Manual calculation for accelerated deco

[victorzamora]

@lowviz and @tursiops: It seems like there's a miscommunication here, but I did some fiddling with numbers and got an interesting (to me, at least) result. 1 atm is a fixed value (29.92mmHg, 14.7psi, 33.9ffw, 10mfw, etc). What's interesting, though, is that lowviz's point is actually valid but in a goofy way.

For the purposes of talking about how deep you go as a ratio of your initial pressure, it seems to equate. What I mean is: I had a question for Dan_P earlier where I equated a 30m dive from sea level (to 4ata from 1ata) to a 24m dive from 2000m of altitude (to 3.2ata from 0.8ata).....they're both 4x increases in pressure. I did a quick proof and found that, for the pressure ratio to stay constant (pressure at max depth divided by pressure at the surface), treating the ambient pressure as an "atmosphere" (different from 1atm for the purpose of this thinking) is actually mathematically accurate.

So if diving from an altitude where the barometric pressure was 0.5atm (14.96mmHg, etc), you could treat one "doubling" or "one atmosphere" (again different from the unit of measure 1atm) as 0.5atm or 5mfw or 17ffw.

Your altitude calculation is back-referenced to sea level atmospheric pressure.

Diving at altitude is nothing more than changing the value that one atmosphere represents. Indeed, water does not change. So at a reduced atmosphere, it takes less depth to equal that 'atmosphere' that is sitting on top of the water. M values have to do with differences in pressure, not absolute pressures.

I read several of the preceding posts and am either impressed at the sarcasm or stunned at the confusion.

1 ATM is a unit of measure, and has exact equivalences in other systems of units.
It does NOT change with altitude, or sea level pressure.
If the surface pressure is 0.8 ATM, that does NOT mean that you get 1 ATM of water pressure above you at 8m; that amount of water only weighs enough to cause 0.8 ATM. You've got to go to 10m to get 1 ATM of water above you. and at that depth your total pressure is 1+0.8=1.8 ATM.

Please note that I said 'atmosphere', not ATM or ATA.

Thought experiment. Assume that we ascend to 380 mm Hg from the sea level 760. Half the atmospheric pressure at sea level. Dive to some great depth at that altitude and measure the pressure in pounds per sq inch. It will be very nearly the same as you would find at the same dive depth at sea level, just as John said.

Now we begin our ascent. The goal is to reach the air above the water with an appropriate gas saturation that will prevent bubbling. Even though there is the same psi at the bottom of either dive, the altitude dive is a much, much 'bigger' dive. I see this as very simply being under many more 'atmospheres' of pressure (at that dive depth when at elevation) as compared to fewer 'atmospheres' at that same dive depth when at sea level. It is nothing more than a change of reference point. You like everything referenced to sea level, it is more intuitive for me to reference to where I want to finally equilibrate. Works out the same either way.

 

[Dan_P]

Things are certainly making more sense. We'll likely never agree, but I at least understand your point.


The above has me concerned, because I think that altitude diving certainly doesn't benefit from deep stops any more than diving from Sea Level, and that you're actually increasing your risk by using the sea-level deep stops at altitude. Increasing your shallow stops makes sense, but it seems to me like the overall ascent profile is just extra flawed when diving from altitude. Again, though, we'll never agree but I believe I at least understand you now.


My point with bringing up your Schrodinger quote was that you created a strawman arguing that UTD's RD underemphasizes deep stops. I have not seen a single reference to that.

Fair enough.
I remain interested in, and open to, further developments in the area of hyperbaric medicine and enticipate with great eagerness more answers to our questions.
It wasn't my intention to create a straw man, I do however agree that my logic behind the Schrödinger-statement and what others may think the correct deep stop emphasis at various level may be, is incorrect.
I would delete the Schrödinger-statement (but, out of principle, at the same time leave it as is).

