15' Safety Stop vs Minimum Deco

[metridium]

Deco theory aside, I think the three stops are what divers should aim for. Obviously, they should start out with the 15' stop in the beginning, then work toward mastering the buoyancy control required for the three stops.

So, in the training sense, the multiple stops would be better because they would be a useful goal and eventual sign of accomplishment.

 

[O-ring]

...but nobody mentioned the other benefit of starting stops deeper. By starting deeper, you leave less deco for the very end of your dive in shallow depths. That means, if you have a problem and need to surface or you run out of gas or conditions worsen, at least you have done some deco instead of saving it all for the 15' stop and then blowing through your only stop in the event of an emergency.

Oh, that and it keeps bubbles smaller or causes bubble seeds not to form...rising fast causes gas in tissues to be trapped and forms bubbles which get bigger as you get closer to the surface. Deep stops help eliminate some of this...

 

[Cave Diver]

O-ring once bubbled...
...but nobody mentioned the other benefit of starting stops deeper. By starting deeper, you leave less deco for the very end of your dive in shallow depths. That means, if you have a problem and need to surface or you run out of gas or conditions worsen, at least you have done some deco instead of saving it all for the 15' stop and then blowing through your only stop in the event of an emergency.

Oh, that and it keeps bubbles smaller or causes bubble seeds not to form...rising fast causes gas in tissues to be trapped and forms bubbles which get bigger as you get closer to the surface. Deep stops help eliminate some of this...

I vote with O-ring for the same reasons. It's better to have gotten some of your offgassing (since this is basic scuba, I wont call it deco) out of the way in the event you have to exit the water prematurely.

 

[Aquamaniac]

Id say the 15' stop.

On the grounds that we are talking Basic Scuba, its a more simplistic solution, therefore less for the "task loaded" new diver to think about.

Dave

 

[MuddyFox]

But this fits into this thread somewhat. A buddy of mine made a spread sheet for a dive to 130fsw for a bottom time of 8 minutes. He used the Buhlman ZH-L16C tissue half-times and modeled the 16 tissue compartments at each point during the dive. After setting a series of constraints, he used the "optimizer" function to find the best method of keeping compartment overpressure differences to a minimum at all times during the dive (to prevent bubbling). He asked the optimizer to find the best stop-pattern at 10 foot increments from 100fsw and then 5 foot increments starting at 20fsw.

This is what the optimizer decided:

Stop 1: 60fsw for 2.2minutes
Stop 2: 50fsw for 2.0minutes
Stop 3: 40fsw for 2.5minutes
Stop 4: 30fsw for 5.3minutes
Stop 5: 20fsw for 7.4minutes
Stop 6: 15fsw for 9.4minutes
Stop 7: 10fsw for 12minutes

It found a maximum overpressure ratio of 133%, 4.90psi difference

Descent: 60ft/min
Ascent: 30ft/min
Tank Size: 80 ft^3
Starting Tank Pressure: 3000psi
TARGET ENDING TANK PRESSURE: 500psi
Air Consumption: 0.35 liter/sec
Nitrogen Mix: 79%
Total Dive Time: 0:54:55

There are many comments to be made about this, the formost being that it is extreme overkill for rec diving. Obviously this would change with every minor parameter change (max depth, air consumption, etc) so a computer using this optimization routine would be useful here. The data also assumes you have nothing better to do on the dive than off-gas, although a certain level of proper off-gassing is necessary. It shows that ascent depth versus time is a logarithmic function, so in answer to UP's question:

A universal ascent profile template for rec diving limits would be UP's favorite:

1minute 30feet
1minute 20feet
3minutes 10feet

This best fits a logarithmic function... at least more so than the single 15' stop for 3 minutes.

 

[jeffsterinsf]

Ok, both regimens require the same amount of time above 30' (1 min @ standard ascent rate + 3 min) where nitrogen uptake is minimal. The average depth for the 3-stop approach is almost 19', compared to 15' for a single safety stop. So comparing the average ambient overpressure in the two, it is about 0.6 bar vs. 0.45 bar. My gut feeling is the amount of nitrogen actually transferred from the body in four minutes at these pressures is not significantly different.

That leaves bubbles. So I'd have to go with my intuition here for the slower ascent (3-stop approach) that would tend to have lower bubble formation rates during the first 2 stops.

All this asumes sufficient skills to maintain the ascent rates and levels so as not to explode through the surface (bad) or bottom of the dive boat (worse).


And yes UP, "All dives are deco dives." Show me one where your partial pressures of aren't elevated over your pre-entry levels and I'll change my story...

 

[Dryglove]

I just recently read a great article on by George Irvine.See http://www.baue.org/library/irvine_baue_talk.html

 

[MikeFerrara]

hmmm...
Another teaching thread that has potential. Should we get the arguing out of the way now or wait till later?

One of the things I find difficult about teaching diving is matching my own views to agency views. I dislike terms like "NDL" or "no decompression diving". What are you supposed to do with the extra inert gas? Take it home with you? In an entry level class it is hard to teach deco procedures. One of the things I do, however, is to point out what a recreational dive table says about a 130 ft dive and then give an overview of how I would do that dive. I give a brief description of the gases used, equipment used, gas planning and my ascent schedule. I don't call it minimum deco. I run the schedule with any one of a number of dive planning softwares. You see, I believe that 130, 100 or whatever is too deep for the vast majority of recreational divers especially given the equipment and level of planning being used. Some see that if a dive is too deep for me, with that equipment and those methods, then maybe they should think twice.

For a "shallow dive" a "minimum deco" schedule will not yield a statistical advantage in avoiding DCI symptoms. I am sure there is a difference in bubble presence and size but does it matter?

Ther are many references available to validate the theory of deep stops and I won't bother to list them here.

I don't know of an agency that dismisses the value of deep stops, however they do not, at this time, teach the theory to divers who haven't yet made a dive. I let new divers know such theories exist and of my beliefe that knowlege of them is a prerequisit to "deep" diving. In the context of recreational diving, my definition of deep is not very deep.

 

[Genesis]

my instructor for my AOW class was when I mentioned that I wanted to do deep stops on our ascent.

She looked at me with a bit of surprise; I told her that I had been doing them on dives over 40-50', and that I'd like to on this one too.

We did; we stopped at one atm up for a couple of minutes, then made the next portion of the ascent very slowly, and did the "hang off the emergency deco bottle" that the agencies insist that you use for teaching (I suppose I would have really blown her mind if I had clipped off a stage, so I didn't.... )

Its just one of those "ought to do it" things from where I am. I haven't been hit, but figure that the deep stops can't hurt and according to the theory that I 've read ought to be helping.

 

[raxafarian]

UP...how long you gonna leave us hanging