Modified ratio deco

[rjack321]

I have enough to do calculating BO dynamically. Doing deco "manually" beyond making sure I don't exceed my BO deco gas is a no for me.

 

[boulderjohn]

what about recalculating gas on a stage cave dive should we not attempt to do that? too distracting?

There is math you need to be able to do, and there is math you don't need to do.

Back in my own "ratio deco only" stage of diving, two friends got mildly bent on a T1 deco dive. One of them was using a computer in gauge mode as a bottom timer, so he was able to produce a dive profile showing what they did on the dive. It showed they made three math errors during the dive. (EDIT: one of them had a Ph.D in a math-heavy area of study.)

If they had not had the computer profile as a check, they would have sworn that they had done the dive perfectly. That is one of the problems with the system--you usually have no way of knowing whether or not you made any errors. I am sure lots of my fellow divers in that era made all sorts of math errors--we just didn't have a consequence or a way to check.

 

[rjack321]

what about recalculating gas on a stage cave dive should we not attempt to do that? too distracting?

There's no way to automate this on a machine actually designed and suited for the purpose - so this is totally apples and oranges. Although this is a fairly advanced skill and many stage divers should probably keep their dives simple and avoid this.

 

[Germie]

Ratio deco was used by at least 1 agency less than 15 years ago. You had to do 'deco by head/on the fly'.
Oh yes, ratio deco was a great tool. But not how it was used then.
I still use ratio deco, but then if I dive with someone else that made a diveplan. If that person tells me the bottomtime and the total divetime of a wreck/wall dive profile, I know directly if he is telling nonsense or that is is within the range I want to dive.
For example, if you go to 60m, you know the diveplan with 2 decogases will be between 50 and 65 minutes with 20 minutes bottomtime.
For a 100m with a travelgas and 2 decogases you know that it will be with 15 minutes bottomtime between 90 and 110 minutes.
For this you don't need to use standardgases, it just is to check quite fast if you made a mistake in your diveplan.
So you still plan, but it is an global check.

I also check with a diveplanner what the extra decotime will be if I stay longer. For example if this is 15 minutes, I know enough. As deco is no absolute science, the 15 minutes can be done with: 8 minutes 6m, then you have 7 left. So do then 2 minutes 9m, 2 min 12m, 1min 15, 1 min 18m and 1 min 21m.
You also can do 6 minutes at 6m and the rest sharing over the other stops. And if you forgot you 18m stop and think about it at 12, do it at 12m. The total divetime will be +15 minutes. This is just a global plan, but it will work if your computer stops working and you only dive with a bottomtimer. (remember, I started in 2010 with bottomtimers and did 4 dives over 100m with only bottomtimes and a diveplan in wetnotes). Then people bought 1 computer. Now you see a lot with 2 full trimix computers.

And so is there a 3rd 'trick' if you forgot all deco information during a dive and only have a bottomtimer with average depth left: stay at 6m till the average depth is less than 22m. In most planned deep dives it will be between 20 and 26m at the end of the deco. So if you don't know anything anymore, then just stay at 6m and wait.

All these things are not safe or the way to go, but it is what works for most cases if you have problems.
Deco on the fly sounds great, but a lot of divers were not that best calculators by head under water. What was easy at surface was not under water and so accidents happened.

What I mentioned above works for the standardprofiles. In a cave it is more complicated. And that is what Patrick sees.
If you dive for a long time at 12m for example, are you ongassing or offgassing at that time? This depends on the profile. The standard dives are way more easy to plan.
I have seen on cave dives a lot of difference between different brand of trimixcomputers even if they all use Buhlmann. As there is still a programmer needed to implement Buhlmann you will see differences. And exhausted testing is impossible, you will see the failures or mistakes in the written code on complicated dives.
And the most difficult part of writing code for a divecomputer is when a diver does other things than the computer tells you to do. So if you skip a stop, a good computer must start recalculating your dive and not easy say: '505'. The first Suunto EON steels where completely useless for technical divers because of the '505' they showed when doing a stop at another depth. So you pay then 1000 bucks for an expensive bottomtimer. It is cheaper to programm only expected stuff and then show an error if something unexpected happens, but as a technical computer producer this is not the way to go.
I have had 20 minutes deco difference between Heinrich Weihkamp OSTC2 and Shearwater, both with the same GF's in a cave dive. So which one is right?

This means that doing cavediving and cave explorations make things more complicated than the normal profiles. So some ratio deco or 'tricks' to check things are not bad. But it will not be the only way to use.

 

[crofrog]

@crofrog at nitrox depths the ratios are generally fine because GF-Lo doesn't matter, the issue is at trimix depths when the GF-lo's are much lower than the current recommendation of 70-80% of the GF-hi.

