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Can varying your SAC rate reduce/increase the likelihood of DCS?

Discussion in 'Ask Dr. Decompression' started by SammyW, Feb 15, 2019.

  1. SammyW

    SammyW Angel Fish

    # of Dives: 200 - 499
    Location: Derbyshire, UK
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    I'm pretty new to diving and trying to educate myself where possible and i wondered if there is a paper or resource someone can direct me to if this has already been discussed.

    My SAC rate on deco is around 50% that of the 'working' portion of my dives, Thats not to say i increase my air consumption during the work portion, Rather it's more like when I'm not moving around much my gas consumption goes crazy low (around 5-6LPM).

    It was after a dive recently where afterwards i thought about all the N2 i had taken on at depth (breathing 10-12 litres per minute) and then coming up to deco depths i spent around an hour there but it got me thinking that surely to offgas efficiently i should maybe deliberately increase my breathing rate to the same as the working portion of my dive where my SACrate is much higher.... Or am i just overthinking things?
    I realise i relax to a zenlike state when I'm just staring at a fish or a rock and not really moving anywhere. :p


    Can you offgas inspired nitrogen in the blood at a fixed rate independent of breathing or does breathing rate directly influence the efficiency of decompression?

    Does a marginal increase in breathing rate on deco, compared to the working portion, assist in off gassing faster?
     
    Schwob likes this.
  2. jvogt

    jvogt Solo Diver

    # of Dives: 200 - 499
    Location: Lakewood, CO USA
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    Not an expert, but...

    Blood perfusion of tissue is the biggest determining factor. In your case, you SAC is changing due to metabolic rate, so yes it is an indicator of off/on gassing efficiency. In my opinion we should do swimming deco whenever feasible.
     
    KWS and SammyW like this.
  3. Pedro Burrito

    Pedro Burrito Moderator Staff Member

    # of Dives: 5,000 - ∞
    Location: Boussens, Canton de Vaud, Suisse
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    A ScubaBoard Staff Message...

    Moved from Basic
     
    RayfromTX likes this.
  4. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
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    As @jvogt said, breathing rate isn't directly affecting offgassing. But breathing rate is correlated with physical activity, and increased physical activity means increased perfusion, i.e. increased transport of dissolved gas out of the tissues and back to the lungs, IOW increased offgassing. EDIT: and of course increased ongassing as well.

    A pretty good thread on the subject: Does higher RMV cause higher DCS risk?

    The tl;dr version can be found in post #42.
     
  5. SammyW

    SammyW Angel Fish

    # of Dives: 200 - 499
    Location: Derbyshire, UK
    63
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    that TLDR explained my thought process perfectly.

    In situations such as hanging on a line in the blue where swimming deco isn't always ideal, what is the accepted practice? Lowering the GFhi? Dancing on the spot? Arbitrarily increasing deco (aka Long Safety stop?

    Like i said, I'm new to all this and keen to understand. :)
     
  6. Mike1967

    Mike1967 Manta Ray

    # of Dives: 50 - 99
    Location: Victoria, Australia
    732
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    I hate you lol
     
  7. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    12,491
    8,867
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    Probably diving wet. Even if so, it's rather annoying.
     
    Mike1967 likes this.
  8. SammyW

    SammyW Angel Fish

    # of Dives: 200 - 499
    Location: Derbyshire, UK
    63
    34
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    i only dive in my drysuit... its been joked to me many times that the only people i can dive with are those on rebreathers. I wonder what it will be when i get a separate bottle for my dry suit inflation.

    But its kinda annoying when I'm having to end dives and sometimes waste so much unused gas because I'm diving with people that have 'regular' SAC rates.
     
  9. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    12,491
    8,867
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    Ok, now I hate you.

    Just to get serious for a few seconds: Unused gas isn't wasted gas. Unused gas is a safety margin. And safety margins are a good thing. If you and your buddy are happy with the dive and have gotten what you wanted out of it, the residual pressure in your tank means zero. zip. nada. zilch. As long as it's above minimum pressure, of course.

    I've had great dives which were ended with way above 100 bar left in my tank. And I've had dives where the end pressure was some 20 bar after the safety stop, which really is a bit on the low side. I seldom finish a dive due to tank pressure; usually it's because I've dived enough, because we've completed our plan or because I desperately have to take a leak. But it's all good.
     
    Nick Steele, TerryC and admikar like this.
  10. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: SE TEXAS
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    Wasnt the reason for doing stationary deco based in the higher PPO2 while doing deco?
     

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