Why 15 ft for a safety stop?

[TSandM]

If someone does get DCS without doing a 3-5 minute safety stop, it is likely they would have gotten DCS even with one on that particular dive.

I don't know about this. Five minutes of deco is five minutes of deco. I don't think you can make the argument that you don't offgas enough nitrogen during five minutes of deco to make any difference; if that were true, we could all lop five minutes off our deco whenever we felt like it and not worry about it. (And maybe we can -- some of the WKPP stuff seems to suggest it -- but I'd be very reluctant to give anybody that advice, wouldn't you?)

 

[MikeFerrara]

I think some of what the WKPP has done definately suggests that we don't have it all figured out yet. Correct me if I'm wrong but I think GI's descriptions of min/max deco tend to sugeest that he thinks that we may tend to do too little decompression on short dives and too much on long dives.

In that context, all my dives are short and over the course of my diving I have added decompression.

 

[murdrcycle]

Wow - I can't believe the controversy this has started. Everyone is making this far more difficult than it needs to be. Assuming a safe ascent rate to the safety stop depth, there are only two possible scenarios:

1. - any time we are under water we are taking on excess nitrogen that our body can't process. If this is true as some have asserted, then a safety stop would do more harm than good. Let's refer to the amount of nitrogen one has in their body, after the dive (regardless of depth), as "x". If, at safety stop depth, the body is still taking on excess nitrogen at the rate of "y" per minute, with "y", by definition, being a positive number, after a 3 minute safety stop the amount of nitrogen in the body would be x+3y, which any day of the week is greater than "x". After a 5 minute safety stop the amount of nitrogen in the body would be x+5y. Therefore, after completing a safety stop the amount of nitrogen in the body is greater than before the safety stop.

-or-

2. - that the body can efficiently handle and expell nitrogen up to a certain depth. Lets call this depth "d", since the actual depth is not only open to controversy in this forum but will, in fact, differ from diver to diver. Once someone goes below "d" depth, they take on excess nitrogen at a rate which their body is incapable of expelling it. When they get back to "d" depth, they have a certain amount of nitrogen built up. Once again let's call that amount "x". Once they rise to above "d" depth the body is now processing and expelling gas faster than they are taking it on. Let's call this rate "y". It is also agreed that at "d" depth one will expell gas slower than at d-1, and at a depth of d-1 the body will process it slower yet than at d-4 or d-10 (those are minus signs, not dashes). Therefore, the rate at which someone can offgas can be expressed as y=(rate of nitrogen expulsion at current depth - rate of nitrogen intake at current depth). This means that at any time one is above "d" depth, they are offgassing. It also dictates that any safety stop must be above "d" depth and must be for a length of time that compensates for the "y" factor in the equation above.

I think that TSandM was the one who put forth the notion that there is a curve, and that as depth decreases the ascent should be slowed. This means that nitrogen is more capable of being offgassed at shallower depths (i.e. comes out of solution quicker) but that the body's ability to expell it is either constant or that it is on a curve as well, but less of a curve, requiring more time to process it. It doesn't matter which of these is true, or in fact if either is true. My assertions do not rely on this at all. All that my assertions rely on is the fact that at some depth the body begins to process nitrogen faster than it is taking it in. And, as long as that is true, and assuming that a "safe" ascent rate was maintained, then as long as the safety stop is above "d" depth, and as long as the time is adjusted for said depth, and as long as the remainder of the ascent is at a "safe" ascent rate, then the requisite offgassing will be achieved.

However, if the "curved" model above is true, it would establish that the ideal thing to do would be to gradually ascend at a very slow rate, lets call it "r" rate, beginning at "d" depth. Ideally "r" rate would be the ascent rate at which the body's ability to offgas matches the nitrogens ability to be processed (the rate at which it comes out of solution). In other words, the ascent rate should allow for the value of "y" in the above equation to be a positive number, and would allow for both "y" and "x" to be zero at the surface. And, if these two rates are not equal, then numerous safety stops of shorter duration and greater frequency would be a better alternative. Now, while this is a more "accurate" solution, I agree that the difference between the method I put forth and the 15 ft safety stop may be scientifically insignificant.

