Effect of slow compartments size in relation to NDL and DECO

[tbone1004]

So these dozen or more guys that you know who got bent, they are all UTD divers?

It's more like knowing that your full tank of petrol will get you 300km and figuring out approximately how far half a tank will get you. And then you check with your passengers to see if they agree.

yes they were

and if you want to use that analogy, what happens when you change speed? and don't realize it, and your passengers are asleep? all of a sudden that distance isn't relevant anymore

@RayfromTX touche

 

[elmo]

I don't want to weigh into the UTD Ratio Deco will get your cat pregnant debate here, but in an attempt to get back to the original "slow tissues" topic, I would like to point out that the example 25min bottom time on 18/45 (?) with 50% and O2 deco gasses (?) example dive "tested" in that study has either the 77min or 109min theoretical Buhlmann tissue controlling when it is safe to surface, and the fast tissues controlling at the start of decompression. That is calculated using Subsurface with either GF 30/80, 15/70 (close-ish match to @sigxbill 's RD 2.0 profile), or VPM-B +4 (best match I could get with VPM-B, but shorter deco than RD 2.0).
The ZHL16 Buhlmann tissues range in half-life from 5min to >10hr. The mid-range 77min and 109min tissues which control bounce deco dives such as the UTD example are not the slowest/fattiest tissues in question.

 

[boulderjohn]

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

Another way of saying this....

They are adding to their bottom time, so they need to compensate for this in their decompression.

On the note of having a computer because a human brain is no good - is that not why we have buddies?
To function as backup brains?

Two divers in our UTD group got bent on a fairly mild (about 150 feet deep for about a half hour) deco dive. One of them was using a computer in gauge mode as a bottom timer, so he was able to get a profile of the dive, which showed the three mistakes their combined brains had made.

1. They miscalculated their average depth. They were taught to run an average in their head by checking their depths every few minutes. The computer profile showed that as they dived, they would descend a bit, check their depth, correct, descend a bit, check their depth, correct, etc.
2. After basing their planned deco on that mistaken average depth, they ascended to their first deep stop at 3/4 of that depth to begin the deco. That ascent should have taken about 30 seconds. It took over 3 minutes, a very common problem with technical dives. (Technical divers who are told to ascend slowly often ascend too slowly, thus adding to their bottom times.)
3. They miscounted their deco stop times. They thought they had done the required (too little) deco for their dive, but they somehow lost count and shortened the last stop especially.

Both their combined brains missed those three mistakes, maybe because only one of them had a Ph.D in computer science.

So these dozen or more guys that you know who got bent, they are all UTD divers?

The 8 cases of decompression sickness that I know of personally were all UTD divers using Ratio Deco. That group includes the two mentioned above.

 

[boulderjohn]

Keeping a team together using computers like the Shearwater is easy as pie.

You start by using similar gases. (I would prefer them to be the same, but where I dive with the need to mix in the field, we come as close as we can.) You agree on a the computer settings for GFs and anything else. You stay together during the dive so that you all have the same bottom time.

When a diver is done with the first stop, he or she signals that. If the teammate(s) is (are) ready, the signal is returned, and up they go. If not, the amount of time still needed is signaled, and they stay at that depth until all members of the team are ready to ascend to the next stop. The computer of the diver who was able to ascend first adjusts to the added time at that depth.

That computer is NOT requiring them to ascend NOW.

In the case I described in post #63, if they had been using computers, all computers would have known the actual bottom depth and time, all computers would have known the actual ascent time to the first deep stop, and all computers would have given a decompression profile consistent with what had happened before.

 

[boulderjohn]

I have posted the story in post #63 before, and pretty much each time a Ratio Deco proponent argues that you can't blame Ratio Deco for their DCS--they made the mistakes, so it was not the fault of Ratio Deco. These same people argue that you can't trust a computer because the computer can make a mistake like that. If you blame a faulty processor for one, you need to accept the fault in the processor for the other. I know that in a complex dive with a lot going on, my brain is all too likely to get confused and make an error, so I don't rely on it. The people who say computers make mistakes but the "computer between the ears" never makes a mistake evidently have much sharper brains than mine.

