Safety stop at 15'..........always

[stevephilanyc]

I always plan to dive so I can do a safety stop--or explore at around 15' to have the same effect. My feeling is it's just a little extra protection.

Steve

 

[Thalassamania]

The thing that keeps getting missed in the tables vs. computer discussion is that computers are nothing but automated tables (or if you prefer, tables are nothing more than a series of computer snapshots). There is a body of information that must be mastered before a diver is able to make an educated decision as which to use or which of which type to use. Once that body of information (which is not taught in classes today) there comes the mechanical methodology of actually using a table or a computer to plan and execute a dive, that is a completely different issue, and yes, Pete is right, it is pretty much you seen one (table or computer) you seen them all ... more or less. Master one and with the expertise that was gained learning the basic information you can likely figure out most others. But what is missing is the basic information, instead most divers only get the mechanics and that, in my mind, is woefully inadequate.

 

[peterbj7]

pressure is a linear function of depth

I give up. Pressure is indeed a linear function of depth, but rate of change of pressure isn't, and that is what we are concerned with.

I've wasted enough time on this.

 

[raymond phule]

and you still dont know the usual meaning of rate of change.

I know what we are concerned with but is that a reason to use the wrong termonology? Edit: and incorrect statements.

 

[Cacia]

I have been watching for some gas physics law that would mean a "stop" is superior to a "slope" to prevent bubble formation, and I don't see where anybody has explained that.

Am I wrong?

 

[miketsp]

I have been watching for some gas physics law that would mean a "stop" is superior to a "slope" to prevent bubble formation, and I don't see where anybody has explained that.

Am I wrong?

The problem is that it depends.
The fast ascent and the stop works on the principle of "bend and mend", in other words let the bubbles form and then let them go out through the lung interface.
However it is also a fact that bubble formation at interfaces then degrades the continuing process of diffusion of dissolved gas out of the medium.
The slow ascent works on the principle of diffusion without bubble formation. Exactly as mentioned above with the coke bottle experiment.
It is possible to get all the gas out of a coke bottle by releasing the pressure at a very slow rate.
It also follows that deco without any bubbles is inherently safer than deco with bubbles.
OTOH if the objective is to get to the surface in the most efficient (fastest) manner then you have to mix techniques. It's a trade off.

 

[captain]

The problem is that it depends.
The fast ascent and the stop works on the principle of "bend and mend", in other words let the bubbles form and then let them go out through the lung interface.
However it is also a fact that bubble formation at interfaces then degrades the continuing process of diffusion of dissolved gas out of the medium.
The slow ascent works on the principle of diffusion without bubble formation. Exactly as mentioned above with the coke bottle experiment.
It is possible to get all the gas out of a coke bottle by releasing the pressure at a very slow rate.
It also follows that deco without any bubbles is inherently safer than deco with bubbles.
OTOH if the objective is to get to the surface in the most efficient (fastest) manner then you have to mix techniques. It's a trade off.

But the coke is saturated so if it takes you one minute or 24 hours to open it, it will not have any more gas loading than when you began to open it. The rec diver may not and probably is not saturated in any tissue groups on an NDL dive so a slow ascent may allow more uptake in some tissues in the deeper depths during the slow ascent than a faster ascent and stop would. It's not so much the bubbles as it is the size of the bubbles. Have you ever had an IV drip, generally there is some air in the IV line that ends up in your vein as bubbles but they aren't big enough to cause a problem,

 

[dave4868]

Thal got it right.
What matters is the percentage change and given a constant vertical speed the percentage of pressure change is greatest near the surface.
Think of it in these terms.
Starting at the surface you need to descend 33ft to double your ambient pressure but to double again you need to go to 99ft.

I agree Thal has it right and you've provided a clear example.

Would it further clarify to say that your example demonstrates the importance of the change in ata relative to the original 1 ata at the surface. The relative change isn't linear despite the linear nature of the pressure change itself, as was pointed out.

I don't have the math skills to describe it, but relative change in the ata is the important consideration, isn't it?

Perhaps someone can say it better.

Dave C

 

[miketsp]

But the coke is saturated so if it takes you one minute or 24 hours to open it, it will not have any more gas loading than when you began to open it. The rec diver may not and probably is not saturated in any tissue groups on an NDL dive so a slow ascent may allow more uptake in some tissues in the deeper depths during the slow ascent than a faster ascent and stop would. It's not so much the bubbles as it is the size of the bubbles. Have you ever had an IV drip, generally there is some air in the IV line that ends up in your vein as bubbles but they aren't big enough to cause a problem,

You're taking a piece of my quote out of context.
I clearly stated in the bit you cut out that it is possible, but not necessarily efficient.
If you had an unlimited gas supply and brought a diver up from say 40m, very slowly you would be able to bring him up without forming a single bubble, and this would be 100% safe. Pure diffusion.
Even if some compartment continues to on-gas initially, at some point in the ascent it will start to off-gas without bubble formation.

I repeat, if you want efficiency then you have to mix techniques.

 

[The Chairman]

Getting BACK to the OP...

Man does quick ascents and blows off safety stops over multiple dives on multiple days of diving. Does anyone think this is GOOD? Can anyone justify THESE actions per any agency protocol? Perhaps the fine splitting of these deco/safety stop hairs belongs in another thread?

Are there ANY suggestions (other than don't dive with strokes) that you can give the OP to handle that very situation in the future? IOW, how would YOU approach the situation? Talk to the diver, or maybe the boat captain/divemaster? If you talk to the diver, how would you handle it? Using a bottle of coke or a beer? How do we get unsafe divers to see that they are putting our fun in danger as well as their own lives?