Not advanced nitrogen/solution theory, just plain old basic algebra.

And, taking into account what I put forth above, that means that when at the safety stop, while in slightly pitching waters in the open ocean with 7 other divers in my group, each clinging to the ascent line with the person at the top at 12 feet and me at the bottom at 20 feet, in order for me to avoid the bends I don't need to wrestle my way through the other divers to 15 feet in some underwater wrestle-royale James Bond style, I just have to extend my stop by "y", and maybe even add another little one at 10 ft.

So tell me again - if basic algebra proves it is right, how am I wrong?

There is only one way - if just about everyone that has posted is wrong, and that may be the case. Maybe the laymans understanding is a bit flawed. But even in this eventuality, my math may be wrong, but the theory is still right.

The one thing that would prove this all wrong is if the body is perfectly capable of handling the amount of nitrogen than it is taking on at any depth, but it is unable to process it because it is in solution. That would then promote the theory that one must ascend, let the nitrogen come out of solution, process and expell it, and then ascend again to repeat the process. However, while that may prove my math wrong, it still proves my theory right.

Anyone who has been on a deep dive (and I haven't, but I have heard about it and even seen pictures - thats how I roll) knows that if you descend, blow up a balloon, and then ascend, the balloon will explode. 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. Do you think that the best method would be to ascend "x" feet (to a safety stop), let it expand (nitrogen come out of solution), and then stop (safety stop), let a little air out (wait to offgas), and then ascend again? Or would it be to make an ascention of constant speed while letting air out at the rate needed to keep the balloon at a constant circumference?

 

[Blackwood]

I can't believe the controversy this has started.

Decompression mechanics is still in the theoretical stages.

any time we are under water we are taking on excess nitrogen that our body can't process.

This is true if we are breathing gas at ambient pressure that contains nitrogen. Without pure oxygen as a breathing gas, we will never come up clean. Your final offgassing (which can be re-stated as the absorbed inert gas pressure reaching equilibrium with the ambient gas pressure) will occur on the surface (hence 'surface intervals').

If this is true as some have asserted, then a safety stop would do more harm than good.

This is not true. Among other things, you have neglected the fact that different tissues absorb inert gases at different rates. Some tissues, like blood, absorb and expel nitrogen rather quickly. Others, like fat, absorb and expel it comparatively slowly. Generally speaking, one can sufficiently offgas fast tissues with a slow, controlled ascent, whereas it may be necessary to stop to account for the inert gas loading in the slower tissues.

2. - that the body can efficiently handle and expel nitrogen up to a certain depth. Lets call this depth "d", since the actual depth is not only open to controversy in this forum but will, in fact, differ from diver to diver.

That's not how it works. Your tissues are always working towards equilibrium. Until they are in equilibrium (saturated with as much gas as they can hold at the given ambient conditions), they'll be either loading up or expelling.

If you go down X feet, you are loading nitrogen. If you stay there long enough, the gases in your tissues will reach Y PSI (equilibrium). Without going deeper, they will never exceed Y PSI, and without ascending, they'll never decrease from Y PSI.


There is little controversy. I imagine that most everyone on this forum will agree that the level at which offgassing begins varies with the dive profile.

I suggest reading up on the RGBM and VPM decompression models.

Start here: Understanding M-Values

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. Do you think that the best method would be to ascend "x" feet (to a safety stop), let it expand (nitrogen come out of solution), and then stop (safety stop), let a little air out (wait to offgas), and then ascend again? Or would it be to make an ascention of constant speed while letting air out at the rate needed to keep the balloon at a constant circumference?

Though this is not analogous to what causes DCS, it's a fun little idea. Clearly, keeping the balloon (with a constant modulus of elasticity) at a constant circumference will stress it less than increasing the circumference.