 

[Neilwood]

One of the great things about this forum is having acknowledged experts in the field who are willing to comment on these issues.

For the average diver it is hard to keep track of what is best but having someone like Simon who knows the subject and can also talk in "normal" terms is an amazing resource. I know that if I were to try to read the study, I would probably glaze over half way.

Thanks for the post Simon.

 

[victorzamora]

It's more like knowing that your full tank of petrol will get you 300km and figuring out approximately how far half a tank will get you. And then you check with your passengers to see if they agree.

I'm sorry, but the math for Ratio Deco is a little bit more complex than an entire team of people in the same car doing simple division. You have to keep track of your average depth, bottom time, apply ratios to get a total ascent time, and then spread that stop time out, and then add the S-curve shaping on top of it all......while doing a tech dive, enjoying the dive, keeping track of your teammates, and making sure nothing goes wrong.

Oh....and to be logically consistent, you should be doing it all with analog depth gauge and timers because "you don't need a computer." Using a computer in gauge mode with an average depth being consistently calculated absolutely counts as using a computer. The second that a "RatioDeco only, computers are dumb" diver tells me they use computers for digital depth and time (and often enough digital average depth) I just giggle to myself.

 

[stuartv]

@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.

Obviously, there is none. Equally obviously, there are algorithms that come closer than others.

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.

That may be how you teach it. How I learned it is that every dive is a deco dive. The ones that OW trains you for are the ones that don't have any mandatory stops on the way up. It's still a continuous spectrum, from no mandatory stop, to one short mandatory stop, to a long stop, to 2 stops, etc.. The only compartmentalization is the way we designate stops every 10'/3m instead of just using a continuously variable ascent rate to "ride the line" all the way up.

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.

I don't NEED to put it in a computer. I also don't NEED to use an SPG at all. I could do it the way my dad did and just leave one tank turned off all the time. When breathing gets hard, turn the second tank on, let them equalize, turn it back off, and now I know I'm down to half a tank. Repeat and I'm at a quarter tank.

I don't NEED to use a BCD. I could do it the way my dad did when he was in the Navy and dive double steel 72s in a wetsuit, to 200', with no BCD at all.

I don't need to put my algorithm in a computer. I could use analog depth gauges and analog timers and plan everything on paper, using Navy tables and a plan written in my wet notes. My tech training did include that.

I do it because it's safer. Two computers are WAY less likely to make a math error than I am - especially if I'm stressed and/or narked.

Do you use an analog depth gauge and an analog timer? If you use electronic ones, do you realize than an electronic depth gauge is actually using a pressure sensor and employing a mathematical algorithm to tell you what your depth is? Do you use it to tell you your average depth, or do you calculate that in your head? If you let it tell you, you realize it is also using an algorithm to calculate that for you, right? So, how do you find that to be okay, but not okay to just let it calculate your ascent schedule also? It sounds like you have enough knowledge and experience to look at the ascent schedule generated by, say, a Perdix and assess the "sanity" of the plan, to know whether you should use it or the one shown on your backup computer (assuming they're different - if they're the same, you'd think they are correct, wouldn't you?).

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.

I'm not a rebreather diver yet. But, I've been studying up on it. And I don't understand why I will have to abandon anything when I make the switch. What computer tells you, on a bailout, "go up now" versus "you may go up"?

Can you give me an example of a scenario where I would bail from CCR and be forced to leave my buddy who is still on his CCR? I have done deco dives on OC, with a CCR buddy, and we've always been able to stay together. We planned our gases such that, to the best of my understanding, if he had to go to BO, we would still have been able to stay together all the way to the surface. I don't understand why it would have been any different if I had also been on CCR.

