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Thread: WOB difference between air and trimix

  1. #11
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    Re: WOB difference between air and trimix

    Quote Originally Posted by Mark Chase  View Original Post
    Ocasionaly I find myself thinking I am quite intelegent then somone posts something like this and i railise I am not even close :D


    Lol I agree.. Net for me from personal experience, the more He in the loop, the easier it breaths on an equivalent depth basis than a dil with less He in the loop...

  2. #12
    Escaped... Lab Rat is an unknown quantity at this point Lab Rat's Avatar
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    Re: WOB difference between air and trimix

    Sorry, should have PM'd Simon...this has more to do with an area of interest of Simon's from a prior project/incident that I think has bearing on this topic.


    -R

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    RBW Member N Bailey is an unknown quantity at this point N Bailey's Avatar
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    Re: WOB difference between air and trimix

    There is a new British standard for testing open circuit regs below 50 m that gives a comparable gas density equation and tables to use.

  4. #14
    New Member Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell's Avatar
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    Re: WOB difference between air and trimix

    Quote Originally Posted by Gobfish1  View Original Post
    In a older post by simon im sure i read 8g/L as the line the sand maybe i read it wrong , Think it was in a wet mules thread ,

    also this is WORKING divers , so how hard are we working them ,

    6/72 Bangs on 20/30min,s more deco ,
    Hello Dr,

    You have a very good memory.

    Estimates of maximum advisable density have appeared from time to time (the 8g/L one was an example) but none were based on an objective practically relevant end point from human testing. The data that will appear in the paper Gavin and I have written are indeed new. They were not collected primarily for the purpose we are putting them to, but they are by far the best we have and it is surprising (and scary) how low"ish" the density threshold is before CO2 problems become much more common during the test dives Gavin performed.

    Those dives were indeed working dives. I don't want to pre-publish the paper here, but the test dives involved a standardised escalating graded exercise protocol with work at 75, 100, and 125 watts (if I recall correctly - we wrote it a year ago and I don't have a copy with me now). 125 watts was when most of the problems with CO2 occurred but this is not a particularly hard workload. For me (90kg, 200lbs) 125 watts is about 6 mets which is level of exercise that a consensus of experts agreed was a plausible sustainable work output for a reasonably fit diver [1]. For context, in one of our recent CO2 scrubber experiments I was able to sustain 6 mets on an exercise bike for the 5 hour duration of the study. The point being that it is not hard exercise as such, but it clearly becomes problematic if you try to do it at depth with a dense gas.

    Your point about the increased deco is a fair one (although the whole helium penalty thing is a can of worms at the moment). I guess what these density / CO2 data will do is allow divers to make more intelligent trade-offs. Obviously we can never be sure about the work requirement of a dive, but on one where you know the work requirement may be high you might prioritize gas density in planning, but in a dive where hard work is extremely unlikely you might prioritize a shorter deco.

    I spoke to Neal Pollock yesterday and he has confirmed that the NOAA / NPS workshop proceedings should appear around the time of Tek Dive USA (ie a couple of weeks from now) and that they will be freely available on line for everyone.

    Simon M

    1. MITCHELL SJ, BOVE AA. Medical screening of recreational divers for cardiovascular disease: Consensus discussion at the Divers Alert Network Fatality Workshop. Undersea Hyper Med 38, 289-296, 2011
    Last edited by Simon Mitchell; 12th April 2016 at 20:54.

  5. #15
    New Member Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell's Avatar
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    Re: WOB difference between air and trimix

    Quote Originally Posted by Lab Rat  View Original Post
    Maybe just new data?

    Either way, thanks for putting this out Simon!

    Also, know that Brett Stolp has a bunch of data on this from the early 80s dives at the chamber, but, as I recall, it is all about max tolerable density and not density at which CO2 retention increases. They did pull blood gasses at depth though and that included the workup dives if I remember right.


    -Richard


    Is it a laminar flow thing through smaller bronchiols and disrupted gas flow into alveoli due to increased density alone?



    where:
    inlet pressure
    outlet pressure
    is the length of tube
    is the viscosity
    is the radius
    is the volume of the fluid at outlet pressure
    is the velocity of the fluid at outlet pressure
    Hello Richard,

    We believe it is mainly about increased density causing an increase in work of breathing with the consequent derangement of respiratory control that leads to CO2 retention (as extensively discussed in our recent review [1]).

    With respect to your equation, you will have noted that there is actually no term for density where laminar flow is thought to prevail, but equations to predict flows, or delta p or resistance where turbulent flow prevails do contain a term for density. The debate then is to what extent laminar and turbulent flow exist in a normal lung, and the consensus is that even though airway dimensions might predict laminar flow, the repetitively branching nature of the airways ("entrance length characteristics") make laminar flow unlikely in most of the lung. Thus, turbulent flow (which is affected by density) prevails and increased gas density will increase the work required to move x amount of gas in and out of the lungs This is also dealt with in our recent review [1].

    Simon M

    1. DOOLETTE DJ, MITCHELL SJ. Hyperbaric conditions. Comprehensive Physiol 1, 163-201, 2011

  6. #16
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    Re: WOB difference between air and trimix

    Quote Originally Posted by Simon Mitchell  View Original Post
    Hello Dr,

    You have a very good memory.

