Why do some agencies recommend using a bottom timer instead of a computer

[Dan_P]

Yes, when you've cherry-picked the statements you've presented here, from the context of this;

I believe it would be challenging to justify characterizing ratio deco as "dangerous" because we have not accurately measured the absolute risk associated with its use and compared that risk with accurately measured alternatives. To deem one approach to deco dangerous in comparison to another you would have to do that and show a difference considered significant. I suppose you can interpret that as agreeing with Dan P's very technical point.

...then I think you're misrepresenting.
It'll be up to Dr. Mitchell to say, thought.

And no, I'm not going for the messenger over the message - in fairness - in your barrage of critizism, I do take note that you're apparently comparing apples to pears without recognizing it.

The difference here is, I started out tech diving with a computer, a slate, a backup computer, wetnotes and a laptop with software, then heard about something else (RD), couldn't believe it was possible to handle it all so much easier, and - get this, it's the important part - decided to find out how it works, rather than conclude on the face of it that it couldn't be done or whichever other premature conclusion I could have perceived.

And thanks for your kind offer, but as much as I'm tempted to teach someone obviously uninterested in learning, for free, I'll go ahead and pass on the opportunity on this one 

 

[Dr Simon Mitchell]

Hello,

Just for completeness, and noting pfcAJ's comments earlier, throughout this discussion I have been referring to UTD RD.

I feel uncomfortable that this debate now seems focused on the nuances of things that I have said. I would like to provide some perspective on this.

As a scientist and physician contributing to a public forum I try to take positions on issues (particularly controversial issues) that I feel I can rationally defend on the basis of the available evidence. You may have noticed I don't get involved in many debates that swing on opinion alone. In keeping with this philosophy, the more incomplete the evidence on a particular issue, the less definitive my statements on the matter become.

This topic, and the answer to Dan's "can you call it dangerous?" question are a good examples. "Dangerous" in this context is poorly defined, but my mental model of the meaning is something like "significantly riskier than a plausible alternative". So, applying my philosophy of being able to defend my answer on the basis of evidence, I have taken the position that we simply don't have enough data to characterise UTD RD as "dangerous". Dan, you should note that this is not the same as categorically saying it is "not dangerous". It could be (dangerous), actually, but we don't know because we have not performed the study required to find out. Changing the question subtly can substantially change the answer. If, for example you asked is UTD RD "less efficient" the answer would be "yes", and if you asked is it "riskier" than alternative approaches of the same length, the answer would be "probably". But by asking if it is more dangerous you have take the question beyond the scope of the current data and to remain scientifically honest / accurate I have to give a much more qualified answer.

There are similar issues with The Chairman's questions. I believe there is enough evidence around deep stops in general and UTD RD specifically for me to make a rational judgement that there are other more efficient decompression approaches, which means that these other approaches are almost certainly safer (to some poorly defined degree) for the same amount of decompression time. It follows that I feel comfortable to state that I would not use UTD-RD, and would not recommend it to my family or friends. That, to me, is a rational response to the available evidence. However, in a similar vein to the "is it dangerous" issue, I don't believe that the relevant evidence is strong enough to characterise people who make a personal choice to use it (for whatever reason) as idiots, reckless, or irresponsible. I might struggle to understand their decision (and harbour suspicions about dogma and personality cults), but I would not characterise them as idiots etc. I have applied this perspective previously during all the debates with Ross. I have often said that I would not consider it stupid if someone using VPM-B successfully were to keep using it. That is despite my belief that the current evidence suggests that bubble models are likely to be less efficient than other pragmatic approaches.

I hope all this makes sense.

Simon M

 

[boulderjohn]

Does use RD result in a higher rate of DCS?

Source pls.

I believe Pete has said in the past that sources within DAN have said there is a particularly high incident rate for DCS using RD.

I don't have such sources. I have only my own experience. I was talking only a couple of weeks ago with one of my diving buddies from my UTD days. We tried to get a count of how many DCS cases we had in our little group, but we could not remember them all clearly, and some happened after I left. There were at least 10 and probably more. All of them were on Ratio Deco.

That was about 8 years ago, and since then decompression diving is mostly what I do. I teach it. In those years I have not only not encountered a single case of DCS, I have not had a conversation with anyone who has ever mentioned having had it.

In summary, in al my years of decompression diving, I have had many personal relationships with people who have gotten bent. In every case, that person was using Ratio Deco. I have had no personal relationships with people getting bent using ANY other system.

