Hypothetical question about decompression

[DwayneJ]

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I'm am proud to be included in what I know to be a group of honorable and smart people. My sincerest thanks.

 

[wve]

No question that deco is theory, but please consider the following, from the DAN website (citation below):

British physiologist Sir Leonard Hill theorized that decompression should be by linear ascent to the surface; he strongly disagreed with Haldane's approach. However, in the end Haldane was able to prove, using goats, that a slow linear ascent was not only ineffective, but unsafe; too much nitrogen remained on surfacing resulting in frequent DCS.​

Full article is available at DAN.

-Bryan

Good article. The other DAN articles cited by Charlie99 and others are also helpful.

To sum them up, the articles recount Hills's argument with Haldane. Hill believed that slow, linear ascents were the best way to avoid DCS, and Haldane thought it was possible to make faster ascents with stops along the way. When they tested both theories with goats, Hill's idea of slow, linear ascents did not work. More recent chamber research using modified Haldane concepts also supports this conclusion.

Interestingly, however, these articles also point out that current no-decompression recreational dive tables and computers -- which also have lots of data to support them -- actually follow Hill's model more closely by stressing slow ascents and only requiring safety stops when one comes within 3 pressure groups of a no-decompression limit, etc.

Now, if you think about it, what we have here is kind of interesting. Because Hill killed a few goats and Haldane didn't, the Navy adopted his model which has been perpetuated, albeit in modified fashion, ever since. On the other hand, hundreds of thousands of recreational divers have unwittingly followed Hill's model -- and lived!

So, for the sake of discussion, I was attempting to split a hair.

To be fair to the respondents who answered No to the original question, the OP did specify that the overall ascent time should be the same in each instance. And in that case, I agree -- especially if one is talking about a dive that everyone would think requires a decompression stop.

However, the OP's question and others' subsequent replies left me wondering:

Does a slow, linear ascent not work because linear ascents never work, or is it just that we simply haven't found the right ascent rate yet? Might a sufficiently long ascent work just as well?

If not, then why not? What is going on physiologically that prevents a slow, linear ascent from working?

There may be a huge difference between what is theoretically possible, and what is actually practical. In the end, I bow to the practical, but I remain curious...

By the way, did anyone ever wonder whether Haldane was better than Hill at picking healthy goats?

 

[rstofer]

Interestingly, however, these articles also point out that current no-decompression recreational dive tables and computers -- which also have lots of data to support them -- actually follow Hill's model more closely by stressing slow ascents and only requiring safety stops when one comes within 3 pressure groups of a no-decompression limit, etc.

I'm having a problem with terminology. Specifically the terms "no-decompression recreational diving", "safety stop" and "no-decompression limit".

I thought no-decompression recreational diving using tables had, as a fundamental premise, the ability to make a no-stop ascent to the surface at any time within the NDL. Further, I thought the allowable ascent rate was 60 fpm, not 30 fpm.

I thought the safety stops were recommended as opposed to required. Because, it seem to me, if they are required, they are decompression stops and this is no longer a no-decompression recreational dive.

I'm not suggesting that the safety stops be ignored nor am I suggesting that a slower ascent rate isn't helpful for recreational diving. What I am trying to do is sort out the terminology: is the safety stop actually required (within 3 letter groups for PADI) or is it just highly recommended?

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.

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?

Richard

 

[SteveAD]

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.

 

[SteveAD]

I'm having a problem with terminology. Specifically the terms "no-decompression recreational diving", "safety stop" and "no-decompression limit".

I thought no-decompression recreational diving using tables had, as a fundamental premise, the ability to make a no-stop ascent to the surface at any time within the NDL. Further, I thought the allowable ascent rate was 60 fpm, not 30 fpm.

I thought the safety stops were recommended as opposed to required. Because, it seem to me, if they are required, they are decompression stops and this is no longer a no-decompression recreational dive.

I'm not suggesting that the safety stops be ignored nor am I suggesting that a slower ascent rate isn't helpful for recreational diving. What I am trying to do is sort out the terminology: is the safety stop actually required (within 3 letter groups for PADI) or is it just highly recommended?

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.

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?

