UTD Ratio deco discussion

[mikeny9]

I actually have read it, no where does it talk about O2 windows, have you read it? They used air for decompression, not Oxygen. The study was more to do about deep stops than anything.

So I ask for the third time, which elements of RD are "based on debunked thinking and outdated paradigms?" Where are those elements debunked? Where is the O2 window debunked? Don't take this the wrong way. I'm not trying to attack you, I'd like to actually know so I can make an informed decision myself.

 

[boulderjohn]

The NEDU study had nothing to do with the supposed oxygen window. That theory was debunked more than a decade ago. There was not a big deal debunking it because at the time only GUE and UTD believed it in the first place because it violates gas laws. GUE dropped the theory about 8 years ago but kept the S-curve in because, as Jarrod explained it to me, it seemed to have worked in the past, and they were keeping it for that reason. They have since dropped the S-curve as well. Only UTD uses it.

The reason there are no real studies debunking it was because only a relative handful of people had believed in it in the first place. When I took Ratio Deco, AG himself said it looked like that theory was not valid, and he came up with two other reasons for still using the S-curve, but I can't remember what those reasons were.

That is why I am so surprised to see that the oxygen window theory is still being used to support the use of the S-curve.

I am pretty sure that no algorithm other than RD has ever used it as a regular part of its dive planning, although I think you could add it into some.

 

[mikeny9]

That's is indeed interesting John.

From what I'm told in the training, the O2 window is used for the higher exposure of Oxygen with PPO2 = 1.6. In the old RD, the tech-1 profiles would do 5 min at 70', and 50 min at 60'. in RD 2.0, it's 3 min at 70' and 3 min at 60'. GUE uses all 2 mins.

It's odd that there is no study debunking the O2 window, yet everyone always just assumes it has no merit. When first experiencing DCS symptoms, don't we go right to 100% O2? If people aren't getting better, they often take Table 6/9 chamber rides down to 60'/90' for 2 hours/5 hours with obviously a much higher PPO2.

I think there's something to it personally. I'll be happy to change my mind if someone shows me something more concrete other than, "we know more than you, go read a study that you've read before that has nothing to do with the O2 window." And I'm the one being accused of not listening?

 

[PfcAJ]

Have to agree with mikeny9, I think very few have taken a RD course and certainly less the 2.0 version. That said, many on SB do have considerable Deco experience/knowledge. The debate will continue and that's always informative, at the end of the day, it's a tool, your choice how you use it.

Good judgement call for them!

 

[dmaziuk]

John, can you address my question about the benefit of starting the dive at altitude and if or why that isn't considered in the calculated deco obligation? Does it just get totally wiped out at depth or is there some reason that I'm not understanding for it's exclusion from the algorithm? I may sound dumb but I really want to understand it.

"Totally wiped out at depth" would be my guess: if your tissue is saturated, it's saturated, regardless of what its starting point was. It may take a little longer, but since it's a log curve, the difference is only significant in the initial "steep" part of it. Like the first couple of half-times. Where it's not saturated yet anyway. Or at least that how I think the math should work out.

 

[PfcAJ]

That's is indeed interesting John.

From what I'm told in the training, the O2 window is used for the higher exposure of Oxygen with PPO2 = 1.6. In the old RD, the tech-1 profiles would do 5 min at 70', and 50 min at 60'. in RD 2.0, it's 3 min at 70' and 3 min at 60'. GUE uses all 2 mins.

It's odd that there is no study debunking the O2 window, yet everyone always just assumes it has no merit. When first experiencing DCS symptoms, don't we go right to 100% O2? If people aren't getting better, they often take Table 6/9 chamber rides down to 60'/90' for 2 hours/5 hours with obviously a much higher PPO2.

I think there's something to it personally. I'll be happy to change my mind if someone shows me something more concrete other than, "we know more than you, go read a study that you've read before that has nothing to do with the O2 window." And I'm the one being accused of not listening?

Those things you're talking about have nothing to do with the idea of the "o2 window". There's nothing special about breathing 1.6 other than its the highest po2 we can reasonably breath in the water, and we want to switch to a gas with the lowest amount inert gas ASAP, so we switch at 1.6.