 

[Dan_P]

When I was a UTD diver diving at altitude, we were told to use Ratio Deco exactly as designed because there was no need to adjust it for altitude. You seem to be saying it is fine to use Ratio Deco at altitude, as long as you make appropriate adjustments. Appropriate adjustments seem to be lengthening the final stops. How long to you lengthen those stops? Does it depend upon the altitude?

John,
I do not mean this to be condescending, or insulting.
I'm all for a fair exchange, but after a number of discussions between yourself and I, I'm not genuinely convinced that you're interested in an even exchange on this subject.
As I have already answered a myriad of questions as to what my views are on this matter, I will assume that prerogative.

If I am incorrect (which I should be pleased to be, holidays and all), and you genuinely do wish an even discourse on matters decompression, I invite you to initiate by way of elaborating on which algorithm you know to be right, and whether you alter that algorithm by way of, say, gradient factors. If so, which gradient factor you know to be correct, and which scientific basis your refer to for evidence to substantiate your choice.

 

[boulderjohn]

If I am incorrect (which I should be pleased to be, holidays and all), and you genuinely do wish an even discourse on matters decompression, I invite you to initiate by way of elaborating on which algorithm you know to be right, and whether you alter that algorithm by way of, say, gradient factors. If so, which gradient factor you know to be correct, and which scientific basis your refer to for evidence to substantiate your choice.

I don't know any algorithm to be the precise right right one. There is obviously no one precise way to do it, or 99.99% of dives would end in decompression sickness. There is rather a range of profiles that can be used that are more or less likely to get divers out of the water safely, a range that probably looks something like a bell curve. At the outer reaches of that curve, a lot of divers will be OK, but some will probably get bent. In the middle, most people will be OK, but even then it is not perfect because we are all different. Some people can dive a wide range of profiles and be fine; some people have to be more in the sweet spot of the curve. The closer you are to the edge of the curve, the greater the probability you will get DCS.

For me and for my students, I want us to be as close to that sweet spot in the range as I can possibly be, and so I read and I read and I read. I do not accept dogma. I weigh the evidence, and I do what it says. Here is a brief history of my journey.

1. I started studying decompression theory long before I started technical diving. I knew the history of people like Haldane and Workman and Hill and Buhlmann and Spencer and Powell simply out of personal interest.
2. When I took my first technical diving class (TDI, but it was very much GUE DIR in approach), I was required to purchase Bruce Weinke's Technical Diving in Depth. That was a real struggle to read, because he assumes you are up to speed in calculus and other advanced topics, and I was not.
3. My tech instructor crossed over to UTD when that organization was created, and our entire tech group had to cross over with him. From that point on, we were required to use UTD's version of Ratio Deco on all our dives, all of which were at altitude. I was trained directly by Andrew Georgitsis on that, but only after I had been using it for quite a while.
4. As I delved into the course materials, much of the decompression theory seemed wrong to me, based on my previous and current reading. I had an exchange with Andrew in which I pointed out things I thought were wrong. He defended them at first, and then we all got an announcement saying that the course had errors in it, and we should wait for the new version of it.
5. As the number of DCS cases in our group began to mount, as friends started taking chamber rides (including a helicopter evacuation from our dive site), I grew more and more concerned. I began to argue vigorously, both with my instructor and with Andrew, that there were problems, especially with using it at altitude. He said there were no problems using it at altitude, and he knew that because he used it at Lake Tahoe with no problems. He also said no one had ever been bent using RD at altitude. I asked about all the people who were bent in our little group, and he said those didn't count. Why not? Because something else had caused the DCS. What? He didn't know, but it had to be something else, because using RD at altitude does not get you bent.
6. I started a thread on ScubaBoard about diving at altitude, and I mentioned all the DCS cases we had had in our UTD group. I got an email from my UTD instructor saying that if I wrote about that any more, I would be reported to PADI as violating the member standard against disparaging another agency, and I might face expulsion. I was not allowed to mention that people using UTD Ratio Deco had gotten bent unless I added that there certainly had to be a different reason for their getting bent, because Ratio Deco could not possibly be at fault. That finished me with UTD.
7. I crossed back to TDI, where I was required to use V-Planner. I did all my dives through Trimix certification and well after it with that program. When I became a TDI tech instructor, I taught using V-Planner.
8. I continued to read about decompression theory, and I learned about the growing disenchantment with deep stops. I bought my first tech computer (I had just been using a bottom timer and written tables before that). It was a Shearwater Predator with the Buhlmann algorithm, and I began to play with it and compare it to V-Planner. I started to become convinced that I did not want to stop as deep as V-Planner does.
9. At about that time, my old UTD instructor and another member of our group did a very high altitude dive in which the instructor's DCS was so bad that he was paralyzed for 3 months, and the other diver's body was never recovered. I do not know how they adjusted their RD for that dive, but I believe they tried to. (Their problems happened in the early stages of decompression.)
10. I am now diving Buhlmann ZHL 16-C with GFs of 50/80. This gives me much shallower deep stops than V-Planner, and much, much shallower stops than the old UTD Ratio Deco. I understand that after the recent study made UTD's Ratio Deco look pretty bad, they have changed it to make the first stop at 66% of average depth rather than 75%. That is still deeper than I want to have that first stop.

