Difference between MB levels and Gradient Factors

[J-Vo]

The only place I have seen it is in Steve Lewis' book. It always seemed like an oversimplification to me. Witch tissue compartment are we talking about? Even on a fairly big TRIMIX dive I've seen my TTS going up on the 30' stop. And it's almost always still going up on the deeper stops below 80' or so. This tells me there are compartments still on-gassing well into deco off-gassing for other compartments.

I did a poor job of expressing my point, on any bounce dive (as apposed to saturation) the slower tissues will be on gassing pretty much the entire dive or until your breathing O2. So where is this "off gas" depth?

I agree slowing down in the shallows is good, but I don't think anyone is going 30ft/min between deco stops anyway. It's that last bit to the surface after deco/safety stops have cleared were it's easy to go too fast.

 

[inquis]

So where is this "off gas" depth?

Given it's a single number, I assume it corresponds to that of the most heavily loaded tissue.

Basically it was a "don't be screwing around until you are above this depth" value that was always annotated.

Presumably, the thinking is that there is no danger from a rapid ascent when no bubbles have formed -- with the implicit assumption that no bubbles have formed when all tissues are theoretically still on-gassing. Seems to work for Sevenrider.

 

[ChrisDee]

While I don't think the G2 is some miracle device as has been described, I don't think it is a bad computer for a very large majority of divers....even with the SOS mode.

The standard G2 is fine for the majority of divers that do not know VPM from ZHL-16 to RGBM. There are not a large number of divers getting bent on the G2. If the diver's only plan is to follow the computer, then who really cares about the proprietary algorithm that makes unknown and undocumented changes to the NDL based on heart rate or skin temperature? Who cares about the MB settings which are really no different than adding conservatism and shortening the NDL? The divers have no concept of decompression algorithms and they have no other plan than to follow their computer. The G2 work just fine in these instances.

While I also like computers that don't go into "lockout" or "SOS" mode (Shearwater and Garmin), it is not an easy task to accomplish with the G2. On the G2 Tek, to go into SOS mode, you are diving ZHL-16 with gradient factors at 100/100 and missing a mandatory deco stop for more than 3 minutes. I dive 50/70. If I miss a stop my Shearwater will not go into gauge mode...but neither will the G2. The Shearwater will calculate the next stop at 50/70 even though you are past the GF line and into the safety margin, the G2 will adjust to the next set of gradient factors that places you back under the calculated GF line. Only when you have so badly messed up your plan that you are diving a pure Buhlman profile, does the SOS mode even kick-in.

I do not know the underlying mathematics in the ZHL-16 algorithm. I understand it is nice to have a computer that still runs calculations when you are on the wrong side of the Buhlman M-value line on your stops. But if you are already on the wrong side of that line...what information are those calculations really providing you? What studies exist that making depth and stop decisions on the wrong side of the M-value line will keep you from getting bent? It seems everybody wants a computer that is still calculating depth and stops when you are in the pink shaded area below, but is that going to reduce the chances of them getting bent compared to just following the missed deco procedures we learned in our technical diving classes or what is taught in basic open water? I do see where it would be nice to know if I surfaced at 110% versus 170%, but would the precautions taken once on the boat still be the same?


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I also use the computer as the controlling factor for a planned decompression dive where I have a written dive plan with the standard too long, too deep and lost deco schedules. The rule I follow is that my written plan should use the same algorithm that my computer will use. I don't plan dives on VPM and then expect to follow my computers that are using Buhlman. If you planned your dive using VPM and assumed that the Suunto HELO2 would match it with its RGBM algorithm...well that is on you. Suunto actually had (maybe still has) an offline planner (Suunto DM??) that matched the Suunto RGBM algorithms used by the different computers (Technical, Fused, etc.). Suunto Technical RGBM did not even match a standard RGBM offline planner like GAP. A "bricked" HELO2 is still a bottom timer which could be used against your written plan. The primary issue started before you ever splashed...plan VPM and try to follow a computer using Technical RGBM.

Big issue is with GF and Sur GF is yo have to do mental gymnastics to understand what computer is trying to tell you. With G2 yo set it, you set it according to your plan. It does what a computer is supposed to do, rather than trying to figure things out real time. BTW, many users of SurGF and GF style have reported that they find spend most time staring at coputer tyring to figure it out. This is not a positive.

Also others set GF high / low, but this isn't what shows up on screen and you people continue to promote this confusion as a positve? Seems all you want to do is bash something you know nothing about.

BTW HOW MANY YOU HAVE MULTIPLE DIVE COMPUTERS? I diving for long time, I have multiple units, I use what I have found to be most reliable and confident use over time. Which is G2. And this coming from actual user of unit NOT someone pretending. Some people want control over "conservatisim" level G2 allows this when others do not. Some people more suseptable to MB issues, G2 best for that. Some people want AI and heartrate and temp factored in,......as well as respiration rate,.....and also want adjustable workload,...... PDIS which makes sense of insane SurGF, more funcunality, eiser to use,...and notheing but criticisims from people who have never used or own same,...SMH.

Thank you Seven for your unbiased opinion.

And a reminder, Uwatec follows the first and leader in Deco science, Every other manufatturer is just a copy of someone elses work.

 

[ChrisDee]

Could you post the studies that show this is beneficial? I have done a lot of research on this topic, and I seem to have missed

It's fair to say that this thread's definitely moved into technical diving territory; basic it is not.

Wasn't the ascent rate of 9m/30ft per minute used by most agencies and planners to work out a dive profile? Was important doing manual Ratio Deco -- maybe still is for some?

The fact is that it doesn't matter that much any longer as your computer(S) measure your ascent rate and updates its calculations accordingly. You follow the computers' calculations. The main thing is slowing down and stopping below the first stop. There's a lot to do on the ascent especially if it's a free ascent with an SMB or two with other divers, so sometimes slower's good.

G2 has variable rate ascent depending on depth. Another fact so many choose to ignore.

 

[ChrisDee]

I think a lot of people say that Pyle stops done without compensation by extending shallow stops are potenetially harmful.

I am not saying anything one way or the other about deep stops in NDL diving. I am just saying we don't know enough to say. In decompression diving, we have more of a sense that they are not so good.

Again you intentionally confuse "efficency" with harm. That is not the issue. And again you fail to support your comments with actual reports that you so demand of others. Deeper planned deco dives = longer deco time, please enlighten us with where this straegy is more "harmfull" ????? And then again in your OPINION, (unsupported) "we have more of a sense that they are not so good." Please prove same.

 

[ChrisDee]

What agency teaches this?

As for what I taught, I taught my students to research all materials to the greatest degree possible, after which we would discuss what they had found and then arrive at a conclusion. I don't recall anyone finding anything at all suggesting that we do anything other than 30 FPM.

From G2 manual:

"The pressure reduction that the body can
tolerate without significant microbubble
formation is higher at depth than it is in
the shallows: The key factor is not the
pressure drop by itself, but rather the ratio
of the pressure drop relative to the ambient
pressure. This means that the ideal ascent
rate at depth is higher than it is in the
shallows."


DEPTH ASC SPEED
m ft m/min ft/min
0 0 3 10
2.5 8 5.5 18
6 20 7 23
12 40 7.7 25
18 60 8.2 27
23 75 8.6 28
31 101 8.9 29
35 115 9.1 30
39 128 9.4 31
44 144 9.6 32
50 164 9.8 32
120 394 10 33

Best one yet JOHN.

Varible ascent speed is a "Golden Rule" at Drager school, been taught for the last 30 years. A fundamental maxim of your "efficency" is high speed depth dependent ascent which are recommended to exceed the 30/ft min. I don't belive you just said what you said,...I suggest you rethink your comments,

You all contine to promote "efficency," but you all don't know what your speaking about, this is clear. "Pyle" has it's place, apparently clear you do not understand.

 

[tursiops]

It not go into gauge mode during a dive.

It has a SOS feature for those times you just can't be bothered to pay attention to what your dive computer is trying to tell you. Like if your not going pay attention to your deco stops, I mean what point to have dive computer then? Seriously. It goes into SOS mode 3 min after surfacing. many find feature usfull, and is selectable. So again why you spread lies????

Perhaps and once again you spread rumor about something you know nothing about. And again your mouths keep moving, SAD.

Your reading comprehension sucks, and you keep trying to defend your G2 even when nobody is criticizing it. Boulder John wrote:

Here is a story about lockout features.

As I said above, when I started tech diving, we just used written plans, with no computers. One one dive in Florida with a maximum depth of 270 feet, my buddy and I were following a plan (with contingencies) derived from V-Planner. My buddy had just bought a dive shop (Fill Express), and Suunto was trying to get him to carry their products. They gave him their hot, new tech computer, the HelO2, to try out. He brought it on the dive to see how it would work. Its RGBM algorithm was close to our VPM to begin with, and he tweaked it to get it as close as possible.

Well, we followed out written plan about as well as we possibly could, but it did not match what the HelO2 wanted us to do. During one of our deco stops, my buddy informed me that the HelO2 had gotten pissed off enough to go into gauge mode. "Well, that's just plain worthless," I decided.

I would never consider using a computer that goes into gauge mode in the middle of a dive.

And I responded to the last line....

Exactly.

And then you go off on a tirade about your G2.

And you decide I am spreading lies? Get over yourself, Dude.

 

[Wibble]

G2 has variable rate ascent depending on depth. Another fact so many choose to ignore.

And?

The rate of ascent is the rate that the diver ascends, it matters not what some computer says you should ascend at, it only matters what rate you actually ascend at. There’s many reasons to do a slow ascent from depth, not least congestion on the shot line.

I want, no need a computer I can trust. Bulhmann works. It’s kept me from DCS over many hundreds of decompression dives (I use 50:80). I don’t trust other algorithms.

 

[boulderjohn]

Again you intentionally confuse "efficency" with harm. That is not the issue. And again you fail to support your comments with actual reports that you so demand of others. Deeper planned deco dives = longer deco time, please enlighten us with where this straegy is more "harmfull" ????? And then again in your OPINION, (unsupported) "we have more of a sense that they are not so good." Please prove same.

For some reason, the site where I published my article on this topic a few years ago is not working. Here is a link to the ScubaBoard thread I started after I published it. It contains a link to the article.

Evolving Thoughts on Deep Decompression Stops

A few months ago I met with a couple former tech students for a cave dive, and one of them told me with amusement of his dive the day before. He had randomly joined a group that included a technical instructor, and when that instructor learned that he used a GF Low of 50, he gave him a very...

scubaboard.com

It references all the known published research related to the topic at the time. If I missed any, let me know. If you have any relevant articles published after, let me know as well. I know David Doolette published an article on gradient factors after that, but he referenced my article and praised it, so you won't get anything different from him.

 

[boulderjohn]

In post #125, you quote me asking you for research: "Could you post the studies that show this is beneficial? I have done a lot of research on this topic, and I seem to have missed" Your response does not cite any of those studies. You ignore the question you quoted.

Then you challenged me to cite research, and I did that in the post above this. So here is the question again: "Could you post the studies that show this is beneficial? I have done a lot of research on this topic, and I seem to have missed it."