five minute stop at a point between 6 and 3 metres is suggested. Optional is a change in ascent rate from that point one bar above the OGC... but after an NDL dive, that change in ascent rate does not constitute actual timed stops but rather slowing down to a soft stroll as compared to a sprint.

 

[Blackwood]

I have no idea what you mean... absolutely none. Please explain it...

What I believe he's trying to suggest is that stress (sigma=E epsilon) caused by the expansion (epsilon) in the balloon (Elastic Modulus = E) is analogous to "stress" (DCS) in tissues caused by the expansion of dissolved gases.

Superficially, that's an okay analogy (expansion of gases causes stress in the balloon and expansion of dissolved gases causes DSC in the body).

But it in no way helps one to understand DCS, nor decompression mechanics. It's more appropriate in discussing AGE.