Deco with too less air, options from the book

just read a book and beside the other option, of getting a other tank, taking o2 etc....
If you have for any reason (don't want to discuss why this shouldn't happen...) not enough air for your deco where to cut??

The book I read tells...don't cut the time on the deeper stops, as that might be harmful for brain and nerves. While missing the upper stops is less harmful.

For me that's very surprising. My first idea would be to think that everywhere are safety margins and cut on them.

For example Deco 2000 Table
48 Meter 19 Minute tells
12 Meter: 2 Minute
9 Meter: 3
6 Meter: 6
3 Meter: 14

Normaly I would also do 1 Minute at 15 Meter, in emergency of course I would not do it.

One of my ideas would be, fall back on the table values of 17 min at 48 meter
12:1
9: 3
6: 5
3: 11 and stay at 3 till the air is finished

Or take the original values and change on everything 1 Meter up and 1 minute less???

What do you think, or what does some software tell?

Your approach to learning decompression scares the hell out of me. Before you start messing around with decompression profiles you need to understand what is happening and why and where. You can not just start changing your decompression profile without understanding what you are doing. This is knowledge you should not get on the internet nor can you get this knowledge from reading one book. Find a creditable technical diving instructor and start at the beginning with a solid foundation of what decompression is and why we do what we do, where we do it. Then keep in mind that learning this theory is only part of the process: you also need to have the underwater skills (rock solid buoyancy, controlled ascent rates, etc) to carry out the decompression profile. This is not something you can learn on the internet. Incomplete knowledge and lack of skills will kill you. Period.

Bismark:

Your approach to learning decompression scares the hell out of me. Before you start messing around with decompression profiles you need to understand what is happening and why and where. You can not just start changing your decompression profile without understanding what you are doing. This is knowledge you should not get on the internet nor can you get this knowledge from reading one book. Find a creditable technical diving instructor and start at the beginning with a solid foundation of what decompression is and why we do what we do, where we do it. Then keep in mind that learning this theory is only part of the process: you also need to have the underwater skills (rock solid buoyancy, controlled ascent rates, etc) to carry out the decompression profile. This is not something you can learn on the internet. Incomplete knowledge and lack of skills will kill you. Period.

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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.

h90:

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.

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I am sorry you believe my advice to you is simple bashing. However your comments above provide further evidence that you don't know what you are talking about. Learning decompression theory does not require a masters degree in mathematics no more than driving and repairing your car requires an engineering and physics degree. Moreover, simply "running out of air" is ample proof that you would not have the diving skills and knowledge you would need to do decompression diving. Also, is your comment about "diving 48 meters and deco is not a high tech technical diving" based on your personal experience with technical diving or just something you have read on the internet? How about 45 minutes at 48 meters? Sound like something you would like to do or would recommend?

You can't learn this stuff from a single book or from the internet. You need some kind of context to place it in. Your question demonstrates that you do not have this background.

h90:

just read a book and beside the other option, of getting a other tank, taking o2 etc.... If you have for any reason (don't want to discuss why this shouldn't happen...) not enough air for your deco where to cut?? ... The book I read tells...don't cut the time on the deeper stops, as that might be harmful for brain and nerves. While missing the upper stops is less harmful.

Click to expand...

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.

h90:

A rock solid buoyancy and controlled ascent rates are, no question, won't help you if you don't have enough air

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There is ONE reason for not having enough gas - you didnt plan it properly.

Diving 48 meter and deco is not a high tech technical diving, it is for many non PADI people a normal thing.

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Correct - its routine. People actually PLAN the dives and the gas required so the crazy scenario you posted cant happen. There is no excuse for doing a dive and not having enough gas. None what so ever. If you cant plan gas usage you have no business diving. Even if you have taken leave of your senses you then send the yellow back up to get a drop tank. As taught everywhere.

Colliam7:

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, but I 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, 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.