    Estimates of maximum advisable density have appeared from time to time (the 8g/L one was an example) but none were based on an objective practically relevant end point from human testing. The data that will appear in the paper Gavin and I have written are indeed new. They were not collected primarily for the purpose we are putting them to, but they are by far the best we have and it is surprising (and scary) how low"ish" the density threshold is before CO2 problems become much more common during the test dives Gavin performed.

    Those dives were indeed working dives. I don't want to pre-publish the paper here, but the test dives involved a standardised escalating graded exercise protocol with work at 75, 100, and 125 watts (if I recall correctly - we wrote it a year ago and I don't have a copy with me now). 125 watts was when most of the problems with CO2 occurred but this is not a particularly hard workload. For me (90kg, 200lbs) 125 watts is about 6 mets which is level of exercise that a consensus of experts agreed was a plausible sustainable work output for a reasonably fit diver [1]. For context, in one of our recent CO2 scrubber experiments I was able to sustain 6 mets on an exercise bike for the 5 hour duration of the study. The point being that it is not hard exercise as such, but it clearly becomes problematic if you try to do it at depth with a dense gas.

    Your point about the increased deco is a fair one (although the whole helium penalty thing is a can of worms at the moment). I guess what these density / CO2 data will do is allow divers to make more intelligent trade-offs. Obviously we can never be sure about the work requirement of a dive, but on one where you know the work requirement may be high you might prioritize gas density in planning, but in a dive where hard work is extremely unlikely you might prioritize a shorter deco.

    I spoke to Neal Pollock yesterday and he has confirmed that the NOAA / NPS workshop proceedings should appear around the time of Tek Dive USA (ie a couple of weeks from now) and that they will be freely available on line for everyone.

    Simon M

    1. MITCHELL SJ, BOVE AA. Medical screening of recreational divers for cardiovascular disease: Consensus discussion at the Divers Alert Network Fatality Workshop. Undersea Hyper Med 38, 289-296, 2011
    Ello Sir

    Thankyou for the reply
    The 6g/L sort of blow,s my 10/50 out the window somewhat,(not all the way) Maybe thats why the 8g/L stuck in my mind , worked ok with my numbers lol
    The workload is interesting to me and as im just under 90kg and also ride my bike several hours or more at a time,
    The can of worms / helium penalty thinng we will just have to live with, and yes looks like a trade off will be in order gas density or less deco
    something to take account of when planning are dives,

    I looking forward to the paper as and when we get to read it,

    Hope your well Simon ,
    thanks for taking the time to post ,

    best Steve


    PS

  7. #17
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    Re: WOB difference between air and trimix

    just out of my head, to answer the first question in post #1 regarding air versus trimix:


    rEvo 40m air 75l/min WOB: 2.00 / 2.17 J/l (horizontal/vertical)


    rEvo 100m 10/90 75l/min WOB: around 1.45 J/l (or something close to it)


    so there is obviously a big difference in WOB when using trimix


    paul
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    ...."Yes you have to pre-breathe to activate the scrubber sorb, anyone who says different doesn't know what they are talking about!"...
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  8. #18

    Re: WOB difference between air and trimix

    Quote Originally Posted by Simon Mitchell  View Original Post

    Quote Originally Posted by Lab Rat  View Original Post
    [...]

    [...]
    With respect to your equation, you will have noted that there is actually no term for density where laminar flow is thought to prevail, but equations to predict flows, or delta p or resistance where turbulent flow prevails do contain a term for density. The debate then [...]
    Not to disagree, but I think maybe it's worth pointing out that the quoted equation is not the equation for laminar flow but rather a solution (of Navier-Stokes) for a specific set of conditions that include, but are not limited to, laminar flow.

    Those conditions (but not laminar flow alone) mathematically (as opposed to physically) yield the same simplifications as assuming the gas density to be 0...

    In particular, they consider a steady state solution (no change over time - how that applies to breathing would need justification). Remove that, there will be a density term.

    Cheers,

    Matthieu

  9. #19
    RBW Member germie is an unknown quantity at this point germie's Avatar
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    Re: WOB difference between air and trimix

    yes, there is a big difference in WOB between air and trimix. On deep dives you have not only the END/EAD (N2 and O2 narcotic or only N2 narcotic) but there is a EADD too. That is for a lot of divers an unknown issue: Equivalent Air Density Depth. The denser the gas is, the higher the WOB. So on a 6/72 diluent on 134m, the END (N2 narcotic) is 28m, END (N2 and O2 narcotic) is 33m, but the EADD is 48m. The END/EAD is within acceptable limits, but you have a higher eadd and wob than diving on air diluent at 35m (real depth) So if you want to do really deep dives, you have to know that the WOB will be more.
    The WOB is not only influenced by the EADD, but by the scrubber too. The higher the EADD, the more molecules has to pass the scrubber, and the way how this is packed influences the wob too. There are apps to calculate the eadd for you (easy if you don't want to do maths ;) )

    And on such deep dives or deeper there can be another problem too: ICD.

    WOB seems further to be a contributing factor not only for CO2 (and then narcosis), but for IPE too (but there is no absolute sign it is, only a seems).

  10. #20
    New Member Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell's Avatar
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    Re: WOB difference between air and trimix

    The proceedings document containing the paper with evidence based guidelines for gas density have been released.


    I have posted a link to where you can download them in this thread:


    http://www.rebreatherworld.com/showt...g&goto=newpost


    Simon M

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