Modern Ratio Deco usage?

[Remy B.]

Changing what I normally do, and adding additional deep stops like the ones I put below would it do any difference or be actually very bad ?

Decompression model: VPM - B/E

DIVE PLAN
Surface interval = 2 day 0 hr 0 min.
Elevation = 0m
Conservatism = + 3

Dec to 60m (3) Trimix 18/40 18m/min descent.
Level 60m 26:40 (30) Trimix 18/40 1.25 ppO2, 27m ead, 32m end
Asc to 42m (32) Trimix 18/40 -8m/min ascent.

Stop at 42m 0:45 (33) Trimix 18/40 0.93 ppO2, 18m ead, 21m end
Stop at 39m 1:00 ( will this be a bad idea or catastrophic )
Stop at 36m 3:00 (36) Trimix 18/40 0.82 ppO2, 14m ead, 18m end
Stop at 33m 2:00 ( will this be a bad idea or catastrophic )
Stop at 30m 2:00 (38) Trimix 18/40 0.72 ppO2, 11m ead, 14m end
Stop at 27m 3:00 (41) Trimix 18/40 0.66 ppO2, 10m ead, 12m end
Stop at 24m 3:00 (44) Trimix 18/40 0.61 ppO2, 8m ead, 10m end
Stop at 21m 3:00 (47) Nitrox 50 1.54 ppO2, 10m ead
Stop at 18m 3:00 (50) Nitrox 50 1.39 ppO2, 8m ead
Stop at 15m 4:00 (54) Nitrox 50 1.24 ppO2, 6m ead
Stop at 12m 5:00 ( I will make it 7:00 ) (59) Nitrox 50 1.10 ppO2, 4m ead
Stop at 9m 8:00 ( I will make it 11:00 ) (67) Nitrox 80 1.51 ppO2, 0m ead
Stop at 6m 33:00 ( I will make it 40:00 )(100) Nitrox 80 1.28 ppO2, 0m ead
Surface (101) Nitrox 80 -6m/min ascent.

Off gassing starts at 45.8m

OTU's this dive: 133
CNS Total: 51.3%

 

[Kevrumbo]

@boulderjohn mentioned that Dr. Mitchell is now recommending a GF of 50/80. Two years ago, I think Dr. Mitchell suggested 40/70. Do you know the reason for this change? It seems to imply that faster tissues can handle a higher level of supersaturation and the last stop can be shortened to surface at a higher GFHigh of 80 vs. 70.

That would be the rationale, along with less CNS/O2 exposure at GF hi of 80 versus 70.

The caveat though IMHO, is still qualitatively how much supersaturation can the fast tissues sustain & tolerate without bubbling out; IOW, will the trend continue toward higher GF lo's or even a full 100% return to the critical tension M-values of Buhlmann ZHL16?

 

[Beau640]

Anyone have a link to the post/article where Simon is talking about using 50/80 now? I've read his posts in the past where he talks about 40/70 but haven't seen anything recently with him discussing now using 50/80. There is quite a jump/change in schedule from 40/70 to 50/80.

 

[Kevrumbo]

Changing what I normally do, and adding additional deep stops like the ones I put below would it do any difference or be actually very bad ? . . .

There's nothing bad per se with any of these profiles -it's when you start approaching bottom times of 60 minutes or more at 60m or deeper depths, or doing expedition type/consecutive deco dives per day for weeks at a time, this is when you have to start paying attention to the effects of deep stops on slow tissue supersaturation.

Just for comparison to your profiles, here is an open circuit profile in Ratio Deco that I used to do a lot on multiple week long trips to Truk per year from 2008 to 2013:

60m for 30min BT
~ascend 10m/min
Deep Stops:
45m 1min (75% max depth)
42m 1min
39m 1min
36m 1min
33m 1min
30m 3min (50% max depth)
27m 3min
24m 3min
21m 9min (Eanx50 gas switch w/ S-Curve profile shape)
18m 9min
15m 3min
12m 3min
9m 6min
6m 25min (Oxygen switch; 10min O2:Backgas break 5min)
~slow 1m/min ascent from 6m on O2 to surface.
Total Run Time: 107 minutes

The major difference -and the reason I believe I started having DCI type I problems in 2013-14 -was that these dives were on bottom mixes with high FN2: 20/20 Trimix or Air. My slow tissues and immune system finally lost stress tolerance over years of inert gas supersaturation-desaturation cycles in doing these types of decompression profiles with high FN2 gases. . .