After more than 100 years of study, decompression scientists are getting closer and closer to the sweet spot of that curve. I will keep reading about it. I will keep making informed decisions. What I will not do is accept a rigid plan on faith rather than reason.

 

[Dan_P]

I don't know any algorithm to be the precise right right one. There is obviously no one precise way to do it, or 99.99% of dives would end in decompression sickness. There is rather a range of profiles that can be used that are more or less likely to get divers out of the water safely, a range that probably looks something like a bell curve. At the outer reaches of that curve, a lot of divers will be OK, but some will probably get bent. In the middle, most people will be OK, but even then it is not perfect because we are all different. Some people can dive a wide range of profiles and be fine; some people have to be more in the sweet spot of the curve. The closer you are to the edge of the curve, the greater the probability you will get DCS.

John, thanks for your post.

You are right and I agree that there is no written-in-stone answer to decompression. Or rather, it or they evade us still.
We are also in full alignment as to our attitude towards both pursuing knowledge and intolerance to dogma.
And I am genuinely sorry to hear that you've had a bad experience with UTD.

You're right, neither of us know for sure what's ultimately correct in decompression.
Personally, I do sacrifice some accuracy in mine, for the sake of a standardized form that I think excels in practical application, for a number of reasons that I obviously choose to prioritize.
That said, it was never my faith that Ratio Deco offered a more "optimal" decompression, but rather a "standard" one.

I have always held that there are probably algorithms out there that most likely get divers out of the water cleaner for most depths/times/gases, but I have simply chosen to use a system of depths, times and gases that works together and I can expand and adjust as I learn.
I'm not saying that has no downsides - I choose to accept them, and I choose to accept that the next diver doesn't.

RD 1.0 started deeper than RD 2.0, and also shaped the s curve with more emphasis on the deep stops - so generally, quite a lot of a shift in terms of deep stop emphasis, but all that aside, I appreciate a system that is easy to work with in terms of depth/time-ratios, gas requirements/logistics, OC/CCR interoperability, training progression, etc., to a degree where I will accept other shortcomings, but:

As for the altitude question, I have honestly never heard that there should be no adaptation to RD at altitude. Hence my genuine surprise.

It is correct that the Italy Project, though supported by UTD, didn't bring the results that deep-stop proponents were probably expecting. RD1.0 was measured against a GF 30/80 and didn't do any better in terms of bubbles.
It didn't do as well in terms of inflammation, and even though we don't know what those mean, I'm guessing it's fair game to put it in the "not great"-category.

The emphasis on deep stops was changed quite a lot, with Ratio Deco 2.0 (shallower first stop, and more shallow s-curve distribution), but the holistic approach was maintained.

