Effect of slow compartments size in relation to NDL and DECO

[sigxbill]

An important point to consider is that the computers are set to set to 50 meters (164') exactly, but for RD2.0 I had to round up to next depth (170') on the table - which put RD at a disadvantage from the start. A profile like this would allow for RD averaging to the 48 meter table, which I am including in an expanded data set for comparison below.

Notice that my 48m RD2.0 profile (which is the one I would actually dive for this dive), only slows down just a bit for the gas switch stops, but can closely mimic Buhlmann profiles.

@victorzamora - it is the low gf that determines deep stops - not the high gf. Doesn't it concern you that 30/70 and 30/80 develop such different deep stops? It does me. It just shows me even more that it's all just theory. Even this latest study doesn't prove anything conclusively. You still have to choose your flavor of risk.

I think you may be missing the point. There are other advantages to RD besides believing in O2 windows or less over pressurization of fast tissues. RD keeps the team together - even during deco. I can't count how many times we've planned deco dives based on tables, and then when we got to the stops, the team split apart to follow their individual computers! Here is a great explanation by Dan:

Hello Tassi,

Thanks for the question. My personal view on it may not benefit the next diver, and it stands on my own accord entirely, so please take this just as one personal view.

Ratio Deco builds on standard gasses.
If one doesn't know what specific depth one is going to, I can use the that framework to prepare for a sequence of dives in the exploration domain, which would in some cases make an "optimal" gas and deco approach difficult (how do you mix the "perfect" gas for a dive that you don't know the depth of yet?), but rather build the dives on gas logistics, Ratio Deco and maximal operative depth, in a manner proactive rather than reactive in-water.
Further, it works well in a mixed team, across rebreathers/open cirquit systems as well as any number of personnel.

Personally, if I travel to a different country or continent, and meet a person I want to go diving with, and we don't speak any of the same languages, which happens, that's samlessly facilitated as well, in terms of decompression. There's no way to quantify such utility with doppler technology.

To be sure, Ratio Deco doesn't numerically factor in, say, fatigue, hydration, exertion, temperature, nor do the alternatives, to my knowledge. But it does certainly build a familiarity and readiness towards adaptation, which may or may not be helpful in some circumstances.

In the classroom, I see the educational benefits in progressive addition of further "layers" when using the Ratio Deco-methodology as quite significant. As an open water diver, one learns the base upon which technical dives build it's approach, and so, as a diver, I attain recognition across levels of education and familiarity in application to diving.
Viewed separately as a training tool, I find it beneficial to add the deco sphere of Ratio Deco to the awareness of a diver, that they naturally learn to manage not just the dive as it may unfold, but at the same time consider and evaluate deco versus gas logistics and make choices based upon those factors.

These benefits of course wouldn't be quantified in a comparative study, which is why I think they're benefits that transcend how two deco plans for one dive work, compared to oneanother.

There's of course the cost factor, which naturally shouldn't dictate our choices, but in fairness, my 330 has cost me less than $1 per month in use towards computer and software, training cost aside (I omit that cost as I assume it would be a factor near enough constant across solutions/agencies, and even if it isn't, Ratio Deco surely wouldn't be the only factor differing across them).
Further, I don't depend on external hardware or software, and I don't risk taking on a habit of "outsourcing" my thinking.

These factors may not apply similarly or be weighed equally by all, and it's fair enough to point that out.
I am merely explaining examples of what I mean by benefits that transcend comparison of an "optimal" deco or gas for one dive versus a "standard" deco or gas for that same dive, specifically.

I hope my expansion brings clarity to my statement quoted, if not, I will be happy to take any questions you might have.


Best Regards,

Dan

Bottom line comes down to two things: 1. how much overpressure gradient are you willing accept for fast tissues, and 2. do you believe in the oxygen window. If you don't believe in the O2 window, and are willing to approach m-values (also theoretical) for fast tissues, then don't slow down until you get shallow.

believe what you want to believe.

enjoy,

 

[victorzamora]

@victorzamora - it is the low gf that determines deep stops - not the high gf. Doesn't it concern you that 30/70 and 30/80 develop such different deep stops? It does me. It just me shows even more that it's all just theory. Even this latest study doesn't prove anything conclusively. You still have to choose your flavor of risk.

