Why 15 ft for a safety stop?

[nereas]

Thal, from what I have read myself, your 24 ft value pertains to extended habitats.

For ordinary scuba, wherein you cannot possible stay down for longer than a couple of hours at a time, 33 ft and the 2:1 ratio that TSM speaks of is more appropriate.

Although I do not disagree with your findings regarding habitats, I believe the models that use 2:1 are prefectly valid for realistic scuba diving. Although I would certainly never ascend directly to the surface from 33 fsw, due to the prior effect of having been even deeper, still if my dive never went deeper than 33 fsw, I would be unlikely to do a safety stop, and deco is never prescribed for 33 fsw, even if it were possible to stay down long enough other than with a habitat at 33 fsw.

Thanks for the info.

 

[nereas]

...
My final comment is that, since tissues in the body vary in the amount of blood flow they get, and in their composition, it is thought that various areas of the body absorb nitrogen at different rates. This is modeled by tissue "compartments" in decompression theory. Very slow compartments are thought to equilibrate over very long periods of time. It is quite possible to do a short, deep dive, come up to 30 feet, and still be ABSORBING nitrogen in the slowest compartments of the model. But we will never spend enough time in the water to load those compartments to where a direct ascent to the surface will exceed the acceptable ratio for remaining asymptomatic, so in essence that absorption doesn't matter to us.

Exactly, we will never spend enough time, unless we are in habitats, which we are not. We are scuba divers.

 

[nereas]

Murdrcycle, I believe Thal has given you a valid choice of 1 min at 20 fsw followed by 2 mins at 10 fsw. Although 15 fsw for 3 mins would give about the same benefit, as it were, according to him.

I myself prefer to avoid the 10 fsw stop altogether, however. My shallowest stop is always 15 fsw, which is 1.5 ATA (rounded up) total ambient pressure.

15/33 + 1 = 1.45

 

[Thalassamania]

The surfacing ratio doesn't change if your compressed gas comes from a habitat or from a scuba tank, you can (and will) get bent if you spend enough time at 33 feet. While it is unlikely that a scuba diver will have sufficient gas supply or thermal protection to remain at 33 feet long enough, it is possible ... not to mention that using a 33 ft value displays a misapprehension of decompression models.

Murdrcycle, I believe Thal has given you a valid choice of 1 min at 20 fsw followed by 2 mins at 10 fsw. Although 15 fsw for 3 mins would give about the same benefit, as it were, according to him.

I myself prefer to avoid the 10 fsw stop altogether, however. My shallowest stop is always 15 fsw, which is 1.5 ATA (rounded up) total ambient pressure.

15/33 + 1 = 1.45

I agree with you, for a "safety stop" 15 feet is much more practical. Required decompression stops are an entirely different matter.

 

[nereas]

Well I have never been inside of a habitat, and so I will gladly defer to you on all issues related to that, Thal.

I have often wondered why 20 fsw on O2 works so well, and your 24 ft explanation nicely validates this.

 

[Buoyant1]

The "deep stop" is generally accepted to be a minute at half depth. It may not be covered in your OW, might be in your AOW - depending on your Inst, as many of the agencies don't really endorse it I don't think, unless I missed a change?

Im seeing a LOT more people suggesting that practice. When I did my recent wreck specialty down in the Keys, my instructor had me do half depth for a minute or two before our regular stop at 15. I think anything that keeps those bubbles from forming is a good thing no matter if the agencies don't endorse it or not!

 

[TSandM]

A couple of things . . . First off, O2 is used at 20 feet because that's the 1.6 ppO2 that most agencies prescribe as the safe maximum. It doesn't have to do with 20 feet being a particularly desirable depth for decompressing, it's just where you can use the gas.

Second, although slowing the ascent somewhat at deeper points than a strict diffused gas model would give you is probably a good idea, it is never good to lose sight of the fact that, at those depths, you may still be ongassing in slower compartments. So deep stops should be kept brief, and mainly serve to slow the ascent rate. You can hang out as long as your gas will let you in the range from 30 feet to the surface (again, this is positing a recreational diver on a single tank of gas), but that is NOT true for half depth stops in the 60 or 50 foot range.

 

[nereas]

But realize, TSM, with Thal helping us we are now outside of the realm of "the agencies" and we no longer care what they think. We care only what we can discover on our own.

The O/P (Murdrcycle) is trying to discover what is sacred about 15 ft.

And Thal is helping us discover what he believes is sacred about 24 ft.

As for me, right now I worship 20 to 15 ft -- on pure O2 at 20 for deco, and for NDL at 15 with ordinary back-gas nitrox.

I suppose that the gist of Thal's info is that 24 ft would work just as well with O2 as does 20 fsw (forgetting about ppO2 demons). Fortunately, there is not much practical difference between 20 ft and 24 ft anyway.

 

[Thalassamania]

Please, let's not forget the oxygen demons, the difference between 24 fsw and 20 fsw on pure oxygen could be a life or death issue.

 

[Charlie99]

Nothing is "sacred" about 15'.

Like most things in life, ascent profiles are a balancing act. As for decompression, the optimal ascent profile is a balancing act beween going shallow to further offgas (or to reduce further ongassing) vs. staying deeper to maintain high enough ambient pressure to avoid bubble formation.

The specific ascent profile one chooses has yet another tradeoff --- simplicity vs. accuracy. "30 FPM to 15', then 3 minutes at 15' and slowly surface" is a simple algorithm to remember. At the other extreme of simplicity vs complexity, one could run a full RGBM or VPM model and adjust the conservatism factor until one gets the optimal ascent profile for the amount of gas you have and the amount of time you are willing to spend on the ascent profile. The limit on simplicity is that you need to find a rule/algorithm that works for the set of dives it applies to. The limit on accuracy is the general inaccuracy of the models and the day-to-day variation in one's physiology. At some point, further complexity doesn't add any value.

My ascent profile is a bit more complex than 3@ 15', but not much. If I were to do just a single stop, it would be 20-25' for a short 100+' dive, and right at 10' for a long 40 or 50' dive. I find that in practice, my normal ascent has two (sometimes 3) deep stop, and than I do a 20' to 10' "slide" --- gradually getting shallower throughout the safety stop. People I dive with regularly don't bother asking if I'm ready to surface until I'm at 8' or so.