Shallow Decompression Dives

[rjack321]

Given that all dives are decompression dives, whether taught that way or not, and that DCS is rare it seems to me there is too much emphasis on the need for extensive "special" deco training. Much of what I've read in other venues seems to be more for marketing of a course than something needed.

Now, I'm not an "old time" NAUI diver. But, my initial training was NAUI and even at that level I understood how to use tables to plan a dive, any dive. That a planned stop had to be at some other depth than 15' was of no consequence from a profile planning point of view. A planned dive is a planned dive.

Since then I've been introduced to a variety of thought themes from a variety of teaching agencies. Each has had its' own take on dive planning. But, physics is physics and the characteristics water don't change. So, these have done more to reinforce the idea that from a profile point of view all dives have the same factors; none of which are complicated.

Where the thought train goes astray is mixing in the needs of certain kinds of diving, usually deep, and the desire for faster decompression, usually requiring gas switches. It further goes astray when desired dive methods, such as team diving, are thrown into the mix. Now, we are going beyond the physics of diving to other matters that we have chosen to affect the dive.

So, to me, this is an issue of clear, analytical thinking, not of any special training.

Arctic,
Do you have any technical training? With NAUI-tech or another agency? What levels?

 

[rjack321]

Just remember, it may seem counter intuitive but, making a shallow dive that is rigorous enough to get you into a decompression status is a far less forgiving situation than making a deep dive that results in the same decompression obligation.

I'm not sure I 100% agree with this. At least your fast tissues that are likely to cause type2 are similar (saturated). You will have a greater gas load overall so on that point I agree. A mixed bag

 

[Rhone Man]

I think the general idea is that:
- short deep dive, you can off gas the excess nitrogen fairly quickly at shallower depth.
- long shallower dive, better have a good book, because it is going to take a l-o-n-g time to get all the nitrogen out of those saturated slow tissues.
But I am no expert.

 

[Thalassamania]

The point is that for shorter deeper dives the surfacing ratios are soooooooo much larger. That's the key.

 

[Kevrumbo]

Just remember, it may seem counter intuitive but, making a shallow dive that is rigorous enough to get you into a decompression status is a far less forgiving situation than making a deep dive that results in the same decompression obligation.

I'm not sure I 100% agree with this. At least your fast tissues that are likely to cause type2 are similar (saturated). You will have a greater gas load overall so on that point I agree. A mixed bag

A mixed bag indeed . . .but you've got to recall as well that the last 33'/10m to the surface is the greatest delta pressure change if you've got a significant deco obligation (I know it and found out the hard way in Truk which btw, has a very nice Recompression Chamber).

If you've got buoyancy problems in that last 33'/10m, or even in the last 10' in my type I DCS case, you may have even bigger problems after surfacing. (That's part of the rigor & difficulty that Thal is generally referring to. . .)

 

[ArcticDiver]

Posted via Mobile Device

One of the advantages to planning every dive as a decompression dive is that from a risk management viewpoint your algorithm will take care of you. Certainly no algorithm is 100% guaranteed. But the odds of a DCS free dive are better regardless of dive depth.

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[Kevrumbo]

Just remember, it may seem counter intuitive but, making a shallow dive that is rigorous enough to get you into a decompression status is a far less forgiving situation than making a deep dive that results in the same decompression obligation.

An unfortunate, unexpected and very unforgiving shallow dive DCS type II incident:

. . .taught some open water courses . . .doing 16 dives in 13 days. The last two dives were less than 1 hour and less than 30 fsw. . .took two full days to rest and to out-gas. After 54 hours of surface interval, we took an afternoon cool-off dive, one we've done hundreds of times from our back steps: 42 ft for 40 min, surface to surface [total elapsed time], with a time of about 8 minutes to ascend from 31 ft.

Full account here

 

[DA Aquamaster]

I am a bit old school but think it makes perfect sense to teach the full range of gas planning in OW and certainly in AOW. It made sense in 1985 when I got certified but it makes even more sense now. For example then, if you stayed above 100' and limited yourself to one tank per day, you had to have an exceptional SAC to exceed the Navy NDL's, so a reserve to 500 psi was usually enough. Now, with more conservative tables and larger single tanks it is much easier to get yourself into a deco situation on air or even with nitrox.

And the fact is that you see many recreational divers doing 5-10 minute deco stops without the redundant equipment or dive planning needed to do it safely.

I agree with those people that have suggested the TDI deco procedures course. I'd recommend taking with with Advnced nitrox, but even without it you will get the required training in gas planning and use of deco tables.

 

[ArcticDiver]

I am a bit old school but think it makes perfect sense to teach the full range of gas planning in OW and certainly in AOW. It made sense in 1985 when I got certified but it makes even more sense now. For example then, if you stayed above 100' and limited yourself to one tank per day, you had to have an exceptional SAC to exceed the Navy NDL's, so a reserve to 500 psi was usually enough. Now, with more conservative tables and larger single tanks it is much easier to get yourself into a deco situation on air or even with nitrox.

And the fact is that you see many recreational divers doing 5-10 minute deco stops without the redundant equipment or dive planning needed to do it safely.

I agree with those people that have suggested the TDI deco procedures course. I'd recommend taking with with Advnced nitrox, but even without it you will get the required training in gas planning and use of deco tables.

Good suggestion as long as a person realizes that most of these courses are focused on the use of specific tools, not the underlying hyperbaric science. They also ring in a healthy dose of the How To with methods to speed up decompression with gas changes. Again, a bit light on the hyperbaric physics side of things. Not saying that training on how to use the tools(tables, etc) developing muscle skills aren't important; just that often the physics side of what we do is treated a bit on the light side.

We spend a good deal of time talking about one aspect of hyperbarics, DCS, and not so much about DCI, which is the one that, from all we know causes the most injury. From a risk management point of view redirecting our efforts to focus on reducing these injuries would seem to be appropriate. This is especially true since by and large we know what causes injuries but are not nearly as sure about DCS.

Injury reduction is all about probablilities and risk management based on what we know and what effort will return the most improvement for the effort expended.

 

[Reg Braithwaite]

...is it really sensible to be teaching OW students all the extensive gas planning that accompanies deco diving? Sure, explain to them why gas planning becomes important when you do deco diving but I'm not sure it really is necessary to put them through the graft of calculating air consumption, cylinder size, air usage on ascent corrected for average depth during ascent, air usage on stops etc...

If you were teaching people woodworking, wouldn't you teach them how to figure out how much wood a project will require before they start cutting and sawing? Gas planing for simple dives is relatively straightforward, especially if you focus on the most common tanks a new diver will encounter, AL80s. I can't imagine someone being able o use tables for planning a no-stop dive but being unable to figure out how much gas they need for the same dive.