This was a typo corrected in my previous post. 50/70 is what I'm diving with now, personally....and it's what I did my math on. That explains the dramatically shallower first-stop depths on that profile compared to 30/80.

 

[victorzamora]

As for an ascend schedule on a 25 minute dive to 50m using 18/45 - as were employed in the trials - I'd land on something on the lines of this:

So I've included this as "DanP RD" on the latest graph.

On a specific note, if it were a 24 minute dive and not a 25 minute dive, the first three stops "by the book" would be 1, 1 and 2 minutes

I'm including this as DanPRD_24min. However, I don't think that 3min LESS deco helps your point. I think that MORE deco, and barely being on that edge, skews the data in FAVOR of UTD's RD.

 

[RainPilot]

GF Lo won't inherently imply deep stops, its the relationship to GF Hi that does that. As a silly example, a GF of 30/31 would have you doing your first stop nearly at the surface. It would also be damn close to being an NDL dive but thats why I said it's silly.

The ratio of GF Hi to GF Lo is what will eventually determine the ascent slope.

 

[tbone1004]

GF Lo won't inherently imply deep stops, its the relationship to GF Hi that does that. As a silly example, a GF of 30/31 would have you doing your first stop nearly at the surface. It would also be damn close to being an NDL dive but thats why I said it's silly.

The ratio of GF Hi to GF Lo is what will eventually determine the ascent slope.

not true. 100 minute dive, 100ft, EAN32, 100% O2 deco gas, done in Deco Planner 3.1.4

GF-low determines your first "ceiling" which is the depth of the "first" stop
The smaller the number, the deeper the stop, it has nothing to do with the GF-high. I can change GF high from 5 to 150, and the ceiling is 40ft. If I change that to a GF-low of 60, then my ceiling is 30ft, again with a GF-high of 5-150 it doesn't change. If I change to a GF-low of 90, then the ceiling is 20ft and is valid from 5-150.

GF-high determines theoretical tissue loading at the surface and the line between them is the line you can't cross.

There is nothing that says you can't have a GF-high that is lower than a GF-low at least theoretically. The Shearwaters will spit back an "invalid gf" error if you try to do that though.

Image below taken from a Dive Rite article that I think Baker originally made. Anyway, what you can see is that the slope of the line is drawn between the two GF-numbers. Basically what happens is when you choose your GF's, you draw that line, and you plot points on that line for your stop depths. I think this graph is a bit backwards because you read it right to left, but doesn't matter.
So this dive is at 80ft, and you hit an inert gas pressure point, in this case it's 80ft. These numbers are a bit bullsh!t so are the times, so I'm sure this doesn't actually represent a real dive profile.
So you stay at 80ft, for x amount of time until your inert gas pressure as a function of ambient pressure drops to a value where you can ascent to 70ft so you slide up and in the graph slide to the left to 70ft. Stay there for some amount of time until you can slide over to 60ft. Continues on until you hit the surface.

Now, when you move the GF-Low to a higher number, the graph gets "steeper" to the point that you can "slide" to a shallower depth faster. When you move the GF-High to a lower number, the graph is still getting steeper, but in this case it means you have to stay at a certain depth for a longer amount of time.

so how does this relate to real world. Going to steal @victorzamora 's graph because I can.
You see the 50/80 curve where because it's a "high" GF-Low which means the GF line is steep, you are able to ascent from 165 up to 60ft quite quickly *note that because he has a 50% gas in Deco Planner, it makes him stop for 1 min at 70ft for the gas switch. If you plot that graph without EAN50, it will have him do first stop at 60ft*
Where you see the yellow 50/70 curve start to get extended is because the lower "GF-High" makes that M-value line steeper so you have to stay at the ceiling depth for longer before you can go up.
The 30/80 line has essentially the same total decompression obligation in terms of time because the GF-Because the GF-Low is much lower, you hit that dotted GF-line earlier on your ascent so you have to wait.

