Modern Ratio Deco usage?

[PfcAJ]

You say that like it's a good thing.

boulderjohn said he uses 50/80 and was using 40/80 for the planning on the dive he was talking about. Simon Mitchell says he uses 50/80.

I have yet to see anyone make a solid case that 50/80 is any better of a choice than 80/80, and I have personally specifically asked a couple of well-respected technical diving authorities that specific question. 20/85 definitely seems like it would be stopping you too deep and spending too much time down there - based on the modern science that has been getting published recently.

I mean, if you are okay with hitting the surface with a GF99 of 80, then why are you not okay with having a GF99 of 80 at 60' depth (or any other depth)? What is it about deco theory, in your mind, that leads you to conclude that limiting yourself to a GF99 of 20 for the initial part of your ascent is safer? If a GF99 of 80 is safe on the surface, why is it not safe at depth? The last part of your ascent is the most hazardous, right? If you can do that part of the ascent with a GF99 that is ramping up from, say, 70 to 80 as you go, why is it any more of a problem to go from 70 to 80 during a deeper part of the ascent, where the pressure gradient is actually changing more slowly?

As for use of RD, I personally would not trust myself to do any math underwater that I can avoid - say, by carrying 2 computers. I plan my dive ahead of time using Multi Deco and I know getting in what my worst case scenario is for deco, so if both my computers died, well, first of all, I wouldn't have a depth gauge or timer. But, I could do something makeshift if I had to, for that. And I would know pretty closely what my deco stops should be just from planning them ahead and writing them down. Relying on RD seems inherently less safe than how I do it now.

Relying on being able to do math after potentially being narced (where science seems to have shown that the effects linger, even after ascending) seems less safe than relying on a computer to do the math for you.

If you're willing to rely on an electronic device to calculate your average depth for you, what possible reason could you have for not being willing to let it calculate your deco stops for you? Does anyone think that computers retain their ability to calculate easy stuff, like average depth, but lose their ability to calculate a deco stop?

If you rely on a mechanical depth gauge, a mechanical timer, with redundancy for both, and you ascend based on a plan worked out in advance based on max depth and bottom time, more power to you. I'll grant you that that may be safer than simply using 2 computers (from 2 different companies, so they are proof against having a bug in common) - unless, of course, your actual dive ends up varying significantly from your plan. But, if you use an electronic device for any of that and you are telling yourself that it's safer to calculate your deco in your head, after being at depth, no less, than to let that electronic device calculate it for you, well, I don't follow your logic.

I generally agree that a GF low of 20 isn't the best. However, on the types of dives we're talking about (where RD fits) I don't think it really matters. The difference between 20 and 40 is minor.

GF99 of 80? Huh? I think you're trying to say GF high, but I'm not sure. A gradient factor discussion is a whole 'nuther thing.

Regarding doing math, it's nothing more than "deco time equals bottom time" or "deco time is double the bottom time". You do know your bottom time, right? If you're 10ft shallower, subtract 5mins. 10ft deeper? Add 5mins. It's not a big drawn out thing despite the tone some folks have in this thread.

I'm happy to use a computer. I use a computer (so long as it doesn't have a proprietary algorithm),I'm happy to use a bottom timer without avg depth, and I'm happy to use one that has avg depth (I use those too). In any case I'm keeping track of my depth, time, and deco obligation as I go through the dive. I prefer not to be surprised underwater

 

[stuartv]

GF99 of 80? Huh? I think you're trying to say GF high, but I'm not sure. A gradient factor discussion is a whole 'nuther thing.

No, I'm referring to the GF99 display that the Shearwater computers have, as a shorthand for saying "the percent of the M-value of the leading tissue compartment". E.g. a GF99 of 80 would mean that your most saturated compartment is at 80% of the M-value.

And, really, the whole point IS the actual GF used. If the GF Lo is too low then you get deeper stops than what people in-the-know seem to be recommending nowadays. And if RD is giving you a profile that closely matches having a GF Lo of 20, then some people might think that is specifically a reason to NOT use RD.

Your earlier post said that, for the example dive, RD was giving you an ascent that is within 3 minutes of a Buhlmann ascent with GF 20/85. That is, IIRC. I'm not going back to re-read it all now. But, how does the RD ascent compare to an ascent with GF 50/80? And when I ask that, I am assuming you and I both know that just because an ascent takes longer does not necessarily mean that it is less likely to result in DCS. So, even if the RD ascent is only a few minutes longer than the GF ascent, that does not necessarily mean that it is even close to as low risk. Maybe it's on par. Maybe it has less inherent risk. But, if it's slower because of a decent amount more time spent deep, then it may very well be inherently significantly more risky. I don't know.