Deco is driven by the concentration gradient (why we switch gas, so you reduce inert gas pressure in your lungs) and supersaturation (why we have to ascend to decompress, to increase inert gas pressure in your tissues). You could conceivably decompress by only one of those two mechanisms, like what happens when you're in a recompression chamber and breathing oxygen (max concentration gradient) or when just doing backgas deco (max supersaturation). Doing both is fast, and fast+safe is generally what we're aiming for.

The idea behind the "window" that doesn't have any evidence behind it is that there was something magic about staying at 1.6 for a period longer than what's required by supersaturation limits. Obvi switching to a gas with the lowest possible inert gas fraction (and therefor the highest oxygen fraction) is good, but the idea of extending the stops really has nothing backing it up.

 

[Kevrumbo]

Here is the rationale behind the original RD DeepStop Protocol at 75% of Max Depth (and now since contraindicated by the NEDU Study because of the unexpected side effect of later critically supersaturating the slow tissue compartments subsequently upon surfacing).

To understand our [Ratio Deco] Deep Stop strategy, we must understand MAX DECO, and to understand that we start by looking at the dissolved gas theory, or the Buhlmann Model.

The Buhlmann Model is a theoretical dissolved gas model in which you have 16 half-time compartments. From the fastest compartment @five minute half times, to the slowest @240 minute half-times. These compartments load and unload inert gases in an exponential manner, or in a "half-life" theory, meaning that they load or unload 50% in the first of the six time segments. Then 50% of the remaining 50%, so essentially an additional 25% in the next time segment, and then 50% of the remaining 25%, so an additional 12.5%, and so on until the tissues in that particular time compartment reach saturation or desaturation in the 6 half-lives. In the Ratio Deco Strategy, we consider the tissues to be saturated or desaturated when they reach 97%, so we use five half-times instead of six.

The compartments are named after their "half-times", so the fastest compartment is the five-minute compartment, and according to the Buhlmann Model it will take 30 minutes to saturate or desaturate that five minute compartment, 25 minutes according to Ratio Deco implementation of five half times. Then the next fastest compartment is the 10-minute compartment, then the 15-minute compartment, and so on.

So for our purposes of creating the Proper Ascent Profile we need to initially determine our "Deep Stop" protocols. In order to do this we will look at the first five tissue compartments: 5-minute, 10-minute, 15-minute, 20-minute, and 30-minute compartments; these are considered as the fast tissue compartments, the ones we start to unload first. In the deep part of the deco these compartments will be used to determine deep stop depths and time to ensure they are allowed to off-gas properly.

For desaturation we will look at the fastest compartment first, the 5-minute compartment, and then the next fastest compartment and so on. For saturation purposes, we will look at the slowest compartment. The slowest of these fast tissue compartments is the 30-minute compartment, which will take 150 minutes to saturate and desaturate according to our Ratio Deco method. . .

Now these five fast tissue half-time compartments will also have a "Max Stop" depth. This is considered to be the depth at which the compartment will first start to unload or desaturate. In other words, when you are on the bottom, you are saturating the compartments at different rates and they are reaching different levels of saturation. For example, after a 10 minute bottom time, the 5-minute compartment will have gone through two half-lives and will be saturated to 75% of ambient pressure. The 10 minute compartment will have gone through one half-life and therefore it will be at 50% saturation of ambient pressure, and the 15-minute compartment will be less than 50% of ambient pressure.

When you start your ascent, you will not start unloading these compartments immediately, as the ambient pressure will be greater than the dissolved gas pressure in each of the compartments. However, at some point during the ascent the ambient pressure becomes less than the dissolved pressure in a particular compartment, and that compartment starts to unload, or desaturate, or decompress. We call this the "Max Stop" depth of that compartment.

As you continue to ascend, the difference in pressure between the saturated gas in that tissue compartment and the ambient pressure is called the driving gradient. The greater the difference, the greater the gradient, and the more you off-gas.

