Deco with too less air, options from the book

[Charlie99]

In the theoretical situation you described, you want to do the most efficient deco (as measured in gas rather than in time) with the limited gas you have.

One way to get a qualitative feel is to simply make several runs of your profile on a deco program such as V-planner. Rather than fudge numbers by decreasing bottom time, I would leave bottom time as you plan, and decrease the conservatism setting and watch what happens.

On the few runs like this that I've looked at, the overall curve of ascent vs time stays pretty much the same, except that maybe one or two very deep stops are dropped and the whole ascent is compressed in time. Optimizing the ascent for gas rather than time would further push it a bit towards the shallow side.

 

[h90]

I have a different take on the OP's question than Bismarck, although I may be off base. I don't see the question as reflecting someone who is trying to learn decompression theory and practice solely off the net, or from a single book, after which he/she will jump in the water, drop to 160 feet, and start using abbreviated decompression schedules (H90, if that is your intent, then I have to fully agree with Bismarck's reaction.). Rather, I see the post as an attempt to better understand something recently read, that does not intuitively make sense. Perhaps, the specific wording of the question creates a bit of a misimpression, and I don't know if English fluency plays any role here (h90, it would be helpful if you add a bit of meat to your profile, by the way). But, asking questions to better understand concepts, or to satisfy curiosity, should be part of what SB is all about. My read of the question is, 'THEORETICALLY, if for whatever reason, you find yourself in a situation where you cannot complete a decompression schedule as planned because of a gas availability issue, is it better to do the intended deeper stops as planned, realizing that you may have to cut short or eliminate some of the very shallow stops because you will run out of gas? Or, is it better to get shallow sooner, by cutting time at deeper stops, and use more of your available gas at a shallower depth?' A very reasonable question, certainly one that is frequently asked during technical dive training, but also one that is not by any means out of bounds for the curious recreational diver to ask. And, a reasonable answer is (as with so many things), 'It depends.' On the backgas you are using, on the amount of remaining gas you have available, on the dive environment, etc. In general, if you have a decompression obligation that you know you cannot meet, and the likelihood of having support resources available to alter that situation (e.g. you have no buddy, no support divers, whatever) is minimal / nonexistent, most divers will want to get as shallow as they reasonably can, as soon as you can, SAFELY. For example, if you are diving air as a backgas, you may be able to ascend to a shallower depth sooner (and spend more time and remaining gas at a shallower depth), than if you are diving a helium-based mix. But, that is a general precept, not an absolute statement. If I know I cannot complete my obligation, if I know I am low on gas, if I am still losing gas, for example, where do I want to be when I run out - at 90 feet, or 60 feet, or at 20 feet. I do not want to get bent by rapid ascent from depth, where I simply ignore planned deeper stops. But, I also do not want to drown or get bent attempting a CESA from 90 feet. That is a matter of practicality. On the other hand, what stop depths will do me the most good, balancing higher air consumption at depth against the physics of gas diffusion / bubble formation? That is where running a series of simulations with decompression software can be informative. Using desktop decompression software as a learning tool is perfectly reasonable. It is NOT a substitute for appropriate training, it does not replace a good instructor, it is not a surrogate for reference texts, and it is possible to wrongly interpret the calculated information out of context, etc. I agree fully with Bismarck on that point. But, you can learn from posing a series of 'what if' questions and running simulations to see what answers you get.

Yes on such questions I have some problems explaining it in english so it can be understood....
As more realistic example: in Croatia the "Francesa di Rimini" (not sure for the spelling) wrack is at 47 meter and some Dive Center go there with guides frequently. Usually they dive with a 15 liter tank and a short deco. It is seen as advanced recreational diving there (most have CMAS background not PADI). A lot safety there and as far as I know for all the years not a single accident ever happend.
But what if an very unlikey situation happens? (an accident). I can calculate the air usuage on land, I doubt I can calculate it under water while making the deco.
On the deco software it looks the best to move everything slightly up and slightly shorten it.
What me disturbs in the book is that thinking of which tissue to protect against bubbles and which not. While I understand the intention: "better have a problem on your skin than in your brain.", I can't figure out how the writer came to the result to better keep the deep stops and drop the shallow ones as a generell rule of the thumb.
I personaly don't do any deco dives which needs more than 2-3 minutes, but I think it is good to have a theoretic knowlege a few steps ahead of what doing in the real diving.

