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Thread: Deep dives with rich He mix - do you change to another mix or not?

  1. #121
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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Personally I do switch gas and flush the loop in order to have affect.
    My reason to do so is this will give me a larger margin to DSI, and most importantly due to diving in cold water, extend the duration of a dive can lead to freezing and will increase the risk of DSI.

    As already mentioned OC divers have made inert gas switching to accelerate decompression for decades without any significant increase in frequency of inner ear DSI. I'm aware of Simon J Mitchell's report and should be taken into consideration, also be aware of Dr. Alf Brubakk made a research back in 2006 regarding gas switching.
    Not sure on the number of experiments made which the statistics are based upon. Anyway he concluded ideal reduction of He fractions during gas switching is within halving the fractions to get the best effect, 70-35-15% He.
    Dr. Alf Brubakk | Global Diving Conference

    It is also well documented many years ago in commercial diving that "bounce diving" e.g. scuba diving increases the risk of DSI dramatically
    Last edited by mixgas; 28th September 2012 at 20:10.

  2. #122
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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Quote Originally Posted by brandonmc  View Original Post
    I dont think anyone is saying gas switching is "dangerous", just not worth the small (real or percieved) benefit of a few minutes off deco in a normal CCR ascent. You may be correct that for the majority of dives IBCD is probably not a big concern with a reasonable selection of gases but why not eliminate any potential risks where we can and KISS? In OC or in a bailout situation we are willing to live with additional "risks" because that is the nature of OC.
    what he said

  3. #123
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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Perhaps I should add a few words to this discussion. I added the IBCD warnings into V-Planner in 2004, and it has been a controversial topic ever since.

    At the time, there were several really deep dives (200m+) that had gone seriously wrong, so I wanted to find out why. I gathered up all the plans and profiles from good and bad deep dives, and looked at the changes in ascent for inspired partial pressure. The troubled dives all had big increases or reversals in pp inert values. It seemed obvious why it had gone wrong.

    So I added the 0.5 setting into V-Planner (it's user adjustable). The 0.5 seemed to fit quite nicely with the dive results I could see. In the years since, I have not seen a dive outcome that would contradict the need to avoid high IBCD spikes.


    The IBCD problem to me would seem to be like most things in nature - That is: A little of something is OK and acceptable, and a lot of it will hurt you.
    Humans seem to have a little natural tolerance for IBCD, and exceeding this will get you seriously injured.

    Now for those who say "I got away with it", then you are just lucky, or still inside the natural tolerance area. For CCR divers the quality or extent of the flush has a big bearing on it too. A weak flush is not going to change the loop content by much, and therefore not really experiencing the planned dil swap.

    Also CCR divers often fall into a planning trap when they swap to OC bail - by comparing the dil mix to the OC mix, when it should be comparing the inspired mix vs the OC mix. That has caught out a few I know of.

    The problem of planning a flush into a deep plan is this: The math in the algorithm can handle this big exchange of gases, and get you sped up. But what about the body's ability to follow? Its unlikely the tissues can keep up, and so a DCS event will follow.
    This is an area of deco modelling for which we have no calibration data (all deco calibration is done with profiles that are gently down, gently up, on one gas - not swapping it all around in the middle). The obvious solution for now, is to avoid this danger area in our planning.

    For the more normal depths of ordinary deco dives, where we can tolerate the gas swaps, we do take advantage of the sped up deco time (called accelerated deco), and we do it every day with success. Of course we are only making smaller pp inert changes and for short time frames. However, the deeper we go, and the deeper the deco becomes, at some point we must limit our pp inert swap pressures. Hence the 0.5 limit.

    I have some diagrams on this here

    regards
    Last edited by rossh; 1st October 2012 at 01:39.
    Ross Hemingway
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  4. #124
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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Quote Originally Posted by rossh  View Original Post
    Perhaps I should add a few words to this discussion. I added the IBCD warnings into V-Planner in 2004, and it has been a controversial topic ever since.