Anyone have a link to the post/article where Simon is talking about using 50/80 now? I've read his posts in the past where he talks about 40/70 but haven't seen anything recently with him discussing now using 50/80. There is quite a jump/change in schedule from 40/70 to 50/80.

Like I've been saying, it's all arbitrary -whatever works well for your own particular dive practice. . .

". . .As I have pointed out before, it is difficult to recommend a definitive practical application of this conclusion to divers for all sorts of reasons. However, the use of GFs does give the diver an opportunity to "de-emphasise" deep stops to an extent. In my own experience, this means that whereas I might have used a GF-Lo of 10 or 20 at the height of the belief in deep stops, I would now use 40 or 50. Given I am frequently the only diving physician in remote locations I usually also dive a conservative GF-Hi (70 - 75) but that is a personal choice. There is nothing definitive in these recommendations. It just seems like a sensible response to the data that we have as of February 2014. . ."

Simon M

Deep Stops (rebreather dive charts) - Page 22

 

[Michael Guerrero]

I recently re-watched a David Doolette presentation on the NEDU study, and if memory serves, the difference in DCS incidence between their deep stop profile and their shallow stop profile was significant (double). However, the actual rate of DCS for both profiles was minuscule.

The traditional schedule produced 0.02% DCS and the deep stops one produced 0.04% DCS--if I'm remembering correctly.

My point is that moving some minutes around here and there is very unlikely to have any significant impact on your outcomes, as all decompression strategies that fall within these two approaches appear to be very, very safe. Even a 100%/100% GF setting would have a low likelihood of causing DCS (I can't remember what the figures were but I think Buhlmann was shooting for a DCS rate below 2% to be considered acceptable).

Don't torture yourself or agonize over "should my GF lo be 40 or 45" kinds of things. Different days will feel different, even if they are the same dive. Even getting bent by doing a dive you've done before isn't necessarily an indication of a bad schedule given all the other factors that go into how your body handles decompression stress. Stay within the accepted parameters, dive your dives, and focus on things that matter much more to becoming a good diver, like your technique and comfort level in the water.

 

[Michael Guerrero]

The caveat though IMHO, is still qualitatively how much supersaturation can the fast tissues sustain & tolerate without bubbling out; IOW, will the trend continue toward higher GF lo's or even a full 100% return to the critical tension M-values of Buhlmann ZHL16?

I think the important metric is how much supersaturation the fast tissues can sustain/tolerate before it actually matters. Or are you saying you're not happy with the critical supersaturation M-values previously derived by decompression scientists?

In furtherance of your second point, if we know extending the shallow stops (conditions permitting) increases conservatism by allowing for more intert gas washout under increased pressure, then why can't I have a GF Lo setting of 80 or 90, and a GF Hi setting of 70 or 60? All I'm really doing is shaping my ascent curve. And if I'm willing to get closer to the Buhlmann numbers early in the dive where the fast tissues control the ceiling, because they appear to be able to tolerate more supersaturation than the slow tissues, and then walk those numbers back to a more conservative level as the slower tissues begin to control the ceiling, shouldn't I be able to do that?

Is the answer "no" because it wasn't part of Erik Baker's original paper?

I've asked Shearwater to add this feature to their dive computers a couple years ago, but haven't seen it happen yet .

 

[Remy B.]

There's nothing bad per se with any of these profiles -it's when you start approaching bottom times of 60 minutes or more at 60m or deeper depths, or doing expedition type/consecutive deco dives per day for weeks at a time, this is when you have to start paying attention to the effects of deep stops on slow tissue supersaturation.