 

[PfcAJ]

I believe Pete has said in the past that sources within DAN have said there is a particularly high incident rate for DCS using RD.

I don't have such sources. I have only my own experience. I was talking only a couple of weeks ago with one of my diving buddies from my UTD days. We tried to get a count of how many DCS cases we had in our little group, but we could not remember them all clearly, and some happened after I left. There were at least 10 and probably more. All of them were on Ratio Deco.

That was about 8 years ago, and since then decompression diving is mostly what I do. I teach it. In those years I have not only not encountered a single case of DCS, I have not had a conversation with anyone who has ever mentioned having had it.

In summary, in al my years of decompression diving, I have had many personal relationships with people who have gotten bent. In every case, that person was using Ratio Deco. I have had no personal relationships with people getting bent using ANY other system.

I don’t think anyone knows how many dives are done using RD (in all its various forms and permutations), so it’s tough to come up with a rate.

Were those cases you mention done at altitude?

Fwiw, I’ve seen DCS in divers using computers and tables, but not once with ratio deco (GUE style. I don’t know anyone trained in UTDs methods)

 

[LiteHedded]

I don’t think anyone knows how many dives are done using RD (in all its various forms and permutations), so it’s tough to come up with a rate.

Were those cases you mention done at altitude?

Fwiw, I’ve seen DCS in divers using computers and tables, but not once with ratio deco (GUE style. I don’t know anyone trained in UTDs methods)

Agreed. Don’t know anyone offhand who’s been bent using the gue system of ratio deco.

Tons bent using other methods but I’m not sure these profiles are relevant to the discussions here

 

[boulderjohn]

I don’t think anyone knows how many dives are done using RD (in all its various forms and permutations), so it’s tough to come up with a rate.

Were those cases you mention done at altitude?

Fwiw, I’ve seen DCS in divers using computers and tables, but not once with ratio deco (GUE style. I don’t know anyone trained in UTDs methods)

Yes, it was at altitude, but Andrew and everyone else at UTD insisted that doesn't matter.

Just to be clear, the majority of our group got DCS using RD.

 

[dmaziuk]

Doing linear extrapolation of a curve is really not hard at all

Doing a linear extrapolation of a known curve is not that hard. The argument here is that one extrapolation of a poorly-understood (to put it charitably) curve appears to be a better fit than the other.

 

[PfcAJ]

Yes, it was at altitude, but Andrew and everyone else at UTD insisted that doesn't matter.

Just to be clear, the majority of our group got DCS using RD.

Remember that post I made a ways back about proper training?

It’s critical.

 

[rjack321]

Doing a linear extrapolation of a known curve is not that hard. The argument here is that one extrapolation of a poorly-understood (to put it charitably) curve appears to be a better fit than the other.

What known curve would that be?

The divers I know, dive with, and who have some reasonable training in RD can all come up with a functional (yet unlikely to be optimal) deco schedule for a 140, 150, or 160ft deco dive (e.g. the Hydro Atlantic in S. FL) for 20-25mins with only a bottom timer. They're a mix of GUE and UTD trained people, not sure to what extent they are following a rule they were taught (or when), or worked out on their own, or they just know schedules that works for them by memory. I don't know anyone using Steve Lewis' neumonics so I don't know how those work in the field.

As a general rule, everyone else I have been diving with has no clue how to adjust their schedule on the fly for deeper/longer/shallower/shorter. I have seen course directors bail on a dive early, then hang on the up-line at 150ft+ until their run time catches up with the shortest stop schedule on their slate. Or go have to go screwing with the +5 function on their Shearwater. Or boast around the shop that their computer died and they have "no idea" how much deco they had left.

So while it may not be optimal, the RD users I know have the best intuitive understanding of depth/time/gas relationships and can quickly and instantly understand when a plan is totally bogus. Whether they can follow the plan and do the ascent they think they are doing is another matter. The extent they stop "too deep" is also not really the fault of RD, that's a consequence of unsubstantiated bubble models lingering in people's minds as the state of the knowledge.

 

[rjack321]

Yes, it was at altitude, but Andrew and everyone else at UTD insisted that doesn't matter.

Just to be clear, the majority of our group got DCS using RD.

So did you ever use decoplanner or multideco or Baltic or (some software) to look at how aggressive your profiles were compared to the underlying model? (which way back when was decoplanner 30/85 GFs plus a bunch of deep stops added)