Modern research/thoughts on Ascent Rates

[rjack321]

To put it in simple terms, my question for scientists would be if we are forming more silent bubbles by doing deep stops would the bubble model theory still hold up regarding tension, and would the tension on those bubbles still place a diver at a lower risk than if the diver ascended higher in the water column, possibly forming less bubbles, but would less tension on fewer bubbles still make it more likely that one bubble could cause a problem. I haven't been following advances in science on the matter because I've been busy trying to recover physically for 4 years, so my understanding might be flawed. I've returned to recreational diving using DCIEM tables. But as I gain experience post-injury by returning to technical diving, a thorough understanding of current vs. past theories would be a central focus of my personal growth.

This a slightly skewed description of the issue with the older low GF approach which is similar across ratio deco, VPM2+ and something like 15/85 Buhlmann.

The issue with the deeper stops is that while your fastest tissues are (hopefully) offgassing, you are still ongassing in the medium/slower tissues at those depths. Which creates a larger dissolved gas load (sum gas load) which is weighted towards the slowest tissues. This gas load has to come out eventually. If you proceed to shorten the shallow stops (something all three of these cited approaches encourage) because you "controlled micro bubble formation deep" then your surfacing gas load ends up higher than it would have been if you skipped the deep stops. That higher surfacing gas load in medium/slower tissues + less shallow time to get rid of it leads to an increased probability of DCS.

You have several possible paths to manage this increased DCS risk:

1. For any given total deco time skip the deepest stops.
2. If you continue to do deeper stops then extend the shallow stops to compensate for the increased gas loads created by the deeper stops.
3. Do both.

Raising your GF low from 10-20 to 50-65% accomplishes #1
Reducing your GF high from 85 to 70-75 accomplishes #2
Diving something like GF 60/70 does both and is now considered the "most conservative".
VPM and ratio deco are not compatible with modern decompression and scientifically supportable conservatism adjustments - other than just winging it and adding time shallow.

In terms of ascent rates, there is very little work done on the significance of how fast you move between the bottom and your first stop. Most people move far too slow, but most people aren't going so slow that 2-3mins of lingering really matters much on dives where you only have ~20-60mins of deco anyway.

 

[Nick_Radov]

VPM and ratio deco are not compatible with modern decompression and scientifically supportable conservatism adjustments - other than just winging it and adding time shallow.

Ratio deco is not any one specific algorithm. It's merely a recognition that the multidimensional decompression curve generated by any algorithm — whether it's Bühlmann ZHL-16C or RGBM or something else — can be approximated by a straight line within certain limits without impacting safety. Deco isn't an exact science so close enough is fine.

If you want to come up with your own ratio deco protocol, then run your favorite dive planning software for a typical baseline profile. Could be 150 ft / 45 m for 30 minutes with one deco gas, 220 ft / 67 m for 20 minutes with two gasses, or whatever you usually do. You'll get a total deco time and can see how that time is divided up across the stops. Then try changing the depth up or down by 10 ft / 3 m or changing the bottom time up or down by 5 minutes. You can chart out how the total deco time changes based on those parameters and see that it's approximately a linear relationship for minor variations as long as you don't change the depth or time too much. So, you can calculate some simple ratios on that basis which enable you to easily plan deco schedules on the fly.

Ratio deco was particularly helpful back in the days before everyone had dive planners at their fingertips in dive computers and smartphones. Where I usually dive in the Northeast Pacific Ocean the conditions vary a lot and we don't know in advance which site we'll be able to hit. So, with everyone following the same ratio deco protocol we could plan a dive in a few seconds on the boat deck and get everyone on the same page. Now it's not as necessary, but it's still useful to memorize those basic ratios so that you can "sanity check" what your dive computer is telling you and understand what the effects will be if you have to modify your planned profile in the middle of the dive.

 

[L13]

...Now it's not as necessary, but it's still useful to memorize those basic ratios so that you can "sanity check" what your dive computer is telling you and understand what the effects will be if you have to modify your planned profile in the middle of the dive.

The "sanity check" is grossly undervalued in the modern world. I try and sanity check everything. If you don't get what you expected, you need to fix something. Sometimes I need to fix my knowledge. Sometimes I need to do it again right. Sometimes I need to fix the equipment.

 

[Wibble]

Ratios -- as in approximate dive times -- are great for planning.

Our dive boat likes <=2h dives, although more by prior arrangement. Thus you know that for various depths it is approximately:

• 30m/100ft is 1:1 minus 30mins (NDL) -- 1h15 on the bottom is 75mins minus 30mins = 45mins deco of which 20ish mins will be at 6m/20ft.
• 45m/150ft it'll be 1:1 -- 1h on the bottom and 1h on deco, of which 30 mins will be at 6m/20ft.
• 60m/200ft it'll be 1:2 -- 40mins on the bottom and 80mins of deco of which 40mins will be at 6m/20ft
• 75m/250ft it'll be 1:3 -- 40mins on the bottom and 120mins of deco

That even gives you a clue to the bailout requirements (deco gas x 2 or more)

Of course you always do a proper plan with MultiDeco/whatever, but you can look at a dive and get a working outline of the dive.

 

[rjack321]

Ratio deco is not any one specific algorithm. It's merely a recognition that the multidimensional decompression curve generated by any algorithm — whether it's Bühlmann ZHL-16C or RGBM or something else — can be approximated by a straight line within certain limits without impacting safety. Deco isn't an exact science so close enough is fine.