Sure, but that just means different ratios could (and should) be selected to remove the deep stops. My only real issue with your statement is this part: "Ratio Deco is at best extremely risky and goes against everything we understand about decompression theory."

The first half of that statement—“extremely risky”—is certainly hyperbole. Based on the empirical data the Navy has published in the last 15 years, almost all of the technical diving we do is "extremely risky." To align with their empirically validated tables, your GF-high would need to be much lower than what most people commonly dive with today. I think it’s hard to categorize Ratio Deco vs ZHL vs [insert algorithm here]—as long as they produce a similar total decompression time, and shallow (30-20ft) stop times—as one being extremely riskier than the others.

If you disagree, I would love to hear your argument on why it is better than properly planning dives with software and then executing that dive with a computer that is running the same algorithm as the software.

I don’t disagree with that sentiment. I agree with you about using Ratio Deco primarily as a sanity-checking tool, and potentially as a backup in the event of computer failures. Although at this point, like many people, I dive with two computers. I plan the dives using DecoPlanner, calculate the Ratio Deco on the way to the first stop, and compare both to what we had planned—then select the longest schedule of the three.

Paying attention to what? Your physical environment? Your teammate(s)? Animal life? Your route? You have lots of things to pay attention to during a dive, particularly a technical dive. Constantly doing math throughout the dive only distracts you from those things. If anything important arises during the dive that seriously occupies your time and attention, what happens to your constant math computations?

There is a lot to pay attention to... Using the computer to enhance your situational awareness is certainly the correct approach. Running the Ratio Deco in your head occasionally and cross-checking it with the computer is, I think, the correct risk mitigation strategy to make sure everything is more or less lining up. Maybe that’s just the pilot in me: scan the instruments (observe), compare those observations to my mental model of how things should be progressing (orient), then decide and act on that information.

I’ve seen people get in the water on technical dives with the wrong gas selected (e.g., 21/35 on an 18/45 dive), and with additional deco gases turned on (O2 and 50%) on a dive that only used 50%. These are both errors that can be trapped by comparing the three different planning approaches as you dive.

 

[tbone1004]

Sure, but that just means different ratios could (and should) be selected to remove the deep stops. My only real issue with your statement is this part: "Ratio Deco is at best extremely risky and goes against everything we understand about decompression theory."

The first half of that statement—“extremely risky”—is certainly hyperbole. Based on the empirical data the Navy has published in the last 15 years, almost all of the technical diving we do is "extremely risky." To align with their empirically validated tables, your GF-high would need to be much lower than what most people commonly dive with today. I think it’s hard to categorize Ratio Deco vs ZHL vs [insert algorithm here]—as long as they produce a similar total decompression time, and shallow (30-20ft) stop times—as one being extremely riskier than the others.




I don’t disagree with that sentiment. I agree with you about using Ratio Deco primarily as a sanity-checking tool, and potentially as a backup in the event of computer failures. Although at this point, like many people, I dive with two computers. I plan the dives using DecoPlanner, calculate the Ratio Deco on the way to the first stop, and compare both to what we had planned—then select the longest schedule of the three.

Two things to clarify. First is Ratio Deco when capitalized is what UTD is currently teaching as that is a branded proper noun vs. the general concepts of using a ratio to determine an ascent profile. The leading deco experts are currently diving 50/70, some of the ratios end up at like 10/115 and I believe some agencies are still teaching 20/85 for the planning portion with software which does go against all current decompression research understandings at this time to have a GF-lo be so low but that is a separate discussion as the ratios could obviously be modified. So in that portion, Ratio Deco, in capital letters when referring to UTD's current teaching of it does in fact go against the current state of the art regarding decompression theory, as would using a GF of 20/85 to plan your dive using software and whatever ratios correspond to that profile.
That however does not make it "extremely risky". What does make it extremely risky is being under the delusion that the human brain can make those kind of repetitive calculations without screwing them up, and that's the real risk with using ratio deco as a primary means of creating an ascent profile.

 

[boulderjohn]

That however does not make it "extremely risky". What does make it extremely risky is being under the delusion that the human brain can make those kind of repetitive calculations without screwing them up, and that's the real risk with using ratio deco as a primary means of creating an ascent profile.

Back when I was training with UTD, I was specifically (and emphatically) told that computers can make mistakes, that is why it is better to use "the computer between the ears," which does not.

 

[tursiops]

Back when I was training with UTD, I was specifically (and emphatically) told that computers can make mistakes, that is why it is better to use "the computer between the ears," which does not.

Right. If only this were true.