half stops

[svs]

Oddly, if I was ascending from a 100' dive, I would much rather do a minute at 50', than add a minute to a 15' safety stop.

A Type-2 (fast tissue) bends hit is a nasty thing.


All the best, James

I may be wrong but my understanding is that bubble in CNS (and I suspect that your major concern is bubble in CNS) can be formed in CNS or come from a bloodstream. Blood microbubbles are the most likely nuance after deep but short no deco dive. Longer 15ft stop will be better choice to clear of blood microbubbles.

 

[TSandM]

. Longer 15ft stop will be better choice to clear of blood microbubbles.

Bubble theory says stay deep to keep bubbles small; small bubbles are much more likely to collapse, and gas is more easily eliminated from the fluid phase than from bubbles. Dissolved gas theory sends you shallow quickly, to push the gradient and accelerate offgassing.

The pure Buhlmann approach of get shallow and sit there is sometimes referred as "bend and mend" for this reason.

 

[fnfalman]

If you do a deep stop on a no-deco dive, then it's called a multilevel dive.

The only way you can stay underwater and not on-gassing is when you are at very shallow depth.

Deco diving is a totally different animal. They have deco schedule to follow. I don't understand why rec divers think that they have to do things tech or quasi-tech. What works or required for tech doesn't automatically translate to rec.

Worry about the bend? Take your time in the ascension process. Go up as slow as your remaining gas allows.

 

[TSandM]

If you do a deep stop on a no-deco dive, then it's called a multilevel dive.

The only way you can stay underwater and not on-gassing is when you are at very shallow depth.

I don't think the issue is that simple.

Many models use a variety of "compartments", with different ongassing halftimes. The halftimes for the "fast" compartments are quite short, often 5 or 10 minutes. This means that, after 30 minutes, the 5 minute compartment is completely saturated at the depth where you are. Offgassing from this compartment will begin the moment you start to ascend at all.

On the other hand, a 60 minute compartment at 30 minutes isn't even half full yet, and if you ascend a few feet, it will still be ongassing, because it isn't even at equilibrium for that depth yet.

Very slow compartments may be ongassing until you are quite close to the surface.

In theory, in no-deco diving, no compartment is going to reach a saturation value where it will not sufficiently offgas during a controlled, direct ascent (at whatever speed is prescribed by the model) to allow the diver to proceed straight to the surface. So all the stops we are talking about are, by definition, optional. (Except for the "mandatory safety stops" in the RDP, I suppose )

It is certainly true that doing deep stops TOO deep is basically multileveling the dive. And on shallow dives, the amount of gradient you can build during a routine, controlled ascent probably isn't great enough to make any midway stop of benefit. A 15 foot safety stop on a 25 foot dive? Probably not useful, except as a mechanism to ensure the ascent speed is controlled.

But for deeper recreational dives, where a stop 2 ATAs (or thereabouts) above max depth is reasonable, it may well help control bubble formation and contribute to reduced decompression stress, which is often manifested as post-dive fatigue.

BTW, for anybody who finds this discussion interesting, I highly recommend Mark Powell's book, Deco for Divers. It's an extremely palatable compilation of information on decompression theory (although it could have used a better editor).

 

[gcbryan]

...

I would bet, if you surveyed people here, there are a lot of us who have, for example, surfaced in the middle of a dive to take bearings. There are definite theoretical arguments against such practices, but again, I don't think there's much if any real data to validate them.

(This quote is taken from another thread talking about reverse profiles just to make that clear.)

I agree that bubble mechanics are as you stated in your post in the current thread and I combine deeper stops with a slower ascent on most dives.

For the sake of discussion however I'm curious why you feel it's OK to surface directly to the surface mid-dive to check bearings (sometimes necessary of course) since you "don't think there's much if any real data to validate" that this shouldn't be done? Yet you consider in your above post that bubble mechanics is valid?

If bubble mechanics is valid then that suggests that direct ascents mid-dive to the surface would not be a great idea. Are am I missing something?

 

[Rick Murchison]

... If bubble mechanics is valid then that suggests that direct ascents mid-dive to the surface would not be a great idea. Are am I missing something?

Depends, doesn't it?
Depends on depth, time, mid-dive ascent profile (rate), time on the surface, depth & time on return, etc., etc., etc.
Bear in mind that a standard practice in Navy and commercial diving was (and I reckon still is) to surface with some pretty big deco obligations, climb in a dry chamber and recompress for deco.
But in general, I think we can say it's "not recommended" as a standard recreational dive practice, except... (list of exceptions)...
As a specific example of an exception, we do lots of dives in Lake Martin, where visibility is always somewhat less than wonderful... but the dives are typically less than 45' max, with a long, slowly rising profile that takes about 30 minutes to come up from 30' as we follow the very gently sloping bottom in to the exit point. As the vis is not so hot, it's real easy to lose track of the leader in just half a heartbeat while watching a fish or investigating a potential treasure. Our standard "lost" procedure is "surface, swim to the flag, follow the line back down to the group." With that dive profile this is a sound procedure no matter whose bubble model you choose to use.
Rick

 

[TSandM]

I don't think surfacing in the middle of a dive is a GOOD idea . . . and the theory would certainly argue that it is not. And, for example, I wouldn't do it on a deep dive -- once I get to the surface from 100 feet, I'm staying there (unless I have to go down a few feet to swim under kelp, or something of that sort). And bubble theory would say that you run a risk of "bubble pumping", or allowing bubbles to form, and then recompressing them so they pass the pulmonary filter and have the opportunity to expand later on the arterial side.

On the other hand, if you manage the ascent to minimize bubble formation and growth (the deep stops we're discussing) then surfacing briefly and then redescending may not be that bad. Certainly, there are many Mexican cave profiles that seesaw up and down, sometimes as much as 50 feet, and you can't avoid the profile unless you simply avoid the dive. And, as I mentioned, many of us have surfaced briefly during a dive to check our navigation, although usually that's not from any great depth. What the theory says, and what you can actually get away with doing, are two different things. One is optimal, and the other is real life

 

[Rick Murchison]

... One is optimal, and the other is real life

"Theoretical..." theoretical is what the good Doctor means...
"Optimal" remains elusive, eh?

Rick

 

[TSandM]

True that, Rick!

 

[1_T_Submariner]

I have been making 1/2 max depth stop just because I'm not is any rush. Just like I hang at 15 0r 20 feet for 5 min at the end of the dive. Sometimes In Boanire I spend 30 min at 15 feet lots to look at. I don't know if the 1/2 stop makes a difference but it doesn't seem to hurt either.