Richard

Richard

You are correct, when diving within the (NDL) limits of the PADI or NAUI tables no stops are necessary.

The OP referenced Decompression diving (something like 200 feet for 30 min) which could require stops totaling over an hour, with specific time requirements at specific depths (and quite likely switching breathing gas at various depths).

And finally yes the NDL number on dive computers is the no deco limit within which an uninterrupted ascent should be safe, but slower ascent rates and safety stops are believed to add an additional buffer zone.

 

[bleeb]

However, the OP's question and others' subsequent replies left me wondering:

Does a slow, linear ascent not work because linear ascents never work, or is it just that we simply haven't found the right ascent rate yet? Might a sufficiently long ascent work just as well?

If not, then why not? What is going on physiologically that prevents a slow, linear ascent from working?

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.

 

[daniel1948]

One of the great things about the internet is that people like me who have no idea what we're talking about can jump right in. (Actually, I think I know a little bit of what I'm talking about. )

... By the way, did anyone ever wonder whether Haldane was better than Hill at picking healthy goats?

If we had no other information besides the goats, this would be a reasonable question. But we now have voluminous studies, including Doppler measurements of actual bubbles in the blood stream. The goats were early data, and useful, though leaving the possibility of the above unlikely flaw. As with all theories, the more confirming data we have, the less likely such alternate explanations become.

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

I'm having a problem with terminology. Specifically the terms "no-decompression recreational diving", "safety stop" and "no-decompression limit".

I thought no-decompression recreational diving using tables had, as a fundamental premise, the ability to make a no-stop ascent to the surface at any time within the NDL. Further, I thought the allowable ascent rate was 60 fpm, not 30 fpm.

I thought the safety stops were recommended as opposed to required. Because, it seem to me, if they are required, they are decompression stops and this is no longer a no-decompression recreational dive.

I'm not suggesting that the safety stops be ignored nor am I suggesting that a slower ascent rate isn't helpful for recreational diving. What I am trying to do is sort out the terminology: is the safety stop actually required (within 3 letter groups for PADI) or is it just highly recommended?

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.

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?

Richard

Here's how I read the terminology and recommendations:

In the end, everything is a recommendation, because none of these agencies has the authority to actually require anything. And because of variability among individuals, there are no absolute boundaries between safe and unsafe practices. There are practices we know will kill you, and practices we know will never cause DCS. But between them are practices that pose some level of risk. How to describe recommendations in the grey area is a problem. Do you "require" everyone to follow a plan that would be "safe" for a 300-pound 75 year-old chain smoker with a heart condition who never exercises? Do you "allow" everyone to follow practices that are generally safe for an ultra-fit and highly-trained Navy Seal?

I agree that "required safety stop" is confusing terminology. That does sound like a decompression stop. Maybe they mean to indicate by this a grey area where most people would not take a hit if they skipped it, but where they consider the risk too great. I don't know.

 

[bleeb]

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.

As was briefly touched on in another recent thread, the ascent rate is fundamental to the table or computer in question.

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

 

[eponym]

I'm having a problem with terminology. Specifically the terms "no-decompression recreational diving", "safety stop" and "no-decompression limit".

[snip]

I thought the safety stops were recommended as opposed to required. Because, it seems to me, if they are required, they are decompression stops and this is no longer a no-decompression recreational dive.

This is, as you said, a terminology issue. It took me some time to realize that "decompression" is used in two different ways, as I now point out to new dive leaders.

In physiological terms, any time you ascend but stop short of the surface (e.g., safety stop at fifteen feet) you will be eliminating nitrogen accumulated deeper and hence you are decompressing.

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.

Hope that helps,
-Bryan

 

[eponym]

And I should probably clarify what I meant by an "unplanned decompression dive" in my previous post. Your dive computer, if you exceed its time limits or ascent rates, may be programmed to coach you through a mandatory stop. If you then ignore that stop, the computer typically shuts down and is unusable for 24 hours or more.

Dive tables may also include unplanned deco instructions. For example, the notes on the SSI tables say that if you exceed their no-deco time limits by up to five minutes, you should stop at fifteen feet for ten minutes or longer, if your gas supply allows.

-Bryan