 

[bleeb]

Maybe the following won't help any and you already knew this, or maybe we're using slightly different definitions of the terms, but I get a bit of a sense of a misconception that seems rather fundamental to your assertions, so here's a kick at the can.

any time we are under water we are taking on excess nitrogen that our body can't process.

Overall, the body can't 'process' any nitrogen. (I'm only talking about N2 here, whether gas or dissolved gas.) As far as human biochemical processes go, it's almost complete inert. Blood moves it from point to point, but changes in quantity in any particular lump of blood or other bodily fluid or tissue depend purely on diffusion. Any time there is a difference in nitrogen concentration, for instance from changes in the surrounding pressure, nitrogen will diffuse from the places where it has a higher concentration to where there is a lower concentration. Diffusion is a completely passive process, as far as the human body is concerned. If tissue A containing one concentration of nitrogen is in contact with tissue B (or lung air) containing a different concentration, nitrogen will move to or from A from or to B, with no, um, human intervention.

Why does this matter to you? On the surface, your body tissues (including your blood) have a concentration at equilibrium with air at surface pressure. When you go down, you're breathing pressurised air, which has a larger amount of nitrogen per unit volume, i.e. a higher concentration. This will diffuse into your tissues over time. The rate of diffusion depends on the difference in concentrations between tissue and wherever it's diffusing from. The less the difference, the slower the rate, so over time, the rate of diffusion slows down as the receiving tissue becomes 'saturated'. This results in the previously mentioned "curve" in concentration in the receiving tissue.

Diffusion rate is also affected by the characteristics of each tissue. Most decompression models used in diving simplify the human body by assuming anywhere from six to twelve different "compartments" (i.e. tissues with different diffusion rate coefficients.) The practical implication is when you go down in the water for x amount of time, y amount of nitrogen moves into one compartment, but z moves into another. When you come part way up, one compartment may now be outgassing (i.e. higher nitrogen concentration than the air being breathed at that depth) while at the same depth on the way back to the surface, another still ongassing. Decompression stop choices (including safety stops) and recommendations therefor are compromises between the effects on each compartment, based on a range of practical dive profiles. This is also one reason why a linear, single-variable formula like the ones proposed, aren't sufficiently accurate to be practical. It takes differential equations (i.e. calculus) to model diffusion closely enough to be useful. Algebra alone can't do it.

Another major issue affecting recommendations as to maximum rate of ascent (and safety stop details) is a concept which comes under a variety of names, including "overpressure" (although that term too is sometimes used in different ways). What I'm talking about here is that a tissue (or bodily fluid) with a certain (elevated) concentration of nitrogen can 'tolerate' a certain reduction in pressure before the stuff starts to come out of solution and start bubbling out. Maximizing the rate of offgassing is partly a matter of getting as close to the surface as possible (where tissue concentration is higher relative to air at the depth's ambient pressure) without being so high as anything starts bubbling. However, a major factor arguing against going too shallow too quickly during the decompression process is the concept of "microbubbles". But that's probably getting too deep for this already somewhat long-winded basic discussion.

 

[murdrcycle]

This is not true. Among other things, you have neglected the fact that different tissues absorb inert gases at different rates. Some tissues, like blood, absorb and expel nitrogen rather quickly. Others, like fat, absorb and expel it comparatively slowly. Generally speaking, one can sufficiently offgas fast tissues with a slow, controlled ascent, whereas it may be necessary to stop to account for the inert gas loading in the slower tissues.

It is absolutely true. Basic math - if you are taking on more than you are expelling then you will be ending the safety stop with more gas than before. To complicate matters further, you will be ending the safety stop with more gas than before and it will be contained in the cells that are slowest to expell it. The thing that isn't, can't be true is that you continue to load up on nitrogen no matter how shallow you are.

*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.

That's not how it works. Your tissues are always working towards equilibrium. Until they are in equilibrium (saturated with as much gas as they can hold at the given ambient conditions), they'll be either loading up or expelling.

I would have thought that they would be loading up and expelling simultaneously, all the time, the ratio of which is the cause of the problem.

Though this is not analogous to what causes DCS, it's a fun little idea. Clearly, keeping the balloon (with a constant modulus of elasticity) at a constant circumference will stress it less than increasing the circumference.

I would think that it is a paradigm of precisely what causes the bends. It is the act of the expansion of a gas inside of a closed circuit, with the expansion causing damage to the vessel.

FOR THE RECORD I am not condoning skipping safety stops. Nor do I intend to go out and do any research through self-experimentation. However, I would like to understand the principles at work in case I find myself in the predicament that I can't safety stop at 15 ft, for whatever reason. Maybe 15ft isn't feasible due to surface conditions - or maybe I am too piss-poor at bouyancy control that I am constantly bobbing up and down between 20 and 10 feet. Or maybe the Secretary of the Navy will call me, after having heard about the impressive way in which I have handled myself on my 7 logged dives, with a special assignment that calls for extraction in heavy shipping lanes. Whatever the reason, I want to know not just that I should stop at 15 ft, but how do I compensate for that if I can't stop at 15 feet. Everything that I have laid out makes perfect sense to me and, using algebra based on information gathered by all of you, appears to be sound logic.

In fact, without having any real numbers to show that it is scientifically insignificant, I would have to say that a safety stop at 15 ft doesn't sound nearly as good as a safety stop at 30 ft followed by an ultra-slow ascent to somewhere between 20 and 10ft for another safety stop.

 

[murdrcycle]

Overall, the body can't 'process' any nitrogen. (I'm only talking about N2 here, whether gas or dissolved gas.) As far as human biochemical processes go, it's almost complete inert. Blood moves it from point to point, but changes in quantity in any particular lump of blood or other bodily fluid or tissue depend purely on diffusion. Any time there is a difference in nitrogen concentration, for instance from changes in the surrounding pressure, nitrogen will diffuse from the places where it has a higher concentration to where there is a lower concentration. Diffusion is a completely passive process, as far as the human body is concerned. If tissue A containing one concentration of nitrogen is in contact with tissue B (or lung air) containing a different concentration, nitrogen will move to or from A from or to B, with no, um, human intervention.

I must not be reading this correctly. It appears to me as though you are saying that the body does not "process" nitrogen at all but simply moves it from one cell to another until a homeostasis is achieved. I know I must be reading this wrong because that can't be true or else there would be no such thing as a surface interval - you can stay on the surface all you wanted and your total nitrogen level will never drop, it will just move from one cell to another until it is diffused evenly throughout your body. Or maybe I am reading it right and that is the hole in my theory. Is DCI caused by excess nitrogen or is it caused by the existing nitrogen being compressed?

When I say "process" nitrogen, I mean the method by which it expels it. My guess would be that it probably expels most of it through respiration, but it is just that - a guess. However, whether it is through respiration or via the Nitrogen Gnomes that live in our bloodstream, I don't think that it has any bearing on what I have said.

 

[bleeb]

I must not be reading this correctly. It appears to me as though you are saying that the body does not "process" nitrogen at all but simply moves it from one cell to another until a homeostasis is achieved. I know I must be reading this wrong because that can't be true or else there would be no such thing as a surface interval - you can stay on the surface all you wanted and your total nitrogen level will never drop, it will just move from one cell to another until it is diffused evenly throughout your body. Or maybe I am reading it right and that is the hole in my theory. Is DCI caused by excess nitrogen or is it caused by the existing nitrogen being compressed?

Actually, you've got most of it. Given long enough, your body and diffusion would indeed move it around until you have the same concentration everywhere. The catch is (as you were getting to in your next paragraph), your blood is also in contact with the outside air through your lungs.