    At the time, there were several really deep dives (200m+) that had gone seriously wrong, so I wanted to find out why. I gathered up all the plans and profiles from good and bad deep dives, and looked at the changes in ascent for inspired partial pressure. The troubled dives all had big increases or reversals in pp inert values. It seemed obvious why it had gone wrong.

    So I added the 0.5 setting into V-Planner (it's user adjustable). The 0.5 seemed to fit quite nicely with the dive results I could see. In the years since, I have not seen a dive outcome that would contradict the need to avoid high IBCD spikes.


    The IBCD problem to me would seem to be like most things in nature - That is: A little of something is OK and acceptable, and a lot of it will hurt you.
    Humans seem to have a little natural tolerance for IBCD, and exceeding this will get you seriously injured.

    Now for those who say "I got away with it", then you are just lucky, or still inside the natural tolerance area. For CCR divers the quality or extent of the flush has a big bearing on it too. A weak flush is not going to change the loop content by much, and therefore not really experiencing the planned dil swap.

    Also CCR divers often fall into a planning trap when they swap to OC bail - by comparing the dil mix to the OC mix, when it should be comparing the inspired mix vs the OC mix. That has caught out a few I know of.

    The problem of planning a flush into a deep plan is this: The math in the algorithm can handle this big exchange of gases, and get you sped up. But what about the body's ability to follow? Its unlikely the tissues can keep up, and so a DCS event will follow.
    This is an area of deco modelling for which we have no calibration data (all deco calibration is done with profiles that are gently down, gently up, on one gas - not swapping it all around in the middle). The obvious solution for now, is to avoid this danger area in our planning.

    For the more normal depths of ordinary deco dives, where we can tolerate the gas swaps, we do take advantage of the sped up deco time (called accelerated deco), and we do it every day with success. Of course we are only making smaller pp inert changes and for short time frames. However, the deeper we go, and the deeper the deco becomes, at some point we must limit our pp inert swap pressures. Hence the 0.5 limit.

    I have some diagrams on this here

    regards
    Thanks for your thoughts Ross.

    Regards,
    Randy

  5. #125
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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Quote Originally Posted by rossh  View Original Post
    Perhaps I should add a few words to this discussion. I added the IBCD warnings into V-Planner in 2004, and it has been a controversial topic ever since.

    At the time, there were several really deep dives (200m+) that had gone seriously wrong, so I wanted to find out why. I gathered up all the plans and profiles from good and bad deep dives, and looked at the changes in ascent for inspired partial pressure. The troubled dives all had big increases or reversals in pp inert values. It seemed obvious why it had gone wrong.

    So I added the 0.5 setting into V-Planner (it's user adjustable). The 0.5 seemed to fit quite nicely with the dive results I could see. In the years since, I have not seen a dive outcome that would contradict the need to avoid high IBCD spikes.


    The IBCD problem to me would seem to be like most things in nature - That is: A little of something is OK and acceptable, and a lot of it will hurt you.
    Humans seem to have a little natural tolerance for IBCD, and exceeding this will get you seriously injured.

    Now for those who say "I got away with it", then you are just lucky, or still inside the natural tolerance area. For CCR divers the quality or extent of the flush has a big bearing on it too. A weak flush is not going to change the loop content by much, and therefore not really experiencing the planned dil swap.

    Also CCR divers often fall into a planning trap when they swap to OC bail - by comparing the dil mix to the OC mix, when it should be comparing the inspired mix vs the OC mix. That has caught out a few I know of.

    The problem of planning a flush into a deep plan is this: The math in the algorithm can handle this big exchange of gases, and get you sped up. But what about the body's ability to follow? Its unlikely the tissues can keep up, and so a DCS event will follow.
    This is an area of deco modelling for which we have no calibration data (all deco calibration is done with profiles that are gently down, gently up, on one gas - not swapping it all around in the middle). The obvious solution for now, is to avoid this danger area in our planning.

    For the more normal depths of ordinary deco dives, where we can tolerate the gas swaps, we do take advantage of the sped up deco time (called accelerated deco), and we do it every day with success. Of course we are only making smaller pp inert changes and for short time frames. However, the deeper we go, and the deeper the deco becomes, at some point we must limit our pp inert swap pressures. Hence the 0.5 limit.

