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Thread: Calculating efficiency

 


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    Will_Evo's Avatar
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    Calculating efficiency

    Hey everyone,

    I have a topic for discussion that stems from some work me and a friend have been doing to create our own digital logbook. We started off basic at an excel spreadsheet and are now in the process of creating an actual database with features galore(well my friend is, we have far surpassed my abilities on a computer at this point). The question comes from our want to have raw data for comparison. Already we are incorporating our SAC rate, and will likely include more once we have finished reading Bob Bailey's article called Understanding Gas Management, but we very interested in calculating efficiency. To be specific we think it would be cool to see an efficiency as compared to your SAC, say sitting in your lazyboy with a regulator in your mouth exerting as little energy as possible. By doing this (or something like it) we figure this lazy SAC could serve as a personal benchmark for how you should strive to breath underwater and can be expressed in a percentage per dive of the goal.

    We figure the way to do this would be to have say, me for example, sit somewhere relaxed, eyes closed and breath from a regulator and find the time it takes for me to consume 1000psi of air. Of course you would need to do this 5 or 6 times to get a working average but once done you've now got a (I'm calling it lazy, you can call it whatever you like) lazy SAC that is specific for you, and now a datapoint you can use to compare your SAC on any following dive and express that in a percentage.

    Well, thats the theory and the goal right now, I posted this to see if what we are thinking is making any sense, if there are any variables we are missing, etc... Please speak up if you've done this before, or heard of anything like this.

    -Evo

    My friends SB name is Minus21

    PS: We are not trying to be extremely precise, we just like the idea of have comparable data for each dive. We also don't know if computers do this kind of stuff already, or what they log when connected to a PC so we are kind of starting from scratch.

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    Charlie99's Avatar
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    You need to breathe at whatever rate is necessary to provide O2 and remove CO2 related to your current exertion level. It is NOT a good goal to try and breathe at barcalounger rates when swimming hard against a current.

    The efficiency you should be working on is learning how to accomplish whatever you are doing underwater with the minimum expenditure of energy.

    If you really want to get into the efficiency game, then I would suggest that you measure your SAC
    1) under rest/deco/safety stop/lying on the bottom conditions;
    2) while continuously swimming moderately (roughly 1 knot speed.)
    3) swimming near max speed (or 2 knots if you can measure it).

    Then try to improve your technique to bring down the SAC for 1 knot and max speed swimming.

    The other measure of efficiency you could work on is to figure out for various depths what speed gives you the minimum air consumed for a given distance.
    MarcvsTheBard likes this.

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    knotical's Avatar
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    You're not alone in your thinking. SDI is one agency who recommends something similar to what you propose, specifically: "…sitting watching TV or reading...".

    However, I agree with Charlie99. Data gathered in-water will be more relevant to scuba diving than anything measured while dry. Unlike Charlie99, I measured my SAC while resting on the surface (then I didn't have to do any mathematical calculations to account for depth).

    Either way, as you're already aware, that's just the start of your exploration. Enjoy, and let us know how it progresses.
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    elan's Avatar
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    Just go diving If you need more air - buy a bigger tank or sling a stage.

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    Will_Evo's Avatar
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    Charlie99-

    I see what you are saying, and perhaps it would be more accurate to take these resting readings in the water at the surface, but I'm not trying to turn this into a huge science experiment. We are simply looking to compare a resting totally relaxed SAC rate to the entirety of a dive, whether that includes swimming against currents, sitting on the bottom, or using your power inflator to fill your BCD. All of that and more contributes to the efficient use of our air on every dive.

    Knotical- Awesome, we figured this has been discussed before but were still pretty new to Scuba with 13 dives each, and because of that our data is botchy at best, especially the dives with the instructor because of the drills and his timing as compared to ours. Right now we have fairly consistent SAC Rates on our last 3 dives and are hoping to get a few more in here at Wake Island before we leave. We also happen to have the luxury of hopefully grabbing a few tanks and getting our resting SAC before we leave (Its a small club out here so this kind of thing is possible).

    Elan-Oh we have been, we've done 13 dives in two weeks in between 12 hour shifts, but its on those shifts that we get bored and start talking about these kind of things:popcorn:


    We also started talking about how cool it would be to be able to monitor your SAC/efficiency as you change gear, progress over time, and other variables. We'll see and I'll keep you updated. We are going to try and get a resting SAC this week.


    -Evo

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    Bubbletrubble's Avatar
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    If you want to put in some (but not a lot of) effort...
    Why not just calculate your SAC/RMV for every dive?
    The info you need is: capacity of tank, service pressