Why 15 ft for a safety stop?

[Steve SDI/TDI HQ]

OK - so if DCI doesn't begin to occur until 33 ft or more, then that means that at 33 ft the human body can off-gas at the same rate at which the nitrogen would collect, meaning that no additional nitrogen will develop. Therefore, it can be assumed that at anything less than 33 ft the human body is able to off-gas faster than the nitrogen is building up.

Obviously every human body is different. However, assuming the above, 15 ft. is pretty arbitrary, and in fact any time that you spend at less than 33 ft. allows off-gas time. Therefore, it would seem to me that a stop at 30 ft followed by another at 20 or 15 would make more sense, or even better would be to slow your descent so gradually at the 30 ft mark that it takes 4-5 minutes to surface from 30 ft. That would allow for the most gradual return to normal as possible, and it is safe because as long as you are less than 30 feet you aren't taking on any more nitrogen.

Does that make sense?

No, it does not... sorry.

The off-gassing point for a dive is a function of bottom gas, duration and depth... you are making wrong assumptions. Just keep it simple: A safety stop at a point between 6 and 3 metres (depending on sea state etc.) is a good practice. It may be that the time spend at this depth after a sport dive within NDL allows the body to eliminate some of the accumulated gas from the dive and may help manage the risk of DCI. Quite apart from that, it helps control ascent rate which is a real concern.

 

[ShakaZulu]

Would a safety stop at 15ft still be valid under these conditions?

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[Blackwood]

Remember, the primary reason for a safety stop is to compenstate because most divers ascend too quickly..

Which is a particularly good reason to recommend a staged ascent (such as minute each at 30, 20, and 10) versus "go here and wait."

we know that the greatest pressure change is from 33' to the surface.

The greatest pressure change is from your maximum depth to the surface. 33FSW to the surface is the smallest depth change across which pressure halves, nothing more, nothing less. Similarly, the surface to 66FSW is the smallest depth change across with the pressure triples. But who cares?

The important concept is that the rate of change of pressure is inversely proportional to depth (alternately stated: the shallower you are, the quicker the pressure decreases with ascent).

OK - so if DCI doesn't begin to occur until 33 ft or more

I think you are misinterpreting or, rather, mixing concepts.

Haldane arbitrarily chose "2" as the pressure gradient across which gas will bubble out of solution and Buhlmann formulated tables to prevent the inert gas pressure within a diver's tissues from exceeding double the ambient pressure (while getting as close as possible for 'optimum deco').

There is nothing beyond theory that states that saturating your tissues with 1.9999999ATA of Nitrogen and then ascending to a 1.0ATA environment can be done with no risk of DCS.

 

[murdrcycle]

I don't see where my point is disproven at all. I see where it is shown to be difficult to put into practice, but not disproven. I am not talking about cell behavior or anything like that - that doesn't come into play. All that comes into play are ingestion and expulsion rates.

Of the things that we do know, we know that nitrogen in the blood is directly related to DCI. Maybe not causal, but directly related. If DCI doesn't develop until 33+ ft, then we know, absolutely beyond the shadow of a doubt, that at depths of 33 feet or less the body is capable of dealing with the additional nitrogen - in other words, we can get rid of it at a pace as fast or faster than we are developing it. If we also know that as depth decreases the amount of nitrogen absorbed decreases, then we can say with absolute certainty that if at 33 ft the body is capable of getting rid of it as fast as it is taking it on, then at anything less than 33 ft the body is capable of getting rid of it faster than it is taking it on, unless the body's ability to remove it increases with depth, which we know is not true. Therefore, at anything less than 33 ft the body is removing nitrogen faster than it is taking it on (i.e. off-gassing). The more depth is reduced, the faster this happens, but if depth is reduced too fast the nitrogen expands before it gets a chance to escape.

Pretend you are drinking beer. Or better yet grab a few and put it to a test. Slam 3 beers. You have just decended past 33 ft - your body is taking on more toxins than it can handle. The effect is inebriation. Now you slow down to a pace that doesn't get you any more or less drunk, but keeps your buzz at the same level. You have just ascended to 33 ft. Your body is now taking on an equal amount of toxins as it can get rid of, but it isn't able to chip away at the 3-beer-buffer that you created to begin with. Now, the optimum thing to do here isn't to just quit drinking (shoot to the surface), unless chunk-blowing (death) is imminent or unless the 3-beer buffer was more than your liver could handle and you now require water (you went too deep for a recreational dive, skipped a deco stop, and now need a chamber). If you do, then you have all of that alcohol (nitrogen) trapped in your cells and you will wind up having a hangover (DCI). If you just cut your intake in half very suddenly (go to a 15 ft safety stop), you start wanting to nod off and fall asleep because it is a drastic change for you body (your body has reached a depth at which it could finally begin off-gassing, but instead you continue to go higher before you stop, creating a sudden change for your body).

What effect does this really have? It may be harmless - if you can stay awake and drink plenty of water, then your hangover will be minimal or non-existant. But maybe it won't. It definitely puts more stress on your ability to continue to function.

However, if you slowly decrease (ascend slowly) your drinking (nitrogen intake) while increasing your food and water intake (happening by default, since the off-gassing is happening), you run a greater chance of avoiding the hangover (DCI). However (sorry for the upcoming double-negative) the slower that you reduce your drinking (the slower you ascend), the longer it will take to sober up and expel all of the alcohol from your cells (off-gas) and the more beer (air) you require for the transition. Therefore if you are getting too tired (running out of time) or running out of beer (air), then quicker action is needed and a "happy medium" may be the only alternative.

Before anyone says that beer and diving aren't the same, keep in mind that these are basic scientific and mathematical principles that we learn in elementary school. These principles hold true for any scenario in which there is a physical induction, a processing, and a removal. The same will work for rain in a gutter, air through a carbeurator, etc. In essence, both the drinking example and the diving example have the following similarities:

1. Both involve the intake of a toxin
2. In both circumstances the body must process and expell the toxin
3. In both circumstances the toxins will get stored in the cells
4. In both circumstances the toxins must get removed from the cells (in drinking, via water-replacement and time, and in diving by drinking water and off-gassing)
5. In both circumstances, an inappropriate amount of the toxin in the cell at the end of the activity will lead to undesireable effects (either hangover or DCI)
6. Both are fun
7. Both are expensive
8. It isn't cool if you puke during either one

 

[murdrcycle]

There is nothing beyond theory that states that saturating your tissues with 1.9999999ATA of Nitrogen and then ascending to a 1.0ATA environment can be done with no risk of DCS.

Then forget 33 ft. Someone else mentioned 33ft as the depth at which it is agreed that above which you are safe from DCI. Call the depth "X" - it really is inconsequential to the outcome.

 

[DeepSeaExplorer]

It depends on a lot of factors such as the amount of exposure, the gas, depth, acent rate, etc. but I see no reason that you couldn't get bent deeper than 33ft.

On recreational dives, that may be hard to imagine because you only have one stop. But, if you extend the logic to deeper dives, the story changes.

For example, on deep dives with lots of exposure, it's possible for the first deco stop to be at 110ft. If you blow off those deepest stops, I suppose in theory, you could get bent before you even get up to 33ft, though you probably wouldn't notice the symptoms early enough to associate the two.

If you got instant feedback, you would know exactly what you did wrong and could attempt to compensate. The problem is that you usually don't get the feedback until you're back on the boat.

 

[murdrcycle]

Remember, the primary reason for a safety stop is to compenstate because most divers ascend too quickly. If you don't ascend too quickly then you don't need to insert an artificial shallow stop to compensate.

All right - now we are on the same track.

What I am basically saying is two things: an ultra-slow ascent (once you got past point X) would be better than a normal ascent with a 15 ft safety stop, and that a safety stop at 20 ft or 10 ft (or basically anywhere above point X) is sufficient, provided that you ascend properly from that depth and adjust the time appropriately.

 

[nereas]

All right - now we are on the same track.

What I am basically saying is two things: an ultra-slow ascent (once you got past point X) would be better than a normal ascent with a 15 ft safety stop, and that a safety stop at 20 ft or 10 ft (or basically anywhere above point X) is sufficient, provided that you ascend properly from that depth and adjust the time appropriately.

Just so that you understand where 10 ft comes from, this is a rounded value representing 1/4th of 1 ATA pressure change. 33 fsw / 4 = 8.25 ft rounded up to 10 ft. 10 ft is used as a final deco stop by many deco tables, after a 20 ft stop.

The popular 15 to 20 ft safety stop approximates the half-way point between 33 fsw and the surface. A 10 ft stop, alone, may not be beneficial at all, without a 20 ft stop first.

You have introduced the notion of a 10 ft stop, however none of the diving literature for recreational NDL scuba does. The safety stop is recommended as 15 to 20 ft.

An ultra-slow ascent, beyond some tissue point threshhold, would be fine, yes. For NDL diving, it should last 3 to 5 minutes, begin somewhere around 33 ft, possibly 50 ft for deep NDL dives, or 20 ft for shallower NDL dives (we don't really know), and end up at 15 ft, before you surface.

When deco divers egress, they normally stop first at approximately 1/2 of their max depth, then egress in 10 ft intervals (1/4 ATA) for various increasing times, until they reach 10 ft, and finish there. Some make their last stop 15 ft. Some make their last at 20 ft on pure O2.

And deco divers have a favorite expression: ALL dives are deco dives.

 

[Steve SDI/TDI HQ]

All right - now we are on the same track.

What I am basically saying is two things: an ultra-slow ascent (once you got past point X) would be better than a normal ascent with a 15 ft safety stop, and that a safety stop at 20 ft or 10 ft (or basically anywhere above point X) is sufficient, provided that you ascend properly from that depth and adjust the time appropriately.

And you'd be wrong. This is bad advice and full of too many assumptions.

 

[Blackwood]

If DCI doesn't develop until 33+ ft

The only depth at which one will not load up with inert gas is 0 feet.

The reason we aren't loading up nitrogen as we speak right now is that our tissues are already holding as much as they can at this pressure.

Anytime you increase the pressure of the gas you are breathing, you create a gradient to your tissues. They'll take on inert gas until they're saturated at that pressure. Now decrease the pressure, and you've created a gradient, but in the other direction. They'll expel inert gas until they're saturated at that pressure.

Pretend you are drinking beer.

Good idea.

Beer cans and other carbonated beverages offer an interesting way to discuss DCS.

A can of beer has gas dissolved in the liquid. Open one and leave it open for a few days. What happens? It goes flat. The gas comes out of solution.

Now drop a can of beer before you open it. What happens? It fizzes. You've excited it and the gas is coming out of solution faster.

Now put a can into a paint mixer before you open it. What happens? It blows over. Beer gets on your ceiling.

All three cans contained dissolved gas. All three cans released the dissolved gas. How fast or how slow that happened governed the result.


Same with diving. You breathe compressed inert gas. Your tissues absorb some. You lower the pressure of your breathing gas. Your tissues expel some. How much there is and how quickly that happens governs the results.

You are right. A dive to 5 feet won't cause much loading and you probably wouldn't get bent rocketshipping to the surface. But there is no guarantee.