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High SAC reported at the beginning of dive

Discussion in 'Advanced Scuba Discussions' started by Andrei Astra, Nov 13, 2019.

  1. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

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    My wife used to consume a significant amount of gas at the beginning of each dive. I believe this was a combination of physical effort before jumping, nervousness, clearing her ears, and BC filling. I don't think cylinder cooling played much, if any, role. Her gas consumption would always even out and decrease during the dive. With considerably more experience and confidence and more efficient clearing and BC use, her initial gas consumption has decreased significantly. This phenomenon likely happens fairly frequently in newer divers.

    @Andrei Astra, in your case, I'm guessing this is more likely a result of your computer's mechanism for calculating and reporting SAC. @Diving Dubai has very extensive experience with Eon Steel, perhaps he can comment on your observations.
     
  2. kelemvor

    kelemvor Big Fleshy Monster ScubaBoard Supporter

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    I almost always see high sac at the beginning of my dives as well. I think it's a real phenomenon rather than a computer artifact. My assumptions were the same as what you guys have said. Reducing the weight I carried greatly improved it but not eliminated it. Reducing weight means that usually I don't have to put very much air in my bcd, and I'm less tired from lugging around heavy stuff pre-dive.
     
  3. Aviyes

    Aviyes Barracuda

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    Isn't it due to the fact that the SAC is a calculation using an average? At the start of the dive you have a tiny sample size for calculations on a per minute basis combined with a rapidly changing ATA during descent.
     
    rhwestfall likes this.
  4. DavidFL

    DavidFL Wide-eyed nube in the Pub ScubaBoard Supporter

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    I attended a lecture on Immersion Pulmonary Edema by Dr. Doug Ebersole at TEKDIVEUSA; he mentioned that in every diver (and swimmer) immersion causes a lot of things to happen immediately including redistribution of fluids and extra work pumping them all around. This is the closest reference I could quickly find:

    Water immersion results in compressive forces on the body and increases pressure on the peripheral capacitance vessels. This results in central redistribution of blood volume into the thoracic cavity with increased venous return and biventricular preload. [excerpted from an article on swimming-induced pulmonary edema on the American College of Cardiology website].

    I see relatively high SAC rates at the beginning of every dive when they are calculated by Subsurface; I have thought since seeing the lecture it might just partially be due to immersion.

    Subsurface dive profile exerpt.png

    Subsurface 'overplots' a color on the tank pressure graph line showing whether the SAC at that data point was high or low relative to the average SAC of the dive. This particular dive showing a red segment for the first five minutes of the dive is pretty typical for me; and it is pretty much like that over the last 300 dives or so for which I have a log with the pressure line.
     
    Andrei Astra likes this.
  5. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

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    The Teric discards the first 30 seconds and then reports a rolling 2 minute average. I do not follow SAC on the screen of my Teric, I assume it begins being reported starting at 2:30 of the dive. Someone who follows SAC can comment on this.
    upload_2019-11-13_10-46-14.png
     
  6. Bob DBF

    Bob DBF Solo Diver

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    Change in tank pressure from tempature, if a tank sets in the sun its temperature will be higher than ambient, darker the tank the higher the increase.

    Putting on gear, walking to the beach, swimming to the decent point, all increase breathing and gas consumption. I usually stay on the surface and relax before submerging, it helps.

    Inflating the BC eats gas, if you overshoot it will use more as you adjust.

    The excitement with beginning of a dive. Relaxing on the surface before submerging helps here also.

    And if you obsess over SAC rates, never get involved with lift bags.


    I just check my rate for the whole tank rather than any portion, which could be good or bad depending on conditions.


    Bob
     
  7. Diving Dubai

    Diving Dubai Instructor, Scuba

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    Thanks @scubadada

    @Andrei Astra in short - it's nothing unusual and just nonsense to be ignored.

    As explained to me, its a combination of the computer seeing a "large" pressure drop over a very short time as you initially breath, descend inflate BCD etc, and it extrapolates that measurement forward until it has more data to correct itself and properly work out teh average.

    Hence some manufacturers ignore the first 30 seconds etc of data

    My initial SAC rate is often shown as 40-50l/m where as the average over a dive is more like 10 - 13l/m I think if I was really breathing 50l/m my lungs would burst
     
    rhwestfall, rjack321 and scubadada like this.
  8. kelemvor

    kelemvor Big Fleshy Monster ScubaBoard Supporter

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    You can calculate an average SAC and some computers apparently display an average of some sort to the diver during the dive. What gets logged is the tank pressure. Combined with the knowledge of tank volume, sac is calculated. What you get in programs like subsurface are SAC calculations along the data points in the log file. Should be pretty much the same thing in any software that shows you a graph of sac over time and has anything other than a bar with a single value across the dive.
     
  9. Storker

    Storker Divemaster

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    Are you using 232 bar or 300 bar tanks?

    If your tanks are 300 bar, compressibility (real gas behavior) will lead to an overestimation of your SAC, especially from the top of your tank. Most SAC calcs are done using the ideal gas law, which deviates from reality at pressures above some 230-250 bar for air and EAN (much earlier for TMX). If your tanks are 232 bar, my money is on the gas cooling during the beginning of your dive, leading to a drop in tank pressure. Spend some time at the surface before submerging, and the effect should be less.

    I use 300 bar tanks, and if I calculate my SAC using the ideal gas law, I get some serious overstimation of my gas consumption for short dives with a full tank.
     
  10. Norrm

    Norrm Nassau Grouper

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    Did anyone mention "anticipatory anxiety?" or even pre-dive excitement. You're ready to jump into an unknown, alien environment, anything can happen, anything can be down there.. even if you've dived 1000s of times, it's never the same. I always feel a bit wound right before splashing and it continues and sometimes increases upon descent until after the wreck or reef comes into view and I get down to the business of checking out what's down there.


    Then again your dive profile is from a pool dive. So never mind.
     
    RyanT likes this.

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