 

[Bismark]

Yes on such questions I have some problems explaining it in english so it can be understood....
As more realistic example: in Croatia the "Francesa di Rimini" (not sure for the spelling) wrack is at 47 meter and some Dive Center go there with guides frequently. Usually they dive with a 15 liter tank and a short deco. It is seen as advanced recreational diving there (most have CMAS background not PADI). A lot safety there and as far as I know for all the years not a single accident ever happend.
But what if an very unlikey situation happens? (an accident). I can calculate the air usuage on land, I doubt I can calculate it under water while making the deco.
On the deco software it looks the best to move everything slightly up and slightly shorten it.
What me disturbs in the book is that thinking of which tissue to protect against bubbles and which not. While I understand the intention: "better have a problem on your skin than in your brain.", I can't figure out how the writer came to the result to better keep the deep stops and drop the shallow ones as a generell rule of the thumb.
I personaly don't do any deco dives which needs more than 2-3 minutes, but I think it is good to have a theoretic knowlege a few steps ahead of what doing in the real diving.

A 150' dive on a single tank that puts you into deco? Lets see what some of the others on SB think of this. Does the phrase "putting all your eggs in one basket" come to mind for anyone else?

H90, when things go wrong on this dive with that equipment, they are going to go very wrong.

I would also suggest that you learn how to calculate your air consumption at that depth. It is not difficult.

 

[texdiveguy]

Yes on such questions I have some problems explaining it in english so it can be understood....
As more realistic example: in Croatia the "Francesa di Rimini" (not sure for the spelling) wrack is at 47 meter and some Dive Center go there with guides frequently. Usually they dive with a 15 liter tank and a short deco. It is seen as advanced recreational diving there (most have CMAS background not PADI). A lot safety there and as far as I know for all the years not a single accident ever happend.
But what if an very unlikey situation happens? (an accident). I can calculate the air usuage on land, I doubt I can calculate it under water while making the deco.
On the deco software it looks the best to move everything slightly up and slightly shorten it.
What me disturbs in the book is that thinking of which tissue to protect against bubbles and which not. While I understand the intention: "better have a problem on your skin than in your brain.", I can't figure out how the writer came to the result to better keep the deep stops and drop the shallow ones as a generell rule of the thumb.
I personaly don't do any deco dives which needs more than 2-3 minutes, but I think it is good to have a theoretic knowlege a few steps ahead of what doing in the real diving.

In short..... any dive to 154ft. on just a single tank with no form of redundant gas is not an advisable thing. Hopefully these divers have taken all the risks into account, but sounds like not.

Regardless......a short dive to 154ft. on air that a medium size single tank would afford will only rack up limited deco (again not advisable on only a single tank)....your first deco stop will be at 80ft.. There are several popular deco models out in the field.....they are all different in some regard to each other (bubble development), some more than others! Without me going into great detail, it is to be remembered that the accepted ones have all shown good results, but none are 100%, and do not guarantee the diver protection totally from DCS. With this in mind and considering the above 154ft. dive, it is better to maintain lesser deco time at the deeper stops and longer deco time at the shallower stops.

*Don't engage in decompression diving without proper training and gear, and never dive well beyond your current experience level.

There is a huge amount to consider on decompression dives, each is a formal affair.....it goes well beyond a few books and a deco software program.

 

[Colliam7]

Yes on such questions I have some problems explaining it in english so it can be understood....

Not a problem. Even for those of us for whom English is a native language, fluency and clarity are not always assured.

What me disturbs in the book is that thinking of which tissue to protect against bubbles and which not. While I understand the intention: "better have a problem on your skin than in your brain.", I can't figure out how the writer came to the result to better keep the deep stops and drop the shallow ones as a generell rule of the thumb.

Thanks for the clarification. I think I better understand the specific focus of your question - do deep deco stops vs shallow deco stops affect bubble formation in different tissues (e.g. CNS tissue vs skin, as in your post), and are data available which support the existence differential bubble formation / dissolution rates. One suggestion: in the Marine Science and Physiology forum on SB, there is a sub-forum 'Ask Dr. Decompression'. You might want to post your specific question there, as there may be more specific expertise on the topic available than we can offer here. Also, could you share the title of the particular book you mention, that has caused you to raise this issue?