Practically, what this means is that if you stay at a particular depth long enough (days) all your tissues will then reach equilibrium with the air at that depth. But when you surface (during your surface interval) your lungs and blood are exposed to a lower concentration of nitrogen and it starts leaving your body through your lungs. Your other tissues are in contact with your blood, so once the nitrogen level in your blood starts to drop, the nitrogen in your other tissues will start to diffuse into your blood.

DCI therefore comes from the 'extra' nitrogen that got added while you were deep. When you ascend, you have more than you 'need' at the shallower depth. In simplistic terms, if you ascend to far or too fast, that excess nitrogen can come out of solution and form a bubble (a la shaken carbonated beverage), ruining your whole day.

When I say "process" nitrogen, I mean the method by which it expels it. My guess would be that it probably expels most of it through respiration, but it is just that - a guess. However, whether it is through respiration or via the Nitrogen Gnomes that live in our bloodstream, I don't think that it has any bearing on what I have said.

Nitrogen Gnome: Sounds a bit like a Unix daemon. (Sorry, geek joke.)

Respiration is indeed part of it, although if you just held your airway open and still for an hour or two, you'd still offgas nitrogen. You might not be too happy otherwise, but you'd still offgas. (IIRC, the fraction of nitrogen that passes in and out of your body through surfaces other than the insides of your lungs is negligeable.)

Actually, joking aside, you can think of it like this: After a dive, you (especially your blood) have a nitrogen concentration above what's normally found at the surface. You breathe this air in that has a lower concentration of nitrogen, so it diffuses out of your blood and into the air in your lungs. The concentration of nitrogen in the air in your lungs goes up. After a while, you breathe it out (remember how you don't replace all the air in your lungs with every breath) and replace it with fresh, low nitrogen air, which allows more of it to leave your blood. The converse is happening when you first go down: Your tissues and blood have surface-level nitrogen concentrations. At depth, you breathe in high pressure air, which has more nitrogen. With time (and every breath), it diffuses into you, while what you're breathing out has slightly less nitrogen than it went in with.

 

[MikeFerrara]

It is absolutely true. Basic math - if you are taking on more than you are expelling then you will be ending the safety stop with more gas than before. To complicate matters further, you will be ending the safety stop with more gas than before and it will be contained in the cells that are slowest to expell it. The thing that isn't, can't be true is that you continue to load up on nitrogen no matter how shallow you are.

*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.

I think the problem in your understanding is that you're treating the body as a whole. In relation to a disolved gas model there are multiple compartments with different half times and different M values.

The depth where "off-gassing" or decompression starts depends on how deep we have been and for how long...ie which compartment is leading or closest to it's critical tension. If you had done a very long and/or deep dive you would not dare ascend directly to 15 ft.

If you go from the surface to 15 ft (or any depth) breathing air, you certainly will be ongassing N2. However, at 15 ft, the pressure isn't great enough to significantly load the faster compartments and you aren't going to do a safety stop long enough to significantly load the slow compartments. We do a safety stop after having been deep enough (at great enough pressures) to load those faster (higher pressure)compartments.


I hope I didn't botch that too bad.

 

[Blackwood]

I know I must be reading this wrong because that can't be true or else there would be no such thing as a surface interval - you can stay on the surface all you wanted and your total nitrogen level will never drop

Unless it gets stuck, it will eventually leave the body the same way it got there: by (as you noted) respiration at a lower pressure than that at which inert gases exist in your tissues.

Is DCI caused by excess nitrogen or is it caused by the existing nitrogen being compressed?

On rare occasions existing nitrogen has been known to cause DCS. Some extreme skin divers have been bent simply by going deep on a breath hold and coming back up. Similarly, it is thought that whales and other air-breathing-deep-diving mammals suffer minor DCS.

I would think that it is a paradigm of precisely what causes the bends. It is the act of the expansion of a gas inside of a closed circuit, with the expansion causing damage to the vessel.

Ah, I wasn't thinking about it that way (balloon :: entire body). I assumed you meant the balloon to represent individual cells loading nitrogen. Still, I think it's an over simplification, but a good thing to think about.