Regarding the comment about "10' shallower, 5 less minutes, etc..". Sure, it's not that hard. IF you are using a standard gas (I generally don't - I'm too cheap for that). And IF you get it right on what your factors are for the gas you're using. And IF your data source for you average (or max) depth is correct. And.. etc.. It's not hard to find stories even just by searching here on SB of people who simply made mental mistakes when calculating their RD and got bent as a result. Meaning, actually, they AND their buddy both made mistakes. And then repeated them over and over at each deco stop. And that's even the case when there was no source of stress involved. Getting it right if something has happened that is resulting in unusual stress, then how easy is it to get right?

Have you ever heard of a buddy team on a tech dive where each person was diving two computers, they followed their computers, and they still got their ascent wrong and got bent?

 

[PfcAJ]

No, I'm referring to the GF99 display that the Shearwater computers have, as a shorthand for saying "the percent of the M-value of the leading tissue compartment". E.g. a GF99 of 80 would mean that your most saturated compartment is at 80% of the M-value.

And, really, the whole point IS the actual GF used. If the GF Lo is too low then you get deeper stops than what people in-the-know seem to be recommending nowadays. And if RD is giving you a profile that closely matches having a GF Lo of 20, then some people might think that is specifically a reason to NOT use RD.

Your earlier post said that, for the example dive, RD was giving you an ascent that is within 3 minutes of a Buhlmann ascent with GF 20/85. That is, IIRC. I'm not going back to re-read it all now. But, how does the RD ascent compare to an ascent with GF 50/80? And when I ask that, I am assuming you and I both know that just because an ascent takes longer does not necessarily mean that it is less likely to result in DCS. So, even if the RD ascent is only a few minutes longer than the GF ascent, that does not necessarily mean that it is even close to as low risk. Maybe it's on par. Maybe it has less inherent risk. But, if it's slower because of a decent amount more time spent deep, then it may very well be inherently significantly more risky. I don't know.

Regarding the comment about "10' shallower, 5 less minutes, etc..". Sure, it's not that hard. IF you are using a standard gas (I generally don't - I'm too cheap for that). And IF you get it right on what your factors are for the gas you're using. And IF your data source for you average (or max) depth is correct. And.. etc.. It's not hard to find stories even just by searching here on SB of people who simply made mental mistakes when calculating their RD and got bent as a result. Meaning, actually, they AND their buddy both made mistakes. And then repeated them over and over at each deco stop. And that's even the case when there was no source of stress involved. Getting it right if something has happened that is resulting in unusual stress, then how easy is it to get right?

Have you ever heard of a buddy team on a tech dive where each person was diving two computers, they followed their computers, and they still got their ascent wrong and got bent?

It would be trivial to adjust your first stop depth to end up with a higher GF low, if you were so inclined. Slow you ascent at 50% of you avg depth instead of 75%, for instance.

If you don't use the gas that your table uses that invalidates the table for the dive at hand. Same as if you don't put the right gas in your computer. I wouldn't expect anything else. If you don't (or can't) play by the rules then it's not for you. That's cool.

The cases I'm aware of people getting it wrong usually have some extra factor thrown in. I think the way UTD is teaching/approaching ratio deco is a problem, too. Gue's methods (instruction and application) don't seem to have the same troubles.

Formal training is an important part of all this, too.

 

[stuartv]

It would be trivial to adjust your first stop depth to end up with a higher GF low, if you were so inclined. Slow you ascent at 50% of you avg depth instead of 75%, for instance.

That sounds like you're saying "do your ascent differently than what RD prescribes."

I understand about the tables having to match the gas you're using. My point was simply that having to use standard gases would be problematic for me because I typically get a fill with the "Best Mix" and then get my next 1 or 2 or 3 fills by just getting my tanks topped with (Oxygen Compatible) Air. They get leaner and leaner until they drop past the point I'm willing to tolerate and I pay the bigger bucks to get them filled with my Best Mix again. Of course, if my dive plan changes significantly from one dive to the next, then I might get Best Mix each time. But, a typical trip for me involves several dives in a row with very similar profiles, so I don't worry about having slightly shorter NDLs each dive (or slightly longer decos).

So, even if I started with a Standard gas, I would not be inclined to pay the extra to have it refilled to be the same mix every day. And, as I understand it, RD doesn't have tables to use for any old mix I get. There is basically one table for each range of depth, and that table goes with the Standard gas for that depth range. If there was a table for RD for every single different percent blend of Nitrox, then that might make it *slightly* more palatable.