As for me, I don't buy into dogma. I do use a tool that I find practical, but I harbour no illusions that it's a "perfect" algorithm of some kind.
It's a tool and it came from algorithms - or rather, relationships in them, but it is more crude than an algorithm is.
I use that tool to create an ascend.
To illustrate, say I have 20 minutes to distribute and Ratio Deco distributes 15 of them in a standard framework. I'll need to allocate the remaining 5 - chances are, I'll do so shallow.
That's a call, based on what I've read and learned and accepted or not accepted.

But, from what you describe, it doesn't sound to me as though you mind terribly Ratio Deco as a general concept, rather specifically the altitude question..?
If you don't mind me asking about that, the altitude bit, I am curious, and I do not mean for this to come across as though I'm dismissive:

1) It sounds like you had a terrible time with your instructor - I assume this was the same instructor who got injured.
I did hear of one instructor, who was expelled for breaking standards, and a year or so later had a terrible injury. I don't know if it's the same person, but it does sound like describing the experience as "not great" would be saying the least...

2) I'm a bit out of my depth here (no pun intended) as I don't know any details, but i'm just thinking if their problems started in the early stages of decompression, how does that align with the issues we've spoken about, relating to altitude diving?

 

[Dr Simon Mitchell]

Hi Guys,

The driver for bubble formation in tissues and blood after surfacing from a dive is dissolved gas supersaturation.

Supersaturation is the sum of the partial pressures of all gases dissolved in a tissue (inert gas + oxygen + CO2 + saturated water vapour pressure) minus the ambient pressure.

On an altitude dive the uptake of inert gas into blood and tissues during the dive is virtually identical to a sea level dive because the requirement to breathe gas at increased pressure is dictated by the pressure exerted by the water column above the diver. Water weighs the same at altitude, and so aside from a tiny difference in the weight of the atmosphere above the water, the pressure of inspired gas at depth (and the absorption of inert gas contained therein) is therefore not materially affected by altitude.

HOWEVER, if the diver were to ignore the effect of altitude and arrived back at the surface with the same pressure of dissolved gas in their tissues as on a sea level dive then, based on the simple bolded equation above, the surfacing supersaturation will be greater because the ambient pressure is less. Thus, there will be a greater driver for bubble formation than there would be if the surface ambient pressure were higher.

There are a couple of corollaries in respect of various discussions that have take place on this thread.

First, if you take a fixed decompression procedure and use it on two dives where all relevant factors are equal except the altitude, then the risk of DCS will be higher in the dive at higher altitude. That does not mean DCS will occur on the altitude dives, and if the profile is lowish risk at sea level, then it would be no surprise that it might not cause DCS at altitude. But we should not kid ourselves that the risks are equal.

Second, and this comment is mainly for Dan, the notion that deep stops are more logical or better supported on an altitude dive is almost certainly wrong. The reasons are largely the reasons that have been discussed in relation to deep stops in the wider (non-altitude) context, and their effect on tissue supersaturation patterns on surfacing from a decompression dive. The inevitable effect of taking two decompression profiles of the same length from the same dive, and distributing stops deeper in one of them, is that you protect the fast tissues from supersaturation early in the ascent in the deeper stop dive, but at the expense of greater supersaturation in the slower tissues later in the ascent. The most plausible explanation for the adverse outcomes in deep stop dives is that these slower tissues are the places where bubbles form, and those bubbles are the source of the problem. Put another way, to at least some extent, protecting slower tissues seems more important than protecting faster tissues.

SO, if you emphasise deep stops on a decompression dive at altitude and arrive at the surface with higher gas loading in the slower tissues, then the supersaturation in those tissues will be even greater than at sea level because you are at altitude. Given that the slower tissues appear to be where the problems arise, this seems like the opposite to what you would want to do.

At several points you have invoked "bubble mechanics" as the reason why deep stops are more important at altitude. I presume you are alluding to the presumption that deep stops control bubble formation, but this is a largely discredited concept now, almost certainly because the slower tissues appear to be where most of the bubbles form and these are disadvantaged by deep stops. Indeed virtually all the relevant human evidence in decompression from equivalent dives suggests that more bubbles form when deep stops are emphasised.

