Can someone explain Ratio Decompression?

[Kevrumbo]

Resurrecting this again, and to clarify in light of a recent TDS thread:

RD works as applied, "similar to a Buhlmann profile set to a conservative factor of 30/85 and/or a V-planner profile (RGBM/VPM-B) set to a conservative factor of +2", with profile shaping using S-Curves or linear progression for the intermediate deco stops, and 12min:6min or 10min:5min/Oxygen:backgas break intervals. You can adjust the shape and/or add more time to the stops as needed per your own physiology and trial & error in "how much better or worse you feel" post-dive.

Going to divulge the big secret now: Four years ago, Andrew Georgitsis alluded to me in private that Ratio Deco --as outlined above and in the pdf attachment below-- also works with deep air as well.

But publicly, Andrew Georgitsis will never endorse diving Ratio Deco on air backgas during his class, or under the auspices of UTD --obviously because of the extreme narcosis, safety issues and potential personal, professional & company liability (see my bends hit incident for an example of the detrimental effects of nitrogen narcosis). This does not imply that the algorithm doesn't work with deep air or any other non-standard bottom gas when correctly calculated.

IMO/IME, you can use what bottom gases you've got with the Ratio Deco method, along with the caveat to be prepared in mitigating task risks during the dive due to narcosis (i.g. no overhead penetrations, complex missions etc.), if you don't utilize the necessary standard bottom helium mixes. . .

 

[diddydidit]

Ratio deco makes little sense to me. If you are an easy bend do what you already doing or set your conservatism setting higher like +4 or 5 on V-planner. I have never bought in to the idea of ratio deco as a contingecy plan similar to what TS&M mentioned earlier, I see it more as a marketing plan to sell the DIR GUE system of diving. Not bashing just giving my 2 cents.
I plan all my dives on V-planner and I carry a set of backup tables with at least a 5 and 10 min over stay at depth as well as exceded depths of 10 and 15 feet. I also bring with me a Liquivison X1 in case all **** hits the fan I just read the computer, no math to do while narced or in a real scary possibly panic type situation

 

[Doppler]

Wookie: There's a chapter in my book -- and I know you have a copy -- on the whole deco on the fly "thing." Check it out.

 

[amascuba]

One thing that's worth mentioning is the PDF's that are referenced only apply for about 30 minutes of deco. Beyond that there are other things to account for, such as minimum stop times for deep and deco stops, as well the amount of time that you spend on deco gases below the 70' bottle. That is, until you reach max deco. Then it's a new ball game.

 

[Kevrumbo]

Practical Dive Planning & Execution Using On-the-Fly Depth Averaging Function of the UWATEC 330m Bottom Timer & Ratio Deco:

Successfully used the following profile range & gases for ten dives on my annual Chuuk Lagoon Trip, 25Nov - 08Dec 2011:

20/20 trimix [MOD 60m @1.4ATA, END 45m] with Eanx50 and Oxygen for deco;
Sidemount 11L AL80 tanks with 4.2L AL30 deco bottles.

The deep wrecks of the Fourth Fleet Anchorage (San Francisco Maru; Aikoku Maru; Nagano Maru; Shotan Maru; Nippo Maru; Hoki Maru etc), all fall in the depth/BT ranges below. I just pick the average depth and the corresponding time on pre-dive planning (& on-the-fly as needed):

Depth: BT:
45m 35min
48m 30min
51m 25min
54m 20min
57m 15min

. . . And then the following same deco profile for any one chosen depth/BT combination above:

42-39m short deep stops 5 to 10sec
36m 30sec
33m 1min
30m 1min
27m 2min
24m 3min
21m 4min Eanx50 switch [Note: elected to not shape S-curve profile]
18m 3min
15m 3min
12m 3min
9m 2:2min Eanx50:backgas break
6m 10min Oxygen switch
-->ascend 1m/min for 6min time to surface on O2

(based on Buhlmann GF 30/85, and Ratio Deco 1:1)
SCR: 10 to 12L/min; Deco SCR: 12 to 15L/min
Minimum Gas Reserve Rock Bottom from 45m: 1600 litres total, or 70bar reading in BOTH BOTTOM MIX TANKS.

Eanx50 used: 640 litres
Oxygen used: 350 litres
20/20 used: 2700 to 3800 litres

Water temp 29deg C (skinsuit with hooded 2mm vest; no added lead weights & Z-systyem sidemount harness).

Gas blending cost in Chuuk (Blue Lagoon Dive Ops), for one dive using 20/20mix with Eanx50 & O2 deco on open circuit as above: $190​

 

[Slamfire]

20/20 is not a standard gas, is it? If that is the case, I guess one of your points is that ratio deco can be used with non-std gases and air (as previously posted). I think that was also one of the points of Doppler's old paper about deco on the fly. I have not read the Six Skills just yet, but one copy is on its way to me as a delayed Christmas gift (transcontinental logistic issues ). I'm looking forward to reading that particular chapter on deco-on-the-fly.

Going back to 20/20, a couple of weeks ago I ended up doing a dive with a similar mix, 19/24. This was not my first choice, but rather than skip a dive, 2000 psi of 19/24 in LP108 doubles was an acceptable compromise. My gas supplier mistakenly sent out my fresh T cylinder of UHP Helium on the wrong delivery truck. Because of work and family events, I had no time to go out and chase other alternatives for Helium. But I had some left over 18/37 from a previous dive on my LP108s. After some quick math, I figured out that if I topped that remnant with air to 2000 psi, my resulting mix should be 19/24 and I should have enough gas to do the bottom time and still comply with rule of 3rds (soloing).

I was doing 50% and 100% for deco. The interesting thing I noted on my Shearwater Predator (on VPM-BG +2 conservatism) is that bottom time (BT) was within a minute or two of the TTS (Time to Surface). At BT of 15 mins, TTS was 15, at BT 20 TTS was 20 and so on. In other words, this dive has a 1:1 ratio.

Below you will see the profile for said dive. (Click here to see full size image)

 

[Slamfire]

Before the questions start coming in, let me clarify that the Predator expects an ascent rate of 30 ft/min. In the profile above, from min 25 to min 32 I ascended at a rate of 13 ft/min. Ascent rate in between stops is that slow or slower. This slower ascent rate is the reason why total deco obligation ends up being 59 mins instead of the 50 mins for a corresponding bottom time of 25 mins.

What can I say? This is a wall dive and there is cool stuff to see on the way up. There is no rush and I still have a good amount of gas.

 

[ianr33]

So what advantage(s) does ration deco have over a good computer(s) ? (Apart from cost)

 

[Kevrumbo]

So what advantage(s) does ration deco have over a good computer(s) ? (Apart from cost)

No advantage other than just a complementary understanding of your deco profile. When you successfully do enough of these profiles in actual practice, you naturally "memorize" them by rote whether by Ratio Deco alone or in conjunction with a dive computer (or desk-top computer software onto a dive slate).

 

[limeyx]

So what advantage(s) does ration deco have over a good computer(s) ? (Apart from cost)

The battery never goes flat (well, if it does you almost certainly have bigger problems than DCS