The Ratio Deco lines IIRC have a plot with a GF-low of about 10 IIRC, and because of that the slow tissues are continuing to on-gas which is why they have a longer total decompression time in order to not get bent. This does not mean that their GF-High is lower than say 30/80, but what it means is that they are still taking on inert gas at those deeps stops because those tissues haven't saturated yet and those become the limiting tissues as they get to the surface which explains the extended 30ft and 20ft stops

 

[victorzamora]

GF Lo won't inherently imply deep stops, its the relationship to GF Hi that does that. As a silly example, a GF of 30/31 would have you doing your first stop nearly at the surface. It would also be damn close to being an NDL dive but thats why I said it's silly.

The ratio of GF Hi to GF Lo is what will eventually determine the ascent slope.

RainPilot, that's not accurate. GFLo determines the safety margin on your first stop while GFHi determines the safety margin when you surface. There's a slope in between the two for the intermediate stops.

Saying that GF30/31 would have your first stop at the surface and would nearly be an NDL dive is also inaccurate. First of all, there's not enough information to know. It's DEFINITELY an NDL dive on oxygen at 10ft for 10min, it's DEFINITELY not an NDL dive to 600ft for 10min bottom time. On the dive we're talking about (165ft for 25min on 18/45 with EAN50 and O2), it gives you 61min of deco with a first stop depth at 80ft. 30/99 has the same first stop (1min@80ft) but has only 24min of deco.

You can also do "backwards" GFs like 99/30 with a more aggressive first stop (little safety margin) and a very low oversaturation on your last stop (when you get to the surface).

 

[dmaziuk]

Would now be a good time for

RainPilot, that's not accurate. GFLo determines the safety margin on your first stop while GFHi determines the safety margin when you surface. There's a slope in between the two for the intermediate stops.

provided your computer's implementation checks both (granted, on a deco dive it will... at least while you're in deco).

 

[stuartv]

On a specific note, if it were a 24 minute dive and not a 25 minute dive, the first three stops "by the book" would be 1, 1 and 2 minutes - the extra three minutes there (on the deep stops) would be the same for a dive 14 minutes longer. This is what I mean when I say there's a probable skewering in the results based on the design of the trials (depth and time). The emphasis on deep stops would be significantly smaller if the dive was longer or shorter;

This is the nature of "standard" versus "optimal", both when it comes to deco and when it comes to gas.
For a 30m dive, one can choose a standard gas that is only just within the density parametres one has decided on, but on a 31m dive, choose a lighter one that is - some might say unduly - well within density parametres. The exact same conceptual principle is valid in a deco context, relating to emphasis on deep stops.

If the dive were 1 minute shorter or 14 minutes longer, the RD blueprint would emphasise deep stops significantly less.

And it makes sense to use an algorithm* that works that way because...? It is attempting to model what happens inside the human body, and the human body makes the abrupt jumps in its internal behavior when you dive 1 minute less or 14 minutes longer?

It seems to me that it's more likely that the body works in a progressive fashion. Stay down just a little less and you need just a little less deco, but nearly the same curve. Stay down a lot longer and you need a lot longer deco, with again, a very similar curve. Using a planning process and algorithm that produces a similar behavior seems like a good idea, to me. Go a little deeper? Use a slightly leaner gas. Stay a little longer? Do a little more deco. A planning process and algorithm the produces a significantly different plan for a dive that is almost insignificantly different doesn't make sense, TO ME.

* Don't like the term algorithm? Fine, call it whatever you want. I will still call it an algorithm. You take data in, process it, and produce an output. It doesn't matter whether some of the decision points are subjective and some of the data used by the algorithm is subjective - i.e. I feel colder today than usual, or I don't think I'm as well hydrated today as usual, or I'm working harder today than usual - it's still data and an algorithm that processes it and produces a result. If I can process the same data as you, using the RD algorithm, and produce a result, and you can review it and possibly conclude that I applied the algorithm incorrectly and produced an invalid or erroneous result, then it's an algorithm.

A profile like this would allow for RD averaging to the 48 meter table, which I am including in an expanded data set for comparison below.

I'm no expert, but everything I have learned about deco theory thus has led me to believe that on- and off-gassing does not occur in a linear fashion. Not even close. So, I'm not sure how the "average" (being a linear calculation) of depth has any relevance whatsoever to producing a deco plan.