    I have some diagrams on this here

    regards
    what he said

  6. #126
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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Quote Originally Posted by rossh  View Original Post
    Perhaps I should add a few words to this discussion. I added the IBCD warnings into V-Planner in 2004, and it has been a controversial topic ever since.

    At the time, there were several really deep dives (200m+) that had gone seriously wrong, so I wanted to find out why. I gathered up all the plans and profiles from good and bad deep dives, and looked at the changes in ascent for inspired partial pressure. The troubled dives all had big increases or reversals in pp inert values. It seemed obvious why it had gone wrong.

    So I added the 0.5 setting into V-Planner (it's user adjustable). The 0.5 seemed to fit quite nicely with the dive results I could see. In the years since, I have not seen a dive outcome that would contradict the need to avoid high IBCD spikes.


    The IBCD problem to me would seem to be like most things in nature - That is: A little of something is OK and acceptable, and a lot of it will hurt you.
    Humans seem to have a little natural tolerance for IBCD, and exceeding this will get you seriously injured.

    Now for those who say "I got away with it", then you are just lucky, or still inside the natural tolerance area. For CCR divers the quality or extent of the flush has a big bearing on it too. A weak flush is not going to change the loop content by much, and therefore not really experiencing the planned dil swap.

    Also CCR divers often fall into a planning trap when they swap to OC bail - by comparing the dil mix to the OC mix, when it should be comparing the inspired mix vs the OC mix. That has caught out a few I know of.

    The problem of planning a flush into a deep plan is this: The math in the algorithm can handle this big exchange of gases, and get you sped up. But what about the body's ability to follow? Its unlikely the tissues can keep up, and so a DCS event will follow.
    This is an area of deco modelling for which we have no calibration data (all deco calibration is done with profiles that are gently down, gently up, on one gas - not swapping it all around in the middle). The obvious solution for now, is to avoid this danger area in our planning.

    For the more normal depths of ordinary deco dives, where we can tolerate the gas swaps, we do take advantage of the sped up deco time (called accelerated deco), and we do it every day with success. Of course we are only making smaller pp inert changes and for short time frames. However, the deeper we go, and the deeper the deco becomes, at some point we must limit our pp inert swap pressures. Hence the 0.5 limit.
    I have some diagrams on this here

    regards

    Coud you please give an indication of the depth range for "more normal dives"




    ATB

    Mark

  7. #127
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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Quote Originally Posted by Mark Chase  View Original Post
    Could you please give an indication of the depth range for "more normal dives"
    ATB
    Mark
    Hi Mark,

    Here I'm referring to the normal deco dives we do every weekend - the 60 -80m stuff with modest amounts of deco.
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  8. #128
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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Enormous amount of food for thought in this thread. Has been quite a slog to keep up with all the posts, but very much worth it. The sense I'm getting is that dil switching is not just a two sided coin (fast/ slow) but more of a many faceted n-dimensional dice cube. All said, this has to be my biggest take away:

    Quote Originally Posted by DGraham  View Original Post
    I truly do believe that if you going to do the dive, just do the time. It is not about todays dive, but the dives I want to make with my Grandson in 15 years from now.
    Well said.

  9. #129
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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Quote Originally Posted by rossh  View Original Post
    Hi Mark,

    Here I'm referring to the normal deco dives we do every weekend - the 60 -80m stuff with modest amounts of deco.
    hi i just use 6/80 it deos me no need to change

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    Re: Deep dives with rich He mix - do you change to another mix or not?

    Quote Originally Posted by Skipbreather  View Original Post
    Enormous amount of food for thought in this thread. Has been quite a slog to keep up with all the posts, but very much worth it. The sense I'm getting is that dil switching is not just a two sided coin (fast/ slow) but more of a many faceted n-dimensional dice cube. All said, this has to be my biggest take away:



    Well said.
    Yes.... I agree. No matter how many times this is disgussed, there is always room for learning....

    Spyros

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