 

[dmaziuk]

@dmaziuk correct

good things come to those who wait -- I read your original post and decided not to reply because I haven't read the code so I don't actually know what the bleep any given dive computer is really doing anyway.

 

[Dan_P]

and if you want to use that analogy, what happens when you change speed? and don't realize it, and your passengers are asleep? all of a sudden that distance isn't relevant anymore

Okay, so the parametres of the dive are changing, diver 1 is unaware and the entire team is snoozing -
I'd guess that team has big problems (probably navigation would be the first one in a long line), and I don't think that trying to mask those problems with a gear solution is a solid way forward. Do you, on a general basis? It's clear to me that in this scenario, this team is way out of capacity to function as a unit, even in the absence of failures, mistakes, deco obligations, etc.

We may as well turn to electronic compasses that measure out degrees with two decimals and sound alarms if/when you make a turn, if we subscribe fully to such an arch of logic.

This is exactly the problem with automated solutions. Not the automated solutions themselves, but the use of them to mask inaptitude, referencing the example you illustrate. I don't have a problem with computers per se. If I wasn't able to look at this in a nuanced fashion, how would I ever get a ppO2 reading?

The 8 cases of decompression sickness that I know of personally were all UTD divers using Ratio Deco. That group includes the two mentioned above.

I'll be honest; I can't help but maintain reservations - but I'd be happy to hear more from you about the instances you tell me about - I'm based in Europe, and I don't think the water's all that different. Would you mind expanding on this via pm with me?
I can't really relate to an incontextual narrative, I'm sure you understand.

• They miscalculated their average depth. They were taught to run an average in their head by checking their depths every few minutes. The computer profile showed that as they dived, they would descend a bit, check their depth, correct, descend a bit, check their depth, correct, etc.

• After basing their planned deco on that mistaken average depth, they ascended to their first deep stop at 3/4 of that depth to begin the deco. That ascent should have taken about 30 seconds. It took over 3 minutes, a very common problem with technical dives. (Technical divers who are told to ascend slowly often ascend too slowly, thus adding to their bottom times.)

• They miscounted their deco stop times. They thought they had done the required (too little) deco for their dive, but they somehow lost count and shortened the last stop especially.

I know, circles, but - again, are you really sure these divers were UTD-trained to dive at the level they were diving? We're not talking about doing a quick workshop on RD and off we go sort of scenario here..?
I'll reiterate that pm may well be appropriate if you're willing to share details.

Keeping a team together using computers like the Shearwater is easy as pie.

You start by using similar gases.

Yes. Because, obviously, if you run the same algorithm on different gases, the computer will say different things. That's what I mean. If one diver is on the loop and another is bailing out, how do you maintain certainty that the ascend will be similar so the team can stay together throughout, regardless when in the dive the bailout is executed? And what if you're donating?

Can you give me an example of a scenario where I would bail from CCR and be forced to leave my buddy who is still on his CCR? I have done deco dives on OC, with a CCR buddy, and we've always been able to stay together. We planned our gases such that, to the best of my understanding, if he had to go to BO, we would still have been able to stay together all the way to the surface. I don't understand why it would have been any different if I had also been on CCR.

The scenario I'm relating to is the other way around - if both divers are on ccr and one swaps to o/c. They won't be breathing the same gases anymore, as the loop is mixed of a diluent and O2 during ascend on ccr.
As RD maintains an average ppO2 of 1,2 throughout the ascend, and the CCR does the same, the ascent will be seamless if a team member bails out.

Do you use an analog depth gauge and an analog timer? If you use electronic ones, do you realize than an electronic depth gauge is actually using a pressure sensor and employing a mathematical algorithm to tell you what your depth is?

If you bring a 100m stick and dive to a depht where your depth gauge says "100m", it'll fit.
If you say your computer's algorithm is perfect, you'll be sat around on the boat discussing GFs all day long, rather than diving.