My Journey into UTD Ratio Deco

[NAM001]

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.

Again Im no expert but it appears that it is a self correctiing process that operate on some givens. In this case even time intervals.

 

[CAPTAIN SINBAD]

@Nirvana They do have the time weight concept that you mentioned in one of your earlier posts. You will take the shallowest 5 min segment and the deepest 5 mins segment, average them both and then time weight for being deeper or shallower. The audio text on top says that even if you make an error in the "time weight" calculation then it will be a minor violation that will / should not put you at risk. Not sure how I feel about that yet.

 

[Dan_P]

The following post touches on something that I think is extremely important to keep in mind in a disussion on depth averaging, FO2's, etc.:
that is context.
It's described below as "operates on some givens", which is also a good way to put it, but I'll expand on it a bit and use the phrasing "context".

Again Im no expert but it appears that it is a self correctiing process that operate on some givens. In this case even time intervals.

Time intervals is exactly right.

If I do land on an academically unacceptable deviation of, let's say, +/- 0,5m on my depth averaging, that might still be perfectly acceptable in practical terms.
Because of time intervals.

In most cases, the deviation won't put me in one "bracket" over another - but in the ones that do, it'll only be just so.
If you as an operator land on an average of, say, 23,8m and you have a set point of 24m and 21m, that's 24m.
That's the practical context of it, and I think it's worth keeping in mind, because otherwise it's just super easy to loose ourselves in details that haven't necessarily a significant practical impact.

Let's also frame this matter in the context of diver growth.
Initially, the depth averaging exercise may seem overwhelming, but it's something that the diver develops with.
At Open Water-level, it's overwhelming but of less consequence. By the time a diver has developed into tech-diving, it's second nature.

I personally challenge students at open water level to play with depth averaging in their diving, and confirm on their gauge.
It's a good place to start.
I use the same methodology for confirming gas volume.
1) Confirm which number - ballpark - I should have in gas volume at this point.
2) Confirm what my SPG actually reads.

I now have a two-legged confirmation of my gas volume, as well as a control of my instrument function.

The contextualization process is equally important in relation to the question posted about FO2's.
It's all too easy to look at a Nx32-table, a 25/25-table, see a setpoint (30m) where the NDL-time is marked similarly across the two and deduct - wrongfully - that the [I've phrased this following part inaccurately but tangible] FO2 of a gas doesn't impact the NDL.

That has to be seen in a context.

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.

 

[NAM001]

Out of curiosity,,,, when the do the depth averaging. do yo have to continue to go deeper. 70-75-80-90 or can you do 70-75-90-77. I would seam that the averaging the accumulated with the next depends on increasing depths for each period. Doing that only furthers the ongoing gas intake on all compartments,,,,, where going deep and than going shallow stops or reduces ongassing in some compartments and continued ongassing in other compartments.

 

[Dan_P]

Out of curiosity,,,, when the do the depth averaging. do yo have to continue to go deeper. 70-75-80-90 or can you do 70-75-90-77. I would seam that the averaging the accumulated with the next depends on increasing depths for each period. Doing that only furthers the ongoing gas intake on all compartments,,,,, where going deep and than going shallow stops or reduces ongassing in some compartments and continued ongassing in other compartments.

Personally, I think that's in essence an accuracy-versus-practicality question.
I'd need to go through a number of iterations to illustrate the degree of theoretical inaccuracy in the conflict highlighted above, to determine approximately how significant a discrepancy this would create in practice.

But in terms of where this is really relevant - we'd stereotypically be talking caves, right?
And usually when we don't know where we're going (or we wouldn't need to at least partly rely on [an approach] to determine average depth in-water).
So pretty much, we're talking non-entry level cave, probably exploratory.

At that point, I suppose a diver has gone through the iterations needed to form a more concise answer and choice.
That's an important aspect of it, I think.

I'm not the right diver to answer the question, and in either case, certainly not in any official capacity.

But I think it's a good question.
Personally, if this became a significant thing for me on a dive, I'd probably pad the shallows - if feasible. If that's unfeasible, I'd disregard it as whatever's keeping me from spending extra time in the water is probably a bigger problem than this one.

 

[tursiops]

Pick the "average" you want. There always seems to be an argument for why it is "good enough" even if it is not the same as a real average. Either it is close enough, or practically the same, or safer, or whatever. Jeez.

 

[tbone1004]

@Dan_P isn't the point of using ratio deco something that is supposed to be scalable across all diving? if it breaks down in a cave where you don't know the profile, that starts to look a lot like it breaking down with the use of a CCR where you can't donate from the mouth... Seems if it isn't scalable to the extremes of diving, why talk about it like it is?

 

[Dan_P]

@tbone1004 I don't agree at all with your view on the CCR, or RD, but are you claiming that the depth averaging methodology approach in question "falls apart" under certain circumstances?

If that is a claim you'll stand by as the premise of the question you posted, then you should, for the sake of fairness, also be prompted for substantiation to that claim.

I.e. instead of me going through the aforementioned iterations, I'll rather ask you to show me the parametres whereby the methodology above renders a different result in average depth, or theoretical loading, compared to another methodology - and whether it's of practical significance at all.

I've said what I make of it - it's a strategy; if I'm diving where there's reason for further shallow padding and there's no reason not to, I'll pad the shallows.
I'd be comfortably developed to do those dives at that point, and I can use the same framework as a foundation for it.

Let me ask you this:
By the logic you're employing, shouldn’t we dismiss RD - as well as all tables, algorithms and computers - because they don't have a specific protocol for what to do if you have a suit flood and are getting hypothermic?

 

[tursiops]

whether it's of practical significance at all.

@tbone1004 Don't take the bait. Whatever you say, he'll claim it is of no practical significance.

it's a strategy

And then he'll say it is not a model, it is a strategy, so of course you adjust it as needed to accomplish your goals.....no matter that the adjustment is not a strategy but rather a pure fudge.

 

[RayfromTX]

It would seem to me that this method is far less accurate some profiles than on others so in addition to the running calculation, you also need to make a determination about adjustments you may need to make based on your profile and it’s reflection of your dive plan.

I love doing just those kinds of calculations as a mental diversion. On my bike I do constant background calculations about average speed, power, distances and times. Of course, these calculations are usually inconsequential to my safety.

I might indulge in learning and using rd as an interesting exercise as I do with monitoring my gas consumption and pressure remaining but I always check my estimates against an actual gauge. Similarly I would monitor my Perdix while diving and would always rely on it. RD would be game of the mind that would give me a way of thinking while diving. When SHTF that will be jettisoned while my available brain cells are focused on problem solving.

My level of experience is limited but I am at the level to have to decide things like this. This was an easy call for me.