 

[Blackwood]

What me disturbs in the book is that thinking of which tissue to protect against bubbles and which not. While I understand the intention: "better have a problem on your skin than in your brain.", I can't figure out how the writer came to the result to better keep the deep stops and drop the shallow ones as a generell rule of the thumb.

I'd be very surprised if the theory the author is applying isn't mentioned in the book.

In short, it's commonly believed that the CNS-related tissues are faster (gas loading/unloading rate) than tissues like fat, skin and bone. As such, if the goal is to prevent a CNS hit (Type II), the theory suggests that clearing those tissues deep and not allowing bubble formation that can cause problems when they grow (in the shallows) is a good approach.

 

[HowardE]

What about...

Doing the deeper stops, on schedule...
because as I was taught as well - the deeper deco stops are for the deeper tissues (brain, nerves) as the OP's book says...

Then if the situation comes to play where you're out of gas on the shallower stop... Ascend to the boat (if this is a boat dive) grab an extra tank, and go back down, and add additional time to your deepest remaining deco stop in the dive plan.

 

[MXGratefulDiver]

I suspect the quote in the OP's book was taken out of context. I can think of a few reasons (medical issue, torn drysuit, etc) for wanting to cut a deco schedule short. And my training did include the advice that a type 1 hit is preferable to a type 2 hit ... meaning that I'd probably favor keeping the deep stops and skrimping on the shallower ones. But that also would have to take into account what type of gas I DID have available. AFAIK, there's no cut-n-dry answer to the question ... it all depends on the specifics of the dive and what resources you have at your disposal.

But if you're cutting out deco obligation because of an inadequate gas supply ... meaning neither you nor your buddy considered reserves and contingencies for the dive ... then you're taking far greater risks than just getting bent.

... Bob (Grateful Diver)

 

[Steve SDI/TDI HQ]

Well I would say the knowledge of the theoretic background you'll find only in books as there won't be many technical diving instructors with a master degree in mathematics as the models are not simple. A rock solid buoyancy and controlled ascent rates are, no question, won't help you if you don't have enough air and about that was the chapter in the book and my questions. Your answer is a simple bashing without any new suggestion/information.
Diving 48 meter and deco is not a high tech technical diving, it is for many non PADI people a normal thing. But if you re-read the original posting, you won't find "dive 48 meter and what happens", this here is a theoretic question about what to do in an unlikely case.

I am a technical instructor-trainer AND I have a master's degree in mathematics (applied physics) and I understand just enough about decompression algorithms to say that ANY modifications one makes to an ascent schedule puts them outside the purview of the tables and therefore statistically more likely to suffer decompression stress... however, your original post was about not having sufficient gas to deco fully... simply not an option. Even a moderately well-trained technical diver should know enough about gas management to ensure this would not happen. For the record, in more than 1500 staged decompression dives and more than 700 trimix dives, I've never come close... and on several occasions have had things go wonky with regs etc during deco.

Normal practice is to 1/ understand the decompression curve as part of the dive plan... have contingency tables and bailout schedules memorized 2/ plan exposures around operating limits (gas, experience, equipment, training etc.) and be conservative so that an abbreviated ascent is "possible" within the context of the algorithm being used.

 

[Rick Inman]

I enjoy a good theoretical discussion - the, "what if" scenario that should never happen. This one always leads to the two alternatives of doing the deep stops and blowing off the shallow, or blowing off the deep and rushing up to shallow.

There is a reason they call it, "bend and mend". And there is a reason, when riding the chamber, that they don't just take you down to 20'.

The scenario requires you to be a poor gas planner, solo deco diving, have equipment failure or horrible skills, and all kinds of junk. If you have some kind of medical emergency that requires you to exit the water now, you're just going to blow it all off anyway and head for the hospital.

I had a recent dive where I made a questionable decision and had less gas in my own bottle than I would have wished (still more than enough) , but between the team we always have gobs of extra gas for anyone who needs it.

Every day, every diver's body, and every dive is different. And no one can know for sure what will happen to a diver on any given day should they blow off some amount of deco. Best use of time is to work on making sure that doesn't happen.