Hope this makes sense,

Simon M

 

[lowviz]

@Dr Simon Mitchell

I greatly appreciate your posting in this thread. Thank you.

One question.

Is it fair to assume that a given decompression protocol would yield the same statistical result if the stop depths were simply scaled with respect to the reduced ambient pressure over the water column?

 

[Dan_P]

Hi @Dr Simon Mitchell and thanks for your post.

I understand the message you put across, and would like to take the chance to underscore a point that I've probably not been clear enough on, previously.
First, it makes sense to me to do more decompression on an altitude dive.

Second, as there is already a comparatively larger emphasis on deep stops in the Ratio Deco framework, I allocate that additional time shallow.

I understand from your presentations and those of others that the relationship between bubble expansion from ascending and bubble formation related to slow tissues, is scewered differently in dives with more emphasis on deep stops - leading to reduced emphasis on deep stops compared to previous views (whether RD 1.0, VPM, RGBM, etc.).

On altitude dives, I add arbitrarily shallow stops. Following, this does naturally shift the emphasis of a such profile upwards, in accordance with what you are saying and I believe.
Overall, the result of the practice I apply therefore adds time and shifts emphasis up, when diving at altitude.

I do agree that you are bang-on in pointing out that my logic for doing so, was poorly presented.

Further, I thank you for sharing your insights on the matter.

As for the use of a framework to make easier the practical dive planning and adjustments, I believe this to be a separate discussion on prioritization of accuracy versus practicality.
On a finishing remark, I remain satisfied that in either case, decompression safety is within reason, to the best knowledge available at this time.

 

[EFX]

Regarding decompression; I want to understand how to calculate accelerated decompression. In other words; using Buhlmann's tables I can calculate decompression stops for a dive if back gas is air. In case of Nitrox; I simply use EAD with Buhlmann's tables to figure out deco stops. But in real world; we use rich mixes (EANx50 and higher) to accelerate decompression at shallower depths (12, 9, 6 meters). Simply I can use computer software like MultiDeco to calculate the profile, but I'm eager to know exactly how to calculate this acceleration by hand. Thanks so much.

I'm close to releasing my spreadsheet that will show TC pressures for nitrogen and helium. It will calculate deco ascent schedules if one is needed using two different nitrox deco gasses. You'll be able to select imperial or metric units and fresh or salt water. You'll be able to see the calculations in the VB code of the ss. I'll post it in the advanced forum and here.

 

[Dr Simon Mitchell]

Is it fair to assume that a given decompression protocol would yield the same statistical result if the stop depths were simply scaled with respect to the reduced ambient pressure over the water column?

Hello Lowviz,

No, I don't think so. As I alluded to above, the problem at altitude is that the diver will absorb inert gas at virtuallly the same rate (for a given depth and time) as they would at sea level because every metre of depth still adds the same increment gauge pressure, the regulator / rebreather will still supply gas at that ambient pressure, and the inspired inert gas pressure will be almost the same as at the same depth at sea level. Then on surfacing at altitude the ambient pressure is lower and the supersaturation is therefore higher. To avoid this, the diver needs to do more outgassing before they hit the surface (and quite possibly before they hit the shallowest stops). This will require more time. Simply scaling stop depths would not achieve that.

I think one of the things that people get hooked up on is the role that the small reduction in atmospheric pressure at altitude plays in all this. Put simply, it has little role in influencing inert gas absorption at depth, but it has a potentially significant role in determining tissue supersaturation back at the surface. For example, if we are at an altitude where the atmospheric pressure is 0.8 atm, then at a depth of 50m (165') the ambient pressure will be 5.8 atm absolute instead of 6 atm absolute for the same dive at sea level. This difference of 0.2 atm only represents a 3% reduction in ambient pressure, so inert gas uptake will be virtually unchanged. In contrast, if you decompressed from this dive using exactly the same protocol as you did at sea level, then you would arrive at the surface with potentially 20% greater tissue supersaturation because the ambient pressure is 0.8 atm instead of 1 atm.

Simon M