However, at some point the gradient becomes too great and the inert gas no longer comes out of the tissues as molecules, but it starts to bubble. This point is called the Critical Tension or M-Value in Buhlmann's Model.

The idea of the Buhlmann Model is that you can ascend all the way until you reach the M-Value, so you would ascend past the Max Stop depth, start off-gassing, and continue ascending, driving the gradient steeper, therefore maximizing the off-gas speed until you reached the M-value, and then you would stop, not crossing the M-value depth, and therefore not bubbling. Theoretically this maximizes the difference in pressure between the dissolved gas in the tissue and the ambient pressure, "driving the gradient" and maximizing the off-gas rate without bubbling.

But now we know this is not the case. Bubbles are formed much earlier in the ascent, and this is a shortcoming of Buhlmann's Model (it penalizes for deep stops for which current dual phase models like VPM and RGBM factor them in to reduce the driving gradient, and prevent bubble nuclei/seed growth). What is important is not to get confused between Max Stop depth and M-Value: Max Stop is when you first start to off-gas, and M-Value is when you first theoretically start to bubble.

So once again, in designing our Proper Ascent Profile for our Ratio Deco ascent profile, let's figure out where we want to make our first stop. We already know that stopping at the M-Value line is not good, as we bubble much sooner than the M-Value line predicts, and therefore those bubbles grow in size and frequency requiring us to stop much longer and shallower; it is much harder to decompress from a bigger and more frequent bubbles than from a smaller and less frequent bubble. This harder decompression is what we colloquially call the "Bend and Treat" method.

(continued below)

 

[Kevrumbo]

(continued from above):

Looking at the Buhlmann Model, we see the first compartment to saturate and desaturate is the 5-minute compartment. Using a rule of thumb derived from this compartment we can predict our first stop depth. That rule of thumb is 75% of the depth for the 5-minute comparment. The 10-minute compartment rule of thumb is 50% of the depth, and the 15-minute compartment is 25% of the depth.

Now that we know our first stop depth is at Max Stop depth, we need to figure out the time we need to spend at that first stop depth to maximize the off-gas efficiency. We know the maximum time should be five minutes, as this is the half-life of the compartment which maximizes the off-gas of that compartment depth. We also know that if we stayed longer than five minutes, we would still be off-gassing, but not nearly as efficiently as the first half-life time, and we may still be on-gassing some of the slower compartments. If we stayed for 25 minutes at the max stop depth, 5 minute compartment X 5 half-lives = 25 minutes, then theoretically that 5-minute compartment would be 97% equalized to the ambient pressure, however it would not be the most efficient use of that 25 minutes, and we would continue to be saturating the slower compartments at that ambient pressure.

But if we stayed for five minutes at max stop depth for that compartment then continued to ascend, lowering the ambient pressure, creating a new gradient and maximizing new 5-minute half-lives for each of the shallower stops, we would create a more efficient off-gassing of that compartment. This would occur at each of the depths shallower than that first stop depth and would maximize efficiency until we reach the 10-minute compartment Max Stop depth at 50% of the bottom depth.

So we create a strategy in which we stop at the max stop depth, stay for five minutes and then ascend, stopping for five minutes and then ascend, stopping for five minutes at each 10'/3m depth above that until we reached the max stop depth of the following compartment -the 10 minute compartment. Theoretically we will have maximized both our 25-minute total deco time frame of off-gas for the 5-minute compartment, and have done the least on-gas of the slower compartments, still without compromising or bubbling. So now we are ready to tackle the 10-minute compartment.

So, how do we determine how much time for each of these stops is needed if you have not reached the max bottom time or max saturation of the slowest of the fast tissue compartments (which is the 30-minute compartment)? Remember, we would need to have a 150 minute bottom time (or more) to reach a 97% saturation, or 5 half-lives of that 30-minute compartment. So we consider a bottom time of 150 minutes or more as full saturation of the five fast tissue compartments. In order to decompress them you would not need to do any more than 5 minute stops from 75% of your depth to decompress the fastest of the fast tissue compartments, and then 10 minutes for each stop for the 10-minute compartment from its max stop depth, and so on. This then becomes MAX DECO Strategy.

But what if we don't do a 150 minute bottom time dive, but something far less? Then we do not need to do five minutes each stop for the 5-minute compartment because it's not fully saturated. So, we could essentially break that five minutes down into individual minutes such as 1 or 2; 3 or 4, or 5 minutes. This time includes the ascent to the next 10'/3m stop. So based on this 150-minute bottom time and 5-minute deco per stops, we could simply divide the bottom time of 150 by the deco time of 5 minutes to get a requisite minute value per stop strategy, per amount of Bottom Time.

So, 150 divided-by 5 = 30 minute Bottom Time for each minute of deep stop. In other words, we have now figured out that for every 30 minutes of bottom time over NDL, you will need to conduct at least one minute of deep stop deco, starting at the 75% of max depth or average depth depending if you are deeper or shallower than the average depth.

Referring to the attached table link below (see page 6 & 7), you can see that if bottom time is up to 30 minutes more than NDL, then do one minute per stop. If BT is between 30 minutes and 60 minutes over NDL, then do two minutes per stop, and if BT is more than 60 and less than 90 minutes, then do three minutes per stop. Remember that this time is for each stop, starting at the 5-minute compartment Max stop depth and then for each 10'/3m above until you reach the 10-minute compartment max stop depth. . .

Now to get stop time for each depth above the 10-minute compartment max stop depth or 50% of your max depth or average depth, you would take the max BT of 150 minutes and divide it by 10 minutes (10 minute half-times), 150/10 = 15 minutes, or each 15 minutes of additional bottom time would require one additional minute of stop time at each of the 10'/3m deco stops above the max stop depth for the 10-minute compartment. Basically you would do one minute per 15-minute bottom time.

Keep in mind that this is for all bottom times even if you are within NDL. So that means to create a proper ascent profile when recreational diving and doing bottom times that keep you within NDL, you simply do one minute stops (including the ascent time) from 50% of your recreational dive depth. So for example, if depth is 80'/24m and your bottom time = 25 minutes, you are within NDL. You would do one minute for each stop depth (10'/3m) starting at 50% of your depth until you reached the surface.

So a proper NDL ascent profile would be to leave 80'/34m and ascend at a rate of 33'/10m per minute until you reach 50% of the depth. Then at 40'/12m, it is one minute. Then ascend to 30'/9m (counting the travel time as part of the 30'/9m stop) and do one minute, then up to 20'/6m and another minute, then up to 10'/3m to do your final minute before surfacing. This is a Proper NDL Ascent Profile and is always conducted regardless of how much time you spent on the bottom as long as it is LESS THAN NDL.

However, if your bottom time was more than NDL, for example 25 minutes over NDL, and you're doing a decompression dive, your stop times would be one minute for each 10'/3m starting at 75% of your bottom depth and then three minutes per 10'/3m stop starting at 50% of your depth.

As a side note, if your bottom time was less than 15 minutes over NDL, then you would do nothing for 75% stop depth and start 1-minute stops at 50% of depth. There is no need for the 75% stop depth and start 1-minute stops at 50% of depth. There is no need for the 75% stops in this case, as the tissue loading is so minimal.

Take a look at the rule of thumb tables (WKPP applied deep stop theory) on p. 6-7 of the attached file link below:

http://www.ultimatedivelog.com/articles/8.pdf

So now with Ratio Deco 2.0, the first DeepStop has been changed to 66% of Max (or Average) Depth, for Bottom Times 30min over NDL (?). . .

 

[CAPTAIN SINBAD]

Simple Explanation of the Oxygen Window
by George Irvine
As a dope's explanation of the oxygen window concept for you Marines, the best gas differential would be a vacuum relative to a partial pressure, right? Oxygen is the next best as it creates a similar effect in that the sum of the gas partial pressures is unbalanced by the fact that some of the oxygen is metabolized, more in a fit person. The greater the difference between the oxygen and the other gases up to the max differential described by the metabolism ( maximum window ) , the greater the propensity for whatever is in the cells to come out and be displaced. For a fit person, the widow is wider and by definition so is his vascularity and perfusion, so he decompresses better. These things are all tied together.

You open the widow as wide as possible subject to 1) risk of tox or damage, 2) how long before the vaso constrictive effect offsets the benefit, 3) how long before the asthma like reaction sets in. You then alternate the process back to open up the vessels and lungs again, and repeat. All part of a good deco. Also it can be said that the sum of the inert gases is the other side of the oxygen window minus the metabolism drop of oxygen- there is no benefit to combining inerts - they act like one gas. Oxygen can be pushed to above its partial pressure effectiveness as a result of this imbalance for a "window" that then exceeds what would be the net effect of the partial pressures of the gases, and this is especially important in diminishing bubbles of inert gas as the pressure of the bubble can always be faced with a negative gradient or "tension" on the outside due to the fact that metabolized oxygen is creating a "vacuum" in the total sum of the partial pressures of the gases, leaving a consistent imbalance between bubble pressure and surrounding tension of any given inert.

This is why oxygen (pure, not 80/20) works so well in DCS cases after the fact to reduce bubbles, as well as the fact that saturation with oxygen tends to move that gas to where it is needed even if the vessels are blocked by damage.

Simple Explanation of the Oxygen Window | Global Underwater Explorers

 

[Diver0001]

I think there's something to it personally. I'll be happy to change my mind if someone shows me something more concrete other than, "we know more than you, go read a study that you've read before that has nothing to do with the O2 window." And I'm the one being accused of not listening?

I'm glad to hear this, Mike and I think that gives an opening.

Once again I wasn't trying to be rude to you and I have no bone to pick with you personally. Hell, we don't even know each other.

People almost always need to experience some kind of cognitive dissonance in order to shake them out of an "unaware" state in order for learning to be (re)started. In cases where people are really stuck in a paradigm lock making them feel that cognitive dissonance isn't easy and it isn't comfortable. I wasn't trying to play "king of the castle" with you, I was sincerely making an attempt to help you reach a mental state in which learning could (re)commence in the best way I could think to do on an internet forum.

In part this approach was also to break through the exchange of stand points from the trenches we were getting in this thread without any real progress being made in understanding.

I don't know if the result vindicates the approach but the fact that you said above that you are willing to listen is a BIG step further than you were 5 days ago or so when you joined this thread.

What I would recommend now is for you to listen very carefully to what John is saying. I'll also try to help you understand the significance of the NEDU study you have heard about in the context of RD if you are willing to be patient. There are also other NEDU studies to consider that you might not yet know about. I don't recall if any tested ratio deco specifically (I don't think so) because it's really a niche thing, but the conclusions of several top deco scientists, when you project it on to what you THINK you know about deco theory, will change how you think about this entire topic. I think in the end you'll be much better off if you can get to a point of thoroughly understanding what the research really means.

You have 10 years of catching up to do so please try to be patient as the discussion progresses from this point. A good starting point is to review the NEDU study that you have heard about again. It's all about deep stops, not the O2 window, but the ascent strategy involved in RD makes use of deep stops.

If you were to suspend "belief" for a moment as "assume" that NEDU is right then the obvious conclusion from the deep stops study is that deep stops are not "good" at all in the sense of it making deco more efficient. On the contrary, the addition of deep stops means that MORE time, not less time, needs to be spent shallow in order to compensate for an ascent strategy that keeps you deeper longer. On an intuitive level this might make some sense, but NEDU proved it.

If you are anything like me, the first time you read that study you're going to have a lot of questions. The research was specifically designed to highlight differences between different ascent strategies but they made dives on gasses and deco schedules that no technical diver would do in the real world. It took me reading that study multiple times and reading extensive discussions about it on the internet, including with the people who designed the study, for me to understand that the science really is solid and that conclusion really is relevant. At first I thought it designed to get a certain result, in other words, starting from a conclusion and working back to an experiment that reached that conclusion. That might be your impression too, but it's not the case.

So let's just start with that.

R..