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Air consumption rates.

Discussion in 'Basic Scuba Discussions' started by KenGordon, Jun 19, 2016.

  1. KenGordon

    KenGordon Rebreather Pilot

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    On this board it seems common to see claims of surface equivalent air consumption (SAC or RMV depending on who you ask) of the order or 0.3 cuft/minute or about 9l/minute.

    I have previously measured by own rate sat on the sofa and get something between 9 and 10 l/minute. My lowest whole dive rate is about 12 l/minute (0.4 cuft/minute) on totally easy warm water dives with no current. Almost always it is higher than this.

    Maybe those claiming 0.3 cuft/minute are younger and fitter than me, so I found a 10 year old and forced him to watch cartoons while breathing from a 3l cylinder. His rate came out at 8 l/minute.

    This talk by a well respected diving doctor and researcher,



    talks about ventilation and in particular co2. it covers technical diving but the breathing and co2 aspects apply at moderate depth too.

    I leave it for those watching to form an opinion about trying to breath less.
     
  2. CuzzA

    CuzzA Percoidea Wetwork for Hire ScubaBoard Supporter

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  3. CuzzA

    CuzzA Percoidea Wetwork for Hire ScubaBoard Supporter

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    Another great presentation from Mr. Mitchell. Second one I've watched here on SB in a week or so.

    While I'm not a deep diver yet (Max 100' thus far), there are still some great things to take away from the presentation even for relatively shallow diving.

    Personally, when I started diving I immediately took up spearfishing and noticed every now and then a very brief (less than a minute) and very minor headache upon surfacing from a dive. Obviously spearfishing is work and also comes with an elevated adrenaline level since big toothy animals are a concern. Consequently I started to evaluate how I was breathing. On the dives I got this slight CO2 headache I recalled I didn't pay much attention to how I was breathing, I just breathed like I normally would. I should note that while I've felt very natural diving, my ascents weren't as polished as they are today and perhaps that played a roll too.

    Nevertheless, in an attempt to eliminate this CO2 headache I began taking deeper breaths and fuller exhalations. What a difference. No more headaches and a considerable reduction in my sac rate. I know this doesn't really apply to deep diving and the concerns laid out in the presentation, but I thought it was worth mentioning.

    A couple other glaring things that stood out to me is, 1) How important it is to stop everything if you feel short of breath. We know this from our training on any level of Scuba, but CO2 is the devil and you better not let it take over at depth. And 2) How incredibly silly these record depth attempts are on scuba. I recall participating in the Guy Garmin thread and you have to wonder if he had any consideration or knowledge of the information Mr. Mitchell has presented here, especially negative pressure difference from within your lungs and gas exiting the first stage, as I'm sure he was making a very rapid descent.

    I never even consider this pressure difference but thinking about a few dives, I recall an increase of WOB despite little activity on fast descents as we try to pinpoint dive bomb a ledge and not end up in the sand off the structure we're intending on diving.

    Once again, great presentation.
     
    Last edited: Jun 19, 2016
  4. Kharon

    Kharon Solo Diver

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    Thanks for the link. Don't have time to watch the whole thing now but I saved the YouTube link for later.
     
  5. John C. Ratliff

    John C. Ratliff Instructor, Scuba

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    I'll have more comments after watching the video, which will be later. Let me show you this graph from the U.S. Navy Diving Manual, 1975, which shows and explains respiration:

    [​IMG]

    SeaRat
     
  6. Kevrumbo

    Kevrumbo Banned

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    CO2 retention and its spiraling vicious cycle is the danger along with the sudden debilitating narcosis that can occur:

    Carbon dioxide acts as a respiratory stimulant and can cause depression of the central nervous system (CNS). The effect depends on the level of carbon dioxide in the blood. Deep diving produces elevated blood carbon dioxide levels for several reasons, which include:
    1. the resistance to breathing caused by breathing denser gas [especially Deep Air] through a regulator and against a higher ambient pressure;
    2. reduced ventilation efficiency due to the denser breathing gas; and
    3. reduced transport, and, hence, elimination of carbon dioxide. . .
    Hypercapnia increases narcosis and the likelihood of CNS oxygen toxicity. In addition, it may increase heat loss, alter heart rhythm and predispose to decompression illness. If the carbon dioxide level gets too high, and it can on deep scuba dives -- especially if a diver is very anxious and / or exerting him/herself -- the diver may go unconscious without warning. Certain divers are more susceptible to severe hypercapnia for a variety of reasons and are therefore more at risk.
     
  7. KenGordon

    KenGordon Rebreather Pilot

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    The point I am half trying to make is that a low SAC/RMV is not necessarily a good thing as swapping dive time for complications due to CO2 is probably a bad deal.

    It is possible to reduce air consumption by becoming more comfortable in the water, avoiding finning hard, getting buoyancy right and so forth. Breathing less is not a good way though.

    I find it hard to believe some of the claims made for low air consumption. My advice to beginners is to ignore them and certainly not treat them as targets.
     
  8. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

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    I think that once you have become a reasonably skilled and experienced diver, your RMV is essentially fixed by individual variables. For my last 742 dives, my RMV has been 0.37 +/- 0.04 cu ft/min (mean +/- SD). That means 95% of my dives have an RMV of 0.29-0.45, the actual range is 0.28-0.63. My RMV is remarkable unaffected, though adversely, by effort and by cold. My current average RMV of 0.35 is barely lower than the 6 year average, I don't expect it to change much in the future. I'm not at all sure what personal factors fix your RMV. Having been a competitive swimmer between the ages of 4 and 18 may have been a positive. I have exercised regularly my entire adult life and am a nonsmoker. I am not a particularly small man at 5'10" and 185 lbs. My breathing pattern of a slow, moderately deep inhale, a short pause, and then a slow exhale, has not changed in more than 1000 dives.

    I am aware of a few men, and several women, with average RMVs lower than mine.
     
    Last edited: Jun 19, 2016
  9. John C. Ratliff

    John C. Ratliff Instructor, Scuba

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    Okay, I am a vintage diver, and have not heard of the term RMV related to diving and air usa. To me, it is "root mean value," but this is from either statistics or math. So, enlighten me! Is it, per chance, "respiratory minute volume"?

    SeaRat
     
  10. Kevrumbo

    Kevrumbo Banned

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    Trying to achieve a low RMV/Sac rate WITHOUT Skip Breathing (and its potential for developing Hypercapnia as we'll) is the ideal goal. For instance, the dive guides at Truk have a phenomenal "nominal" Surface Consumption Rate (SCR) of around 5 to 6 litres/min on Open Circuit Doubles -which is why they can almost stay at operational depth with their CCR Clients (the best warm water Open Circuit SCR I could ever achieve inside the deep wrecks in Truk was only 10 L/min).
     

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