If you want to come up with your own ratio deco protocol, then run your favorite dive planning software for a typical baseline profile. Could be 150 ft / 45 m for 30 minutes with one deco gas, 220 ft / 67 m for 20 minutes with two gasses, or whatever you usually do. You'll get a total deco time and can see how that time is divided up across the stops. Then try changing the depth up or down by 10 ft / 3 m or changing the bottom time up or down by 5 minutes. You can chart out how the total deco time changes based on those parameters and see that it's approximately a linear relationship for minor variations as long as you don't change the depth or time too much. So, you can calculate some simple ratios on that basis which enable you to easily plan deco schedules on the fly.

Ratio deco was particularly helpful back in the days before everyone had dive planners at their fingertips in dive computers and smartphones. Where I usually dive in the Northeast Pacific Ocean the conditions vary a lot and we don't know in advance which site we'll be able to hit. So, with everyone following the same ratio deco protocol we could plan a dive in a few seconds on the boat deck and get everyone on the same page. Now it's not as necessary, but it's still useful to memorize those basic ratios so that you can "sanity check" what your dive computer is telling you and understand what the effects will be if you have to modify your planned profile in the middle of the dive.

I am well aware of ratio deco. The total times for RD were typically compared to 20/85 GF and VPM+2 and RD practitioners would aim for those times but mess around with the distribution of that total stop time - usually with lots of arm waving and technical words about the solubility of helium, controlling bubble formation, milk trucks carrying away N2, and whatnot.

But that curve shaping we now know to be incorrect. RD concepts like the "oxygen window" and "S-curves" on 50% are not scientifically defensible in 2023 -as Dr Mitchell and others would say - they are "bro science". Also RD as taught ends up being fairly aggressive, both the total time and the distribution of the stops are not aligned with a conservative dive plan.

There has never been a way to adjust RD to be more or less conservative in any scientifically defensible way. At the same time. like I said, for dives like your examples with 20-60min of deco it probably doesn't really matter that much. The DCS incidence remains low enough so it's hard to point to RD as "wrong" even if it's not necessarily capturing a modern understanding of what is/isn't conservative.

 

[Nick_Radov]

I am well aware of ratio deco.

No, you really aren't. You appear to be aware of one specific implementation of ratio deco that was formerly taught in GUE Tech courses. Now we know that some parts of that old protocol were suboptimal for deco efficiency and gas volume needed. No one is advocating for that specific approach here so we can quit beating the dead horse.

Gradient Factors in a Post-Deep Stops World

World-recognized decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

gue.com

The general mathematical concept of ratio deco as approximating a complex curve with simple linear relationships within certain narrow limits remains as valid as ever. It can be applied to any deco algorithm you like as a convenient planning tool. The trouble is that some divers treated one specific implementation of ratio deco as some sort of magical "received knowledge" without understanding how it was derived from a combination of theory and empirical experience. If you don't like a particular implementation of ratio deco then just make a new one.

The general concepts underlying ratio deco were widely discussed like 25 years ago on the old Aquanaut Techdiver mailing list. Some of the details have changed but unfortunately we're still arguing over the same misconceptions.

Aquanaut Mailing List Archive Service

Aquanaut's mailing list archives.

aquanaut.com

 

[rjack321]

Gradient Factors in a Post-Deep Stops World

World-recognized decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

gue.com

This article is honestly dreadful, not really recognizing modern science. And as I and others have pointed out here many times before, GUE is slow to change. What has worked for their course directors and board members for years and years does not mean its conservative or statistically safe.

As soon as you take the curved output from whatever Bulhmann (or VPM even, or nearly any other) model and slap a "ratio deco" line on it you will be shifting at least some time deeper which is exactly what modern science says to avoid. You can undo that linearization, but you end up back where you started with a shape similar to the original GF 15/85 or 50/80 or whatever you started with.

You can also jiggle the 1:1, 1:2, 1:3 depth setpoints for deriving total time instead of tweaking the underlying total deco numbers. E.g. using 140ft as the 1:1 setpoint. But AGAIN there is no basis for this beyond the "bro science" anecdotes about what is pragmatic and usually works.

As I mentioned already, moving a few minutes around when you only have a 30min obligation or being a bit slow off the bottom and lingering too deep when your total obligation is modest probably isn't noteworthy and well within the statistical noise.

 

[boulderjohn]

This article is honestly dreadful, not really recognizing modern science.

Are you saying that Dr. David Doolette does not recognize modern science? Seriously? Can you name anyone who would know more about modern science than Dr. David Doolette? I can't. You may say Dr. Simon Mitchell, but I think Dr. Mitchell gets a lot of his information from Dr. Doolette.

 

[rjack321]

Are you saying that Dr. David Doolette does not recognize modern science? Seriously? Can you name anyone who would know more about modern science than Dr. David Doolette? I can't. You may say Dr. Simon Mitchell, but I think Dr. Mitchell gets a lot of his information from Dr. Doolette.

shrugs I have read it several times.
what people are being taught and actually doing does not align with anything in there.

 

[boulderjohn]

shrugs I have read it several times.
what people are being taught and actually doing does not align with anything in there.

Well, we obviously travel in different circles. What people I know are being taught and actually doing aligns very nicely with what he wrote in there.

Can you specify what modern science says that is different from what he wrote there?