My Journey into UTD Ratio Deco

[LiteHedded]

Actually, with different half-times for He and N2, you could get an explanation for being able to use the same times for 25/25 and 32. I believe playing with this fact has been used on very long dives (24hours) by Laurent Ballesta.


Not saying what's right or wrong, just saying that it could turn out to make no difference, depending on what "religion" you're following.

i wonder if they also teach to ignore the po2 drop when they teach their SCR class. that combined with 25 being equal to 32% could make for some INTERESTING decompression

 

[tbone1004]

Actually, with different half-times for He and N2, you could get an explanation for being able to use the same times for 25/25 and 32. I believe playing with this fact has been used on very long dives (24hours) by Laurent Ballesta.


Not saying what's right or wrong, just saying that it could turn out to make no difference, depending on what "religion" you're following.

I don't think anyone's religion would make it the same. Historically we have thought about the "helium penalty" where if you had say EAN30 vs. 30/30 the NDL for 30/30 would be shorter or the deco times longer than just the nitrox. The helium penalty is currently being thought of as unnecessary, but even that it would make EAN30=30/30 from a decompression standpoint. I have not heard of anyone believing that helium can shorten decompression times.

If the min-deco was done based off of 25/25, then using it on EAN32 would be analogous to diving nitrox but leaving your computer set to air, but it doesn't work the other way around and the min-deco was done based off of EAN32

 

[Dan_P]

While ratio decompression falls apart over extended depths and long exposures, the limits that UTD imposes at the Tech 1 level (160 ft depth, maximum of 15 mins deco on pure O2, 30 minutes on 50 % with standard gases of 25/25 21/35 and 18/45) would prevent ratios from deviating drastically apart within these parameters.

I'd like to expand on the separation you've probably been acutely aware of making in your phrasing above - and rightfully so - that is, the Tech 1 level ratio (1:1) is an intermediary ratio, in principle separate from the overarching concept of Ratio Deco (the correct phrasing would be "Cascade Deco").
The same holds true for following (deeper) ratios - 1:2 and 1:3.
They align well at their setpoint, but with deviation in depth, gas or time, they gradually become less accurately aligned with "Cascade Deco".

The significance is relatively straightforward;
While an intermediary ratio might seem "unstable" to the unaware spectator (points for observing it!) because diminishing proximity to the setpoint parametres relatively quickly reduces what we might here choose to call "accuracy", it's not meant to be universally aligned with Cascade Deco - if it were, there'd be little need for it - but rather to practically approach training and diving in a gradual progression for the diver/student. That may also be, creating "bite-size chunks", taking into consideration how many tanks a student is trained in using, whether they have been trained for hypoxic mix diving yet, etc.

As a diver and student using the intermediary ratios, you will intuitively (I see you beat me to it on that one) understand how the accuracy diminishes with distance to the setpoint parametres - and that's really not a problem because when you deviate far enough that it matters, there will be another setpoint, or you'll have developed deeper and into Cascade Deco.

That's the core question that aware spectators ought ask - what are the logics driving Cascade Deco - not focusing on an intermediary setpoint that mimics it within a narrow set of parametres.

Paradoxally, it has some common ground with underlaying logics employed by USN (obviously not everything!)
I'm not going to elaborate on the details of it further here, because a) I feel like linking to where one might buy "the book" instead so any zealous Scubaboarders who want to be part of that conversation actually have to make an investment in time and effort to pursue knowledge about the topic at hand first, and b) I'm not certified to teach "Cascade Deco" (that'll be "Full Tech"-instructor level).

The only exception I can think of is when Min-Deco schedule for 32% is also used for 25/25. Am I correct in my skepticism that this 7% difference in oxygen should mean a totally different min-deco schedule for 25/25?

You may well be - as Patoux aptly puts, I couldn't justly give you a more ultimate answer, but I will say this:

First, no way I'm doing an NDL-dive on 25/25 when Nitrox 32 is a reasonable alternative - 25/25 is set in limit between 27m and 39m, and Nitrox32 up to 30m. For a 27m or 30m dive, I'm not going for helium.
Nor do I particularly want a completely separate table for me to remember for 25/25.
I want one for air (NDL, <30m), and one for "gas". Simple.

But, to answer your question, my opinion is that you may well be absolutely correct in your concern. But I think in practical terms, having discussed the above, as well as having had a discussion on the significance of proximity to set points, it will be intuitive to the reader that those depths/NDL-times are both at the very fringe of the 25/25-range and unlikely of practical significance because very few divers will actually go and spend beer-and-helium-tokens on a 25% He fill when they can choose not to without any negative consequence.

Hence, my personal opinion is that making a separate table for that contingency, may be more academically accurate, but also more impractical.

I hope the above answers the question you posted satisfactory.

 

[Kevrumbo]

. . .are you saying that something published in 2012, based on research done in the 90's is more applicable than research published based off of experiments within the last couple of years?

No! . . .(the post was written for rhetorical irony). Read the quote again @tbone1004 :

Bruce Wienke of the RGBM Model had a seemingly compelling argument in this monograph on deepstops:
http://tecvault.t101.ro/Deep_Stops-BW.pdf

The first four paragraphs in the above linked article are particularly damning and now "cringeworthy" (the claim that deepstops can result in the overall shortening of decompression times -no wonder we haven't heard from BRW in a while. . .) all in the context of the unexpected results of the NEDU Deepstops Study.

You savvy?

If the min-deco was done based off of 25/25, then using it on EAN32 would be analogous to diving nitrox but leaving your computer set to air, but it doesn't work the other way around and the min-deco was done based off of EAN32

The UTD Min Deco Air and applied UTD Eanx32 & 25/25 Triox Table (roughly 20% EAD from setpoint 30msw) is loosely based on Merrill Spencer's (1976) groundbreaking revised NDL Tables utilizing Doppler Ultrasound for VGE detection, and further refined for Repetitive Diving by Karl Huggins (1981) and the development of his HUGI Tables.

