first stop = 80, last stop = 6 ??

[rossh]

A web page that describes some common practices, myths, mistakes, errors and other aspects not well understood in decompression. The topics shown were selected based on comments in this list and others.

Common practices, myths and mistakes on deco?

.

[Special K]

Found it very interesting, answered a few questions for me.

[johntemp]

The "20ft / 6m last stop" is an eye opener

Good article

[Mark Powell]

The 'slow ascent off the bottom' section is a particularly clear explanation of a complicated situation.

[tom]

Ross, your article says in a couple of places "NDL dives have other considerations not shown.".

Would you mind mentioning what those other considerations are?

Thanks,
Tom

[And]

Hi

I think you are confusing a computer program with physiology. How does a computer program know when tissues are offgassing, where they are offgassing and at what rate. It doesn't. The human body is more complex than that.

There are people who can do a dive and miss all the stops in your program and not be bent, and there are people who will do all of the stops 'planned' to the far end of a fart, and still get bent.

I'm sorry but IMHO all you are doing is creating more myths. Suggesting that people doing other than what your program says are making mistakes regarding an algorithm which is fundamentally wrong in every sense is a bit rich I think.

Andy

[Drmike]

Originally Posted by And

Hi

I think you are confusing a computer program with physiology. How does a computer program know when tissues are offgassing, where they are offgassing and at what rate. It doesn't. The human body is more complex than that.

There are people who can do a dive and miss all the stops in your program and not be bent, and there are people who will do all of the stops 'planned' to the far end of a fart, and still get bent.

I'm sorry but IMHO all you are doing is creating more myths. Suggesting that people doing other than what your program says are making mistakes regarding an algorithm which is fundamentally wrong in every sense is a bit rich I think.

Andy

WTF?

I dont see any of those points Ross has raised on his 'myth busting page' as being programming or VPM model specific - these are points generally agreed upon by practicaly everyone familiar with deco theory regardless of what deco model they follow.

[And]

Originally Posted by Drmike

WTF?

I dont see any of those points Ross has raised on his 'myth busting page' as being programming or VPM model specific - these are points generally agreed upon by practicaly everyone familiar with deco theory regardless of what deco model they follow.

Are you sure you are reading the same page I'm reading Mike?

Just in case

1. 80% stops

The first stop depth is related directly to the pressure change from the maximum dive depth (distance from bottom). The first stop location is primarily to limit and prevent bubbling quantities and sizes from growing large early on in the ascent.

For most dive parameters, the fastest tissue cell gas loads begin to stabilize after 10 or 15 minutes. The dimension of the first stop (distance from bottom) will be very similar across many decompression dives.

2. Slow ascent

Bubble models will account for a fast ascent and the potential bubbling. They will stop your ascent before bubble quantity / size becomes a problem (this is the primary function of bubble models).

3.Deep stops

The risk of adding a deep stop onto a bubble model plan, is over staying in the deeper sections of the dive. This will require additional decompression to compensate. A Bubble model will provide all the necessary deep stops.

4.Slow final ascent

They need to prevent cases of inadvertent skipped deco. If the program was to consider and include the slow last ascent, the time to surface shown would be a lot earlier. The diver would then be obliged to carry out the slow ascent on all dives. In real life the diver might do some ascents at the regular ascent rate, and therefore the diver will inadvertently skip over a large amount of required deco.

The interesting thing is that I do all of the stuff I am not supposed to Maybe I'm biased. I do start slowing my ascent at 80% of the ATA's, I do not necessarily ascend from the bottom at 10mtrs/min and I do not change my deco accordingly, I do have 'long' deep stops, and I do ascend slowly to the surface. Oh, and I do ignore the 3mtr stop and in fact shorten the 6 mtr stop significantly compared to the computer programs.

I do all of that and I am not dead, or bent, and in fact, do not feel any tiredness after dives at all. So forgive me if I look upon this with more than a little scepticism. What is more some divers can ignore all of what Ross says and will be fine, and some will not. Some will follow Ross's program and be fine, others will not. There are different approaches to do the same thing, which is decompress safely. I think it is wrong to state that certain ways are 'myths', thats all. They are valid and I have yet to see anyone 'disprove' them. Computer programs know nothing about physiology, and we don't know that much about it

Andy

PS Salsette, on 18/45 and 50% deco gas.

[cod3r]

Firstly let me get something out of the way: I'm a RD with 60 dives, no deco, thus a bit out of my "depth"

However I found the info very valuable. In the UK I have inavertedly pushed the NDL (twice). The first time I logically decided to do a very slow ascent, similar to a multilevel profile which usually has the effect of 99mins NDL by 10m or so. However in that instance it added another min. to my deco obligation...

At 30m time runs out fast, I found out. Knowing how to respond makes all the difference.

thx rossh, effort much appreciated, have a green


cod3r

[Mark Powell]

Originally Posted by And

How does a computer program know when tissues are offgassing, where they are offgassing and at what rate. It doesn't.

You are right it doesn't but then it's not just computer programs that don't know this, no one knows this for sure. 100 years ago we thought we had decompression cracked then we realised that there were some cases that didn't fit the model. Each revision of our deco models has been trumpeted as correct but in truth we are probably as far away from understanding what really happens as we were 100 years ago. There is still so much that we don't know. As an example we just don't know for sure why bubbles develop in our tissues. There are theories but none of these have been absolutely proven. We know for sure that it is not just related to pressure differences. You can pressurise a pure liquid to 1000s of bar, leave it to saturate and decompress it back to the surface in seconds with no bubbles. This shows that pressure differences are not the whole story.

Originally Posted by And

The human body is more complex than that.

Absolutely.

Originally Posted by And

Suggesting that people doing other than what your program says are making mistakes regarding an algorithm which is fundamentally wrong in every sense is a bit rich I think.
Andy

Even though I agree with your earlier comments I disagree with your conclusions.

I don't think there is anywhere that Ross says that "doing other than what his program says are making mistakes." He is not referencing V-Planner here but a body of theories upon which V-Planner is based. Not surprisingly his arguments are consistent with the results of V-Planner - we should worry if they weren't! The reason for making this distinction is that Ross has not put numbers into his program, seen the answers and then written these comments as a result. If he had I would have agreed that this is was meaningless. Instead has written these comments based on the original principles.

Now his original principles may not be correct but they are shared by a relatively large proportion of technical divers.

Now to make this thread really interesting I would love to see some discussion of the alternative arguments. Give us some details of why you think this advice is wrong and what the correct answers should be.

For example, why is 80% of ATAs always right, irrespective of the depth, time, etc?

Why a slow ascent off the bottom if not to prevent bubbling?