Just for comparison to your profiles, here is an open circuit profile in Ratio Deco that I used to do a lot on multiple week long trips to Truk per year from 2008 to 2013:

60m for 30min BT
~ascend 10m/min
Deep Stops:
45m 1min (75% max depth)
42m 1min
39m 1min
36m 1min
33m 1min
30m 3min (50% max depth)
27m 3min
24m 3min
21m 9min (Eanx50 gas switch w/ S-Curve profile shape)
18m 9min
15m 3min
12m 3min
9m 6min
6m 25min (Oxygen switch; 10min O2:Backgas break 5min)
~slow 1m/min ascent from 6m on O2 to surface.
Total Run Time: 107 minutes

Deep Stops (rebreather dive charts) - Page 22

Ken, why will you stop at 45m if Off-gassing starts at 45.8, are those 0.8m that crucial to stop for 1min ? are the bubbles not still to small to be a concern and be the 42m stop more efficient ?

 

[stuartv]

why can't I have a GF Lo setting of 80 or 90, and a GF Hi setting of 70 or 60?

Interesting idea. How would you expect that to work?

Instead of Lo and Hi, let's call it GF1 and GF2. Assume you choose a GF1 or 90 and a GF2 of 60.

Should the computer let you ascend until your GF99 is about to exceed 90, and then do a linear interpolation from there to your final stop, so that you arrive at your final stop with a GF99 of something above 60, then hang there until it drops enough that you can ascend to the surface and arrive there with a GF99 of 60?

Or should it make your first stop when you're about to exceed 90, then let you keep ascending where you are riding that line of GF99 == 90 until you get to your final stop depth, then hang there until your GF99 drops from 90 all the way down below 60, so that you can then ascend and hit the surface with a GF99 == 60?

Examples:

Assuming a direct ascent to 30' results in a GF99 of 90.

Scenario #1 (from above - hang times totally made up):
Arrive at 30' with a GF99 of 90. Hang for 4 minutes.
Arrive at 20' with a GF99 of 80. Hang for 5 minutes.
Arrive at 10' with a GF99 of 70. Hang for 5 minutes.
Ascend to the surface, arriving with a GF99 of 60.

Scenario #2:
Arrive at 30' with a GF99 of 90. Hang for 1 minutes.
Arrive at 20' with a GF99 of 90. Hang for 2 minutes.
Arrive at 10' with a GF99 of 90. Hang for 9 minutes.
Ascend to the surface, arriving with a GF99 of 60.

I suppose you could accomplish #2 now, by setting your Shearwater to GF 90/90. Then, when you get to your last stop depth, change the computer to GF 60/60.

And I guess you *could* accomplish #1 by setting the computer to GF90/90 and then doing a little mental arithmetic are your first stop, then changing your GF at every stop, to reduce the GF each time.

Or, in either case, set the computer to GF 90/90, then at your first stop, start using the GF99 display and control your ascent based on riding that value to keep it wherever you want it.

But, the variation on the question I keep asking, now, is, why do you think it's okay to have a GF99 of (for example) 90 in the water, but it's not okay at the surface (so you need a lower GF when you hit the surface)?

 

[PfcAJ]

But, the variation on the question I keep asking, now, is, why do you think it's okay to have a GF99 of (for example) 90 in the water, but it's not okay at the surface (so you need a lower GF when you hit the surface)?

If nothing else I'd prefer to surface with less supersaturation because of lifting gear, climbing ladders, etc.

 

[Michael Guerrero]

Interesting idea. How would you expect that to work?

Exactly like it works today but by incrementally stepping down the leading compartment's supersaturation level instead of stepping it up.

But, the variation on the question I keep asking, now, is, why do you think it's okay to have a GF99 of (for example) 90 in the water, but it's not okay at the surface (so you need a lower GF when you hit the surface)?

Because, based on current research, it appears that the fast tissues are more tolerant of supersaturation than the slow compartments in terms of DCS outcomes. So allowing the leading compartment to reach 90% during the initial part of decompression (this will be a fast tissue) and then slowly decrementing the level of supersaturation as the leading compartment shifts from a fast to a slow tissue, would still protect the slow tissues. When you are at 90% GF99, it's not every compartment, it's the leading compartment. At the start of deco, those are the fast tissues, because they have on-gassed the most and will off-gas the most (not talking saturation dives here). Having a GF99 at the beginning of deco is very different than having one at the end. Getting to a shallower stop at the start of your deco (higher GF99 number at the start) would mean less on-gassing of the slow tissues.

Yes, I could do this manually, but I want a computer to calculate it for me so I can focus on the other things that might kill me.