Hypothetical question about decompression

[rstofer]

Also, I went and dug out a NAUI recreational table. At least as far back as 1989, it's been 30 fpm.

I stand corrected! I inadvertently grabbed my 1987 version. The 1989 version does indeed specify 30 fpm.

NAUI made huge changes to the table in 1989. They eliminated 2 pressure groups, dropped the 140' row and substantially reduced the NDLs. It's a wonder I lived through it.

FWIW, that 1987 table was still in my BC until I converted to a BP/W earlier this summer. Nobody told me it had changed.

Richard

 

[daniel1948]

In recreational open water training, "no-decompression diving" means staying within limits that allow you to make a controlled no-stop ascent all the way to the surface at any time with very low statistical risk of DCS. The stop is optional, but it further lowers your risk of DCS on reaching the surface. It's highly recommended, as it helps deal with contributing factors such as exertion and dehydration during the dive.

So in open water training we're using the term "decompression stop" to refer to a stop that is required (as part of a planned or unplanned deco dive), regardless of the fact that all stops include physiological decompression.

So where does that leave the "required safety stop" if you come within three pressure groups of a "no-decompression" limit? if the distinction between a safety stop and a decompression stop is that the former is "recommended" and the latter is "required," then it looks to me as though the actual no-decompression limit is three pressure groups above the spot indicated as such on the RDP.

Clearly, as you said, it's a question of terminology. But the PADI terminology seems inconsistent.

 

[wve]

WVE

The big difference between Hills dead goats and all the recreational divers who seem to be following hills model successfully is that the majority of todays divers are not getting to the point where decompression stops are mandatory. There is a huge difference between a 50 foot 1 hour dive and a sandhog working in a caisson at >100feet for a 12 hour shift.

While I am certainly NOT advocating this, the majority of recreational (ndl) profiles would be unlikely to result in DCS even with a relatively fast ascent rate and no stops.

I agree.

Although the DAN articles cited say that recreational divers follow Hill's model, I bet they really don't.

It seems to me that most recreational diving is really multi-level. You might hit a maximum depth of 100 ft, but then you slowly ascend and then swim along a wall at 40-50 ft for 20 minutes or so. From an "ascent" standpoint, you have "stopped" at half your maximum depth -- even though you are still moving. Plus, most recreational divers thrown in the 3 minute safety stop at 15-20 feet.

Although the ascent phases are slow, there are "stops" in the profile.

This may be why the actual incidence of DCS is so low.

 

[wve]

One bit of basic physics/chemistry to bear in mind is that since we're talking about gas diffusion here, the basic rate of transfer is exponential. Applying a linear term on the other side of the equation is likely to be either sub-optimal or outruns the offgassing, and the greater the quantity of gas (i.e. longer and deeper), the farther off things get. I have a vague recollection that one of the two H's examined a similar line of thought.

Of course, in itself, this doesn't prove that linear ascents can' work.

Right.

It gets kinda fuzzy at the line where hard science (physics) meets the softer science (physiology). By soft I mean more variables which cannot always be controlled or accounted for -- or even anticipated -- in a scientific study.

 

[MXGratefulDiver]

Is the 30 fpm ascent rate a good idea or is it fundamental to using the tables? Actually, I know the answer to this one: the PADI tables indicate a 60 fpm ascent rate as does their training literature. Same for NAUI.

NAUI's tables and training literature indicate a maximum ascent rate of 30 fpm ... and their tables are based on an assumption of a 30 fpm ascent rate.

One final concern: when a computer indicates an NDL, do they really mean no-decompression limit? Can I directly ascend to the surface within that NDL?

Yes, and yes ... especially because the majority of computers out there have so many levels of conservatism built into them.

However, keep in mind that all of this is based on a set of assumptions that apply only to a theoretical "ideal" human body ... which may or may not apply to your actual situation. That's why on rare occasions someone takes a hit even though they followed all the rules. There is a finite risk of DCS on every dive ... all we can do is manage the risk to keep it within what we consider acceptable limits.

... Bob (Grateful Diver)

 

[MXGratefulDiver]

And low I/Qs go onto my ignor list, with the others.

I didn't realize a member could put himself on "ignor" ...

... Bob (Grateful Diver)

 

[HowardE]

It's not exactly what the OP was asking about, but there is the concept of "ceilings", as opposed to discreet deco stops. V-Planner Live running on the Liquivision X1 computer has the option of continuously calculating the exact required deco depth required, according to its model, in real time, so the the diver can continuously ascend holding the specified depth, rather than spending time at evenly spaced stops. Although the displayed ceiling changes continuously, I doubt very much that it's a linear progression.

I know several accomplished divers who appreciate the ceiling model as opposed to the "sawtoothed" level stops... and while I also thought about this while reading the OP's post. This method of "achieving the continuous curve" of decompression is pretty much a CCR thing.

Here's my tech instructor's opinion on that topic -> http://www.scubaboard.com/forums/4039501-post86.html

 

[nereas]

I didn't realize a member could put himself on "ignor" ...

... Bob (Grateful Diver)

You're almost there. You have bouts of complete irrelevance.

Other times you occasionally post something interesting.

 

[nereas]

I agree.

Although the DAN articles cited say that recreational divers follow Hill's model, I bet they really don't.

It seems to me that most recreational diving is really multi-level. You might hit a maximum depth of 100 ft, but then you slowly ascend and then swim along a wall at 40-50 ft for 20 minutes or so. From an "ascent" standpoint, you have "stopped" at half your maximum depth -- even though you are still moving. Plus, most recreational divers thrown in the 3 minute safety stop at 15-20 feet.

Although the ascent phases are slow, there are "stops" in the profile.

This may be why the actual incidence of DCS is so low.

Safety stops are more likly to be the reason why the actual incidence of reported DCS is low.

 

[wve]

I think maybe you were joking. But I still think the question is worth asking... and answering.

I was joking, although it would have been a valid question for that day and time. I wonder whether anyone thought to ask back then.

... "required safety stop" is confusing terminology. That does sound like a decompression stop.

It does.