These are more conservative tables, but a valid point of contention is UTD's repetitive dive protocol, and use of these tables as reference for 25/25 Triox. (@CAPTAIN SINBAD already has some familiarity with the UTD Nitrox 32 Min Deco Table and repetitive dives using it).

 

[LiteHedded]

word salad.

 

[Compressor]

For those who are not technical divers, can someone list the different methods for calculating decompression obligations:
US Navy?
NOAA
Ratio...
RGBM...


Thanks

 

[JohnnyC]

word salad.

More like a dumpster fire.

 

[NAM001]

I do not understand this average. I would imagine that any relevant average depth would be a weighted average with relation to time, something like:

D = (D1*∆t1 + D2*∆t2 +...+Dn*∆tn)/T

where D is the average depth, Dn is the nth measured depth, ∆tn is the time spent at depth Dn and T is the sum of all time intervals.

However, it appears you are just taking the average up to a given time, summing it up with the depth of the next 5min interval and dividing it by two. In the example above, if we take just the first three lines, we have 10min spent at an average of 75ft (I supposed the depth there is measured in feet) and then 5 min at 85 ft. All this is averaged to 80ft.

By this process, if I did a dive of 25min at 20ft and then 5 min at 90ft, my average would be 55ft. The same average would be found if I did a dive of 25min at 90ft and then 5min at 20ft.

Is the example correct?


edit: I see @huwporter raised more or less the same point on a previous post. Sorry for the repetition.

Im not an expert here but I understand the minimal basics and i dont agree with your simple avg of actual depths. First speaking for my self IMO the process is one that can be done will minimal input clock and depth gage. It appears to be the same type of argument as it applies to validity of standard gasses. IT JUST WORKS and there is math to justify it. What ever error there would be as i see it is in the side of conservative. I think that what yo say is correct. I am unable to follow the math but i think i understand what you are referring to and why, so I will just leave that there and assume it as correct. It is my elief that gasses absorbed at 70' for 5 min added to teh gasses absorbed at 90' for 5 min will average to 80' for 10 min. I believe it will be greater. non linear rates. like taxes you make twice the amount and pay 3 times the taxes. so avgint 2 years income and and taxes divide them they do not fall in place. the resultant avg in higher than the mid point of the incomes. If that were not true than a ndl of 80 min at 50' should be 1/2 that amount at 100'. When it is 1/4 of it. So I guess what i am saying is that 5 min at 100ft is more physiological time than 5 min at 40' so they are not just simple adds and divides. In reality are the fine differences critical in rec diving. I would say probably not as we have rules regarding safety stops and when they can and should not be skipped.

 

[tursiops]

Im not an expert here but I understand the minimal basics and i dont agree with your simple avg of actual depths. First speaking for my self IMO the process is one that can be done will minimal input clock and depth gage. It appears to be the same type of argument as it applies to validity of standard gasses. IT JUST WORKS and there is math to justify it. What ever error there would be as i see it is in the side of conservative. I think that what yo say is correct. I am unable to follow the math but i think i understand what you are referring to and why, so I will just leave that there and assume it as correct. It is my elief that gasses absorbed at 70' for 5 min added to teh gasses absorbed at 90' for 5 min will average to 80' for 10 min. I believe it will be greater. non linear rates. like taxes you make twice the amount and pay 3 times the taxes. so avgint 2 years income and and taxes divide them they do not fall in place. the resultant avg in higher than the mid point of the incomes. If that were not true than a ndl of 80 min at 50' should be 1/2 that amount at 100'. When it is 1/4 of it. So I guess what i am saying is that 5 min at 100ft is more physiological time than 5 min at 40' so they are not just simple adds and divides. In reality are the fine differences critical in rec diving. I would say probably not as we have rules regarding safety stops and when they can and should not be skipped.

 

[Nirvana]

Im not an expert here but I understand the minimal basics and i dont agree with your simple avg of actual depths. First speaking for my self IMO the process is one that can be done will minimal input clock and depth gage. It appears to be the same type of argument as it applies to validity of standard gasses. IT JUST WORKS and there is math to justify it. What ever error there would be as i see it is in the side of conservative. I think that what yo say is correct. I am unable to follow the math but i think i understand what you are referring to and why, so I will just leave that there and assume it as correct. It is my elief that gasses absorbed at 70' for 5 min added to teh gasses absorbed at 90' for 5 min will average to 80' for 10 min. I believe it will be greater. non linear rates. like taxes you make twice the amount and pay 3 times the taxes. so avgint 2 years income and and taxes divide them they do not fall in place. the resultant avg in higher than the mid point of the incomes. If that were not true than a ndl of 80 min at 50' should be 1/2 that amount at 100'. When it is 1/4 of it. So I guess what i am saying is that 5 min at 100ft is more physiological time than 5 min at 40' so they are not just simple adds and divides. In reality are the fine differences critical in rec diving. I would say probably not as we have rules regarding safety stops and when they can and should not be skipped.

What I was trying to do in the post you quoted was to highlight some of the consequences of averaging depths by the method described by the OP, showing why I thought some of its results were odd.

I started by stating that I believe that a depth average should, on principle, take into account the time spent on each depth. That is, the more time spent on a certain depth, the closer the average should be to this particular depth. By following the method presented, however, there are some cases where the average can stray significantly from a depth that was maintained for the vast majority of the dive.

I was looking forward to hearing why the people who use that method think that the phenomenon described is not significant, or what they do to compensate it in calculating average depth.