"Accidental" Deco

[DuboisP]

For me it seems important to think about the gear that people use to do Deco diving and how it is different from the gear that I usually wear on a recreational dive.

Double tanks? double regulators (4+ for the dive team), deco gas?

you can do deco diving with a single tank (in France, we're using steel tanks, with more capacity than a S80) and a single regulator; of course, we're planning...
after, you can use a bi, deco tanks, more regulators.
actually, for a "easy" dive, i'm using a 15L steel tank with 2 regulators, diving deco or no deco.
more engaged, i add a 7L al ( S40) with oxygen or a nitrox > 75%, for deco.
i ordered a 2 * 12L.
at this point, it's no more rec diving.

depending on conditions, we can choose to not deco dive.

 

[boulderjohn]

I am a bit late to this conversation and would like to address an older topic in this thread, the use of Ratio Deco in cases of accidentally going into deco on a planned No deco dive.

As one of the originators of RD, Jarrod Jablonski wrote to me during a private conversation, "RD is not an algorithm; it is a mnemonic device designed for use within a CONFINED set of parameters....Blindly using an inappropriate tool outside relational parameters is very much like blindly jumping off a high bridge into water with an entirely unknown depth. This can work a lot of times but is not necessarily a sustainable activity."

RD was created as a way of reproducing a decompression schedule that would have been produced by a standard program. In order to make it work reasonably well in such a simple format, the parameters have to be very narrow. You can't input a variety of gas mixes into it, as you can with a program like V-Planner or a mixed gas computer. It must be used with standard gases. Air is not one of those standard gases. In a well known (at least among DIR people on SB) case in Truk Lagoon, someone used RD for a fairly deep dive on air and ended up quite bent. It was not RD's fault. The dive was outside of the RD parameters.

Both GUE and UTD use a different system for dealing with the kind of dive described in this thread. It is called minimum deco, and it is similar to RD in nature. The problem with it is, once again, the parameters. The diver is supposed to be diving on EANx 32. When it is explained in the UTD class materials, it says clearly "This is not an air table."

It seems to me that if one goes into deco on a dive in which that was not in the plan, then the proper solution is to use the contingency plan offered by whatever system the diver was using in the first place. If the diver was using the PADI tables, then the PADI emergency deco system would be used. If the diver is using a computer, then the diver should know how to follow the deco directions the computer is providing.

 

[Rainer]

John, I think you're taking a too literal interpretation of what Ratio Deco means. You can certainly find ratios for decompression for any gases (as I'm sure Jarrod would clearly tell you). Many of us have just such algorithms at our disposal for air (and 32%) dives that exceed NDLs by small margins (requiring 10-30 minutes of backgas deco). These are pretty common (and generally agreed upon) in the DIR community. Such algorithms (I get the sense JJ doesn't know what this word actually means; RD is most certainly an algorithm, it itself is just not a decompression theory) are valid if they match (or stay on the conservative side of) experimentally validated theories. Our GUE T1 class even encouraged us to play with software to get a sense of other ratios not formally covered in class (i.e. 1:1 set point at 150').

I'd certainly be able to deco out small stays over NDLs on backgas for the gases I tend to dive. That said, good pre-dive planning and dive execution means I haven't had to do so. If I'm going to stay over NDLs, it's usually by a good margin with other gases for accelerating decompression.

 

[TSandM]

Rainer, I think it's accurate to say that you can CREATE ratios for other mixes and for times and depths outside the parameters of the usually taught RD, but the RD that is typically taught (the numbers I memorized) are for standard gases, and a relatively narrow range of depths and times. It's curve-fitting, and as so often happens, the simpler formula approximates the more complex one very well within a certain range, but departs from it more and more outside of those limits.

Honestly, I think the reason we get away with a lot of the things we get away with, is that there is so much uncertainty in decompression calculation to begin with.

 

[Rainer]

I'm not sure how it works in the C1/2 classes, but as I mentioned, we were explicitly encouraged in GUE T1 to explore other ratios for O2 decompression (which isn't/wasn't formally covered in class). To do that, you need an understanding of how time over NDL relates to mandatory deco owed. For this, you need an NDL table (or "rule"). We used the same table/rule we use for our recreational/NDL/MDL diving, that is, something akin to the 120-rule / Navy tables (backed off for a few depths). It's an air table (from which you can easily generate a 32% table based on the 20% depth credit). There's a pretty simple algorithm that generates deco times for time over NDL across a usable curve (again, 0-30 minutes over). It's pretty much 1:1 deco time for time over (cutting half the 10-20' stop time with O2 if you have it), which requires shaping I'd rather not cover in a public forum.

I think this approach to O2 or backgas deco for either air or 32% (the latter just generated off NDL air tables) is pretty standard in the DIR community. I've used it with teams from both coasts.

This very algorithm is quite applicable if one had to make up for a few minutes past NDLs.

 

[lowviz]

...//.... It's curve-fitting, and as so often happens, the simpler formula approximates the more complex one very well within a certain range, but departs from it more and more outside of those limits. ....//.....

Agreed. If you just graph the US Navy Air tables with a single 10 foot stop, you get:

View attachment 98774

You can see how a straight line may work at shallow depths, but at deep recreational depths, the deco obligation non-linearly roars off of a single stage (10 foot) obligation. (trace the graph from right to left along a constant depth line)

It is hard to get an innate feel for how fast a deco obligation racks up at ever increasing depths. Much too dicey to guess. I think we get away with what we do as most of the omissions are in the shallower sections.

Keeping this suitable for Basic Discussions, the whole idea is that each additional increment of depth makes the deco obligation much worse than the last depth increment did. It gets out of control fast as you go deeper and stay longer. Forget about getting a "feel" for it. You just have to memorize tables or over estimate in some other fashion.

 

[Mr Carcharodon]

Honestly, I think the reason we get away with a lot of the things we get away with, is that there is so much uncertainty in decompression calculation to begin with.

Stated another way a low risk decompression sickness multiplied by a rare occurrence of going into unplanned deco results in a close to zero risk of DCS over a diving career. Using an appropriate setpoint for your mix, and using 1:1 ratios, coupled with padding for known risk factors seems reasonable for small errors. It surely seem preferable to shooting to the surface and hoping they have oxygen on the boat.

 

[DuboisP]

Stated another way a low risk decompression sickness multiplied by a rare occurrence of going into unplanned deco results in a close to zero risk of DCS over a diving career. Using an appropriate setpoint for your mix, and using 1:1 ratios, coupled with padding for known risk factors seems reasonable for small errors. It surely seem preferable to shooting to the surface and hoping they have oxygen on the boat.

an easy rule in case of not knowing your deco (computer out of service, no clock, no gauge) : empty your tank at the 10 ft stop.
in France, oxygen is mandatory on diving boats

 

[DevonDiver]

Don't they have currents, tides, waves and temperatures in France?

Open-ended hanging seems a very bad option for unplanned deco resolution, unless the conditions on every dive can be assured to support that long hang...

 

[DuboisP]

Don't they have currents, tides, waves and temperatures in France?

yes, we have all of them
if you are from Devon, UK, you know how it can be diving in the Channel
(yes, i think you are, after have read your profil)

Open-ended hanging seems a very bad option for unplanned deco resolution, unless the conditions on every dive can be assured to support that long hang...

we're assuring these conditions, even when we don't plan to do deco