30 minutes at 60m followed by 30 minutes at 40m gives the same average depth as 30 minutes at 40m followed by 30 minutes at 60m. I would not even consider using a deco algorithm that produces the same ascent schedule for both dives. Or even one that relied on some subjective decisions to adjust the schedule based on the difference between the two dives. But, that's just me. I am a fledgling tech diver, so maybe that's just my inexperience talking.

RD keeps the team together - even during deco.

There is only one thing - in my inexperienced opinion - that keeps a team together - the actions of the individual members of the team. I don't know of an algorithm that says "you MUST go up." They say "you MAY go up now." If a team becomes separated, I would attribute that to either extenuating circumstances (very likely due to poor planning or inept execution) or at least one poor decision on the part of at least one team member. I would definitely not attribute that to a deco algorithm. My Shearwater has never ever told "leave your teammate, now!"

believe what you want to believe.

I believe that the study being discussed showed that RD 1.0 gave a longer amount of time in the water, as compared to GF30/85, and that the diver subjects still showed a higher incidence of a physical indicator that may be a sign of decompression stress. I also believe that if you did the same study again and used RD 2.0 and GF 50/80, you would get similar results.

 

[scubadada]

For mostly rec divers, who do occasional light deco, it is important to remember how most dive computers work (at least Perdix, Nitek Q). The GF hi completely dictates your surfacing for any non-stop dive. The GF lo only kicks in for deco, and dictates your lowest stop.

 

[Dan_P]

@stuartv
On accuracy versus practicality - I believe it's obvious that "mimicking" physiology with minute accuracy is not the aim of RD; it is to get you out safe, while also achieving other things.
Besides, please do show me the algorithm that perfectly mimics human physiology.

On antipathy towards the principle of compartmentalization - if, say, you stay at a given depth for 9 minutes, it's a NDL dive, but for 10 minutes, it's a deco dive.
By the same logic employed against the compartmentalized scheduling in RD, the above construct is invalid.
Obviously, it's not a good idea to shoot up after 9 minutes, and conversely you'd most likely survive doing so after 10 if for some reason you feel that you need to.
You know this. You can stop on an 8 minute dive if you're freezing and dehydrated, even though the table says 9 is the limit.

When diving, we have different tools and approaches at our disposal. Some are safe, others are unsafe. Among the safe ones, there are some that employ computers, and some that employ bottom timers and standardized gases.

There have been quite a few engineers saying "hey, I can quite easily put the RD algorithm into a computer", but that's missing the point.

The point is, you're trying to achieve a system of gas and decompression that you can easily use (for all your purposes) - consequently, you don't need to put it in a computer.

There is only one thing - in my inexperienced opinion - that keeps a team together - the actions of the individual members of the team. I don't know of an algorithm that says "you MUST go up." They say "you MAY go up now." If a team becomes separated, I would attribute that to either extenuating circumstances (very likely due to poor planning or inept execution) or at least one poor decision on the part of at least one team member. I would definitely not attribute that to a deco algorithm. My Shearwater has never ever told "leave your teammate, now!"

The full perspective includes deep hypoxic rebreather bailout scenarios. You don't want to abandon your gear and procedures and rote learning every time you progress a step in your diving, do you?

So, during that bailout your computer doesn't say "you may go up" - it's saying "go up - now".
Then you don't because your buddy's won't let him ascend yet, as he's now breathing something different from what you are.
Now what?

Try that same scenario while employing standardized gases and a standardized deco framework that maintains an approximate 1,2 ppO2.

Ratio Deco is a framework that you use to build your ascends with, ranging from Open Water Diver (min deco) to hypoxic rebreather dives. It encompasses gas logistics and gas choice, it relates to diver progression and capacity, it works across all systems including rebreather in mixed teams and bailout scenarios - seamlessly - it lets you adjust on the fly, pre-plan dives in literally seconds, save you some bucks on computers, increase your autonomy and most importantly - MAKE you think!

Again, it's one of many safe approaches, and making up one's mind about it on a comprehensive basis really does require understanding of all the links in the chain.