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Colliam,

I thought about answering h90's question in the same fashion as you did, however, I decided that based on the fact that we did not know anything about H90's background and that this question was asked without any other context other than "just read a book about decompression" I elected to go the other route. My reasoning was that h90 was not going to be the only one reading this post and someone at a later stage of training might decide to take the answer as the gospel and apply it with out any further training. Kind of like the "I am not a surgeon, but I stayed at a Holiday Inn last night" approach to diving. I believe that too many divers think decompression diving is simply "read the book or follow the computer and you can't go wrong". These people become the statistics we read about in the accidents forum. The fact that the OP answered with "your just bashing me" as opposed to trying to explain more of what they were trying to accomplish further suggested to me that this person was not so much interested in learning as concerned for their ego. Just my thoughts anyway.

Bismark,

Other posts support your conclusions.

AZ

I had to thank Colliam for his thoughtful answer.
I appreciate and agree with the strong caution of the other posters too but this is "advanced scuba discussions" and a caution and discussion can be provided in the same reply (as Colliam did). Plus.. for a deco discussion, it was actually easy to understand.
I have spent almost my entire professional life analyzing various systems for failure points and planning the appropriate resolutions. Troubleshooting and considering various "what if" scenarios does allow for a better understanding of the system IMO and moves one further from the "read it in a book scenario". It doesn't mean one has to go out and dive that way and if the OP does so, after the cautions, that is his/her problem but the discussion itself is valid.

I'm not taking an adversarial stance here but adopting a "this should never happen" approach is self limiting IMO. Lot's of things shouldn't happen. Before the books, courses and tables people actually had to think about these things. Understanding how and why they arrived where they did can only adds to ones body of knowledge.

h90:

just read a book and beside the other option, of getting a other tank, taking o2 etc....
If you have for any reason (don't want to discuss why this shouldn't happen...) not enough air for your deco where to cut??

The book I read tells...don't cut the time on the deeper stops, as that might be harmful for brain and nerves. While missing the upper stops is less harmful.

For me that's very surprising. My first idea would be to think that everywhere are safety margins and cut on them.

For example Deco 2000 Table
48 Meter 19 Minute tells
12 Meter: 2 Minute
9 Meter: 3
6 Meter: 6
3 Meter: 14

Normaly I would also do 1 Minute at 15 Meter, in emergency of course I would not do it.

One of my ideas would be, fall back on the table values of 17 min at 48 meter
12:1
9: 3
6: 5
3: 11 and stay at 3 till the air is finished

Or take the original values and change on everything 1 Meter up and 1 minute less???

What do you think, or what does some software tell?

Click to expand...

I don't know what I would do in every scenario but my general reaction in most lost-gas cases would be to (a) try to calculate if I could finish my deco normally and do that if possible and failing that (b) ascend to my ceiling and try to make whatever gas I had left last as long as it could.

I don't think there is a good cook book answer to every lost gas incident beacause you probably never lose the same amount of gas each time and you'll never lose it all at once. The chances (especially diving in a team) of having a total loss of all deco gas is incredibly small.

Either way I don't know if my way is the right thing to do, I"m just telling you what my own personal reaction would be given zero time to prepare.

As for your book, you need to carefully consider who wrote it. Some deco theorists are *much* more interested in *appearing* to be experts in the real world applications of their theories than they are in the actual health and safety of divers. In fact, in some cases I think there is literally zero hard evidence that the things being perscribed actually work the way the theorist thinks they will, which leaves YOU in the roll of "Guinea pig" if you choose to blindly do what they say. I would advise skepticism because some deco theorists are trying really hard to get rich on their intellectual property and it gives me alarm bells (personal opinion).

Where does this leave you? In limbo, I'm afraid. Your scenario (skipping deep stops to extend shallow stops vs. doing deep stops and running out of gas sooner on shallow stops) is somthing that hasn't, as far as I know, been tested in proper laboratory conditions against *any* deco model.

R..