In the end, it's moot, for me. I would trust the two computers I'm diving with more than any ascent plan I'd come up with on the fly. Especially knowing that my computers will compensate for, say, a slower ascent than what the RD plan is based one. Or a faster one. I'm not (yet) good enough to do an ascent and maintain an exact 30 ft/min for the whole way. I do pretty well, most of the time. But, sometimes I end up only doing somewhere between 10 and 20 feet per minute. And if that were to happen during the deepest part of an ascent, I feel like that would obligate me to more deco - deco that the RD ascent wouldn't have built in, I don't think. At least, not unless you were doing everything else you needed to do AND noticed that your ascent was 15 fpm instead of 30 and took steps to modify your plan accordingly.

Task loading is a mental thing. Offloading mental tasks like calculating an ascent plan just seems like a good idea. Like having an alternate air source. The majority of the time, it makes no difference. But, the one time it matters, makes all the rest worthwhile.

 

[PfcAJ]

That sounds like you're saying "do your ascent differently than what RD prescribes."

I understand about the tables having to match the gas you're using. My point was simply that having to use standard gases would be problematic for me because I typically get a fill with the "Best Mix" and then get my next 1 or 2 or 3 fills by just getting my tanks topped with (Oxygen Compatible) Air. They get leaner and leaner until they drop past the point I'm willing to tolerate and I pay the bigger bucks to get them filled with my Best Mix again. Of course, if my dive plan changes significantly from one dive to the next, then I might get Best Mix each time. But, a typical trip for me involves several dives in a row with very similar profiles, so I don't worry about having slightly shorter NDLs each dive (or slightly longer decos).

So, even if I started with a Standard gas, I would not be inclined to pay the extra to have it refilled to be the same mix every day. And, as I understand it, RD doesn't have tables to use for any old mix I get. There is basically one table for each range of depth, and that table goes with the Standard gas for that depth range. If there was a table for RD for every single different percent blend of Nitrox, then that might make it *slightly* more palatable.

In the end, it's moot, for me. I would trust the two computers I'm diving with more than any ascent plan I'd come up with on the fly. Especially knowing that my computers will compensate for, say, a slower ascent than what the RD plan is based one. Or a faster one. I'm not (yet) good enough to do an ascent and maintain an exact 30 ft/min for the whole way. I do pretty well, most of the time. But, sometimes I end up only doing somewhere between 10 and 20 feet per minute. And if that were to happen during the deepest part of an ascent, I feel like that would obligate me to more deco - deco that the RD ascent wouldn't have built in, I don't think. At least, not unless you were doing everything else you needed to do AND noticed that your ascent was 15 fpm instead of 30 and took steps to modify your plan accordingly.

Task loading is a mental thing. Offloading mental tasks like calculating an ascent plan just seems like a good idea. Like having an alternate air source. The majority of the time, it makes no difference. But, the one time it matters, makes all the rest worthwhile.

RD mimicks the algorithm, not the other way around. If you can develop a RD ascent strategy to mimick a different gradient factor, cool! Make sure it works across the range you intend to use it with and rock on.

A big part of GUE's tech training is managing that ascent and learning to hit the time benchmarks to ensure you're actually doing the ascent you want to do.

 

[Kevrumbo]

Ratio Deco Deep Stop Strategy is referenced on the "fastest" of the Fast Tissue Compartments, Buhlmann's 5 Minute Half-Life Tissue Compartment which initially controls the ascent from maximum operational depth.

The first deep stop at 75% of maximum depth is a rule of thumb based on this leading 5min TC: that is by 10 minutes bottom time at an operational depth of 39m/130' or deeper, the 5min TC will have undergone two half-lives and will be 75% saturated. By ascending to 75% of maximum depth, and holding that deep stop, the theory claims this Fast Neuro TC will just begin to desaturate but still keep inert gas in solution for venous blood transport back to the lungs for inert washout. If we were to instead decrease ambient pressure by ascending shallower than the deep stop, going directly to the actual M-value depth or critical tension of Buhlmann's Model, there is a risk of supersaturating this fast tissue or worst case resulting in a DCS type II Neuro hit.

However, the implications of the NEDU Deep Stops Study shows that the above deep stop strategy may now in fact supersaturate the Slow Tissues later in the deco profile along with an increased risk of DCS upon surfacing. In other words, the Slow Tissues are still on-gassing while you are decompressing the Fast Tissues with Deep Stops at 75% and 50% max depth.

To compensate, and still choosing to use a Dual Phase/Bubble Model Deco Profile with DeepStops like Ratio Deco, you might have to extend the shallower stop times on Oxygen to effectively offgas & clear the Slow Tissues' supersaturation.

