Ascent Help

[uncfnp]

The way it was explained to me (and I've seen the same in print, numerous times) is that your offgassing is more uniform when horizontal.

Hi Kharon. I have seen this too but my understanding is that it is a small and purely theoretical factor, especially on recreational dives. I have seen studies that suggest that light exercise may aid off gassing. If I am concerned, sometimes I will hang a little heavy on my dsmb and gently fin.

 

[Kharon]

Hi Kharon. I have seen this too but my understanding is that it is a small and purely theoretical factor, especially on recreational dives. I have seen studies that suggest that light exercise may aid off gassing. If I am concerned, sometimes I will hang a little heavy on my dsmb and gently fin.

Yeah, I kind of figured that the difference in offgassing would be nominal and, as you say, much of offgassing is theoretical anyway.

 

[Storker]

I have seen studies that suggest that light exercise may aid off gassing.

Me, too. And I have the impression that current knowledge more than "suggests" it.

much of offgassing is theoretical anyway.

I beg to differ. Offgassing is very much a practical thing.

 

[The Chairman]

Suuntos are fine PDCs, but simply moving your hand from a resting position to adjusting your mask can set off their ascent alarm. Swimming is just way too quick, so I usually breathe myself up. But then, I don't wear much exposure protection either so I don't have to worry about Boyle's Law making a cork out of me.

 

[Kharon]

I beg to differ. Offgassing is very much a practical thing.

Never said it wasn't practical. It's theoretical because there is no way to actually measure the rate of offgassing of each of the very theoretical compartments. It's actually not even a theory scientifically. By scientific definition it's only a hypothesis. There isn't even proof (scientific, experimental, objective proof) that the compartments being used in any single model are the right ones, nor do all models use the same compartments.

The models work pretty well Not saying they don't nor that they aren't practical, but every computer manual, article, discussion of tables I've ever seen has stated emphatically that what is actually occuring isn't completely understood and that any dive, even completely well within parameters and exactly the same as a previous dive, could result in a DCI event. The only way to be sure not to get bent is to not dive.

 

[yle]

DevonDiver, can you elaborate on the "too slow" and "best speed" labels on your Zoop diagram? I'm especially curious about the "too slow"...

To the OP... you paid a lot of money for your Suunto Steel so that it would give you advice while you're diving. It's doing what it's supposed to, but you're going to discount that advice because you just don't like it? The computer is telling you to slow down; up to you if you want to take it's advice, but the advice is based on algorithms developed from a lot of real-world experience.

 

[DevonDiver]

DevonDiver, can you elaborate on the "too slow" and "best speed" labels on your Zoop diagram? I'm especially curious about the "too slow"...

I wrote about the issue on my blog 3 years ago, see link below. It's primarily applicable to technical divers, but over the years I've noted that the 'quality' of my ascent (speeds and stops) does have an impact on my post-dive vitality. Some would attribute this to greater or lesser levels of decompression stress.

OPTIMUM ASCENT SPEEDS FOR SCUBA DIVING

What interests me is that, in general, the major diving agencies teach a very simplistic approach to decompression ascents. Whilst this is arguably suitable for novice divers; a K.I.S.S approach for divers still developing their core buoyancy skills.... I am surprised that the agencies do not refine or optimize that ascent strategy at subsequent levels of training; where the diver might be reasonably assumed to have a higher competency in buoyancy, control and situational awareness...

 

[yle]

I wrote about the issue on my blog 3 years ago, see link below. It's primarily applicable to technical divers, but over the years I've noted that the 'quality' of my ascent (speeds and stops) does have an impact on my post-dive vitality.

Thanks, I just read your post on your blog. It does seem that the concern about decompression on ascent (i.e. an ascent rate fast enough to create a large enough pressure differential to effectively off-gas) would be far more a concern of tech divers than recreational divers. As a tech diver you probably feel that all divers should be aware of this issue, but you have to admit: that's kind of like a NASCAR driver trying to explain competitive driving strategy at the neighborhood go-kart track.

 

[DevonDiver]

As a tech diver you probably feel that all divers should be aware of this issue, but you have to admit: that's kind of like a NASCAR driver trying to explain competitive driving strategy at the neighborhood go-kart track.

Yes and no.

What I noticed over the years was that my post-dive vitality was excellent after technical dives, but often diminished after recreational, especially training, dives. Put simply, I felt fine after a heavily saturated, but well ascended, dive.... but lethargic or sleepy after very 'light' recreational dives with a very unrefined off-gassing strategy.

Decompression stress isn't merely a product of high saturation - it's a product of pressure change on saturated gas. In particular, the formation of bubbles. On a recreational dive, the level of saturation isn't often sufficient to grow bubbles to a harmful size; but the volume (or score) of sub-clinical bubbles (micro bubbles or emboli) can be high.

This is due to many factors; not least the dumbed down ascent strategy and the relatively awkward dive profiles often experienced. In the shallows, pressure changes more rapidly over a given depth. Smaller depth variations cause bigger pressure shifts... and contribute more markedly to bubble seeding.

This, I believe, has a systemic effect (the immune reaction) that presents decompression stress symptoms - fatigue, lethargy, sleepiness.

I typically feel much less vitality after a 90 minute shallow (3-6m) skills session than I do after a 90 minute deep decompression dive. There's no basis for that other than decompression stress as a result of relative pressure fluctuations over the dive profile. One profile is clean but deep, the other is a zig-zag mess - even though the depth is negligible. This is something I notice, without fail, and have done for years.

I feel much more inclined to post-dive sleepiness after a 45 minute, 22m, no-stop dive with a simple 'PADI' ascent (3-6m per min ascent and 3min @ 5m safety stop) than I do after a 45 minute run-time 40m wreck dive with staged deco and 50% gas from 21m.

My thinking is that ascent profiles are equally as important for recreational divers; as they do the sort of dives that can raise decompression stress over the profile.... whereas technical divers typically dive much cleaner, deeper and less pressure fluctuating dives....and, of course, ascend and off-gas in a very refined manner.

Whilst sub-clinical bubbles are not harmful in a medical sense; at least in the short-term, the presentation of decompression stress is something that most divers would prefer to avoid, if possible. My rationale for more refined ascent decompression strategies is about that avoidance.

There's nothing wrong with teaching novice divers to ascent "slowly from every dive".... as this is about insulating them from the danger of too fast ascents. Slower is, of course, very prudent if you are liable to loss control on ascent... or not yet otherwise in complete and consistent control of your buoyancy.

However, as the diver progresses in experience and training, there's no logical rationale that they shouldn't progress to more refined ascent strategies, commensurate with their increase in skill and control.

Personally, I see it as a failure in diver training that recreational divers are not introduced to more sophisticated and beneficial ascent techniques as they progress beyond OW level....and especially if they do recreational deep diving training.

 

[halocline]

Two quick points:
1. Dive computers often react to very slight movements, like raising an arm with the computer on the wrist can easily set off an ascent alarm. While the ascent alarms are very useful safety devices for new divers, they're not perfect indicators of good scent practices.

2. Putting DevonDiver's excellent post on a bumper sticker, good technical diving involves precise control over ascents and this results in feeling better after a dive. Recreational divers should adapt their ascent practices to likewise feel better after a dive, and the recreational agencies should teach them how to do this. I completely agree with this idea, except smart divers can learn it on their own with some reading. One very simple strategy is to do a much longer shallow stop on recreational dives, and then proceed to the surface extremely slowly.

Okay, that's a really long bumper sticker, sorry!