At present, exactly how much to extend this O2 stop time is arbitrary. For starters consider the FN2 of the bottom mix (if not using Standard Trimix Bottom Gas) and how many consecutive days of decompression diving you plan to do along with taking into account the slow tissue loading of residual inert gas over that time (the tissue loading bar graph of the Petrel Computer shows this nicely on surfacing post-dive): So for example, doing three consecutive days of two open circuit dives per day on the deep wrecks in Truk with bottom times & depth ranges of 40 to 60 minutes at 36 to 60 meters breathing the maximum FN2 of Air, I would additionally extend my O2 profile by another 50% to 100% of the original oxygen stop times as calculated by Ratio Deco. On the fourth day, I would now a take day-off from diving to further off-gas slow tissues, reset CNS/O2 exposure and get relief for my lungs from any pulmonary inflammation effects due to prolonged high ppO2 breathing.

Lastly, here's non-RD method, for those wanting to de-emphasize DeepStops while still utilizing the conservatism of arbitrary extended O2 stops: By using & applying Buhlmann GF's for instance, if I were doing two OC deco dives per day for three or four consecutive days-in-a-row on Air or Trimix on a month long Indo-Pacific/Micronesia tech wreck expedition, then I would program my Petrel Computer from Day 1 thru 4 something like 40/80 for Day 1; 40/70 for Day 2; 40/65 for Day 3 and finally 40/60 for Day 4. On Day 5 take a break from diving, and then continue the same three or four consecutive dive days with one day-off schedule/pattern.

How does this sanity check work?

I was recently in a situation where I was doing a deco dive with someone and we were joined by someone I had dived with a number of times in the distant past. During the interim, he had gotten GUE training and done his diving with Ratio Deco. He had decided, though, that he wanted to get with the current trend of using Buhlmann and Gradient Factors, which is what the other diver and I were using. To that end, he had purchased a Shearwater computer, and he was using it with us for the first time on a dive.

We planned the dive using Multi-deco with 40/80 gradient factors, and we wrote the plan and two contingencies in our slates. We agreed to use the slates as backups and use the computers as our primaries. It was not a really big dive, and when we started our ascents, the computers had the first stop at 60 feet. (The multi-deco plan had the first stop at 70 feet, but our actual dive was not as deep as planned.) The three of us started the ascent together, but the other guy suddenly stopped. Throughout our deco, he stayed well below us. When he was done, he said that when he saw the computer telling him to ascend to 60 feet, he knew it had to be wrong because he had been trained that the first stop had to be much, much deeper than that. He used the word "insane" to describe the 60 foot first stop depth that we all had and which was essentially what we had preplanned for the dive using Multi-deco. He did not explain why he had not noticed that insanity prior to the dive.

So, if you are using it for a "sanity check" for the computer and notice a big difference like that, how do you know which one is insane?

In Feb 2015, I used the Petrel on Buhlmann 30/85 GF's for an open circuit dive to 60m for 60min bottom time on Air (Oite Destroyer). The first deep stop displayed on the Petrel was at 36m for 1min, while for RD it was at 45m for 2min. So yes, I can understand the concern for the shallower first deep stop -for me this was 20% more shallow than I was used to. (I simply slowed the ascent starting at 45m from 9m/min to 6m/min, and continued to the deep stop of 36m as indicated by the Petrel Computer).

The run times for 30/85 GF's and RD were similar: 60min bottom time, 60min Eanx50 time and 60min O2 time (10min O2:Air Break 5min). The Petrel had 19 minutes of DeepStops (starting at 36m: 1,4,4,5,5 totaling 19 minutes) while RD as back-up calculated out at 25 min (starting at 45m: 2,2,2,2,2,5,5,5). And extending out the O2 stop, I did an additional 30 minutes to clean up the slow tissues, for a total run time of 230 minutes and a final surfacing GF of 60.

 

[The Chairman]

I don't use Ratio Deco for many, many reasons. It's way too easy to make a mental miscalculation that will get you bent. There's really no science behind it as it "mimics" other deco strategies. Why mimic, when you can just use the real thing? Finally, it's too restrictive. If I'm going deep nowadays, I'm going there on a rebreather. My dueling Petrels do just fine.

 

[LiteHedded]

All my course materials (which might be a bit out of date) had 75% of avg.

it's different now
variable ascent rate instead of the deep stops. i can send you my newer materials if you want

 

[PfcAJ]

it's different now
variable ascent rate instead of the deep stops. i can send you my newer materials if you want

Do want. Mine has 30fpm to 75% then 10fpm to 50%, then 5pm to gas switch.

 

[boulderjohn]

Do want. Mine has 30fpm to 75% then 10fpm to 50%, then 5pm to gas switch.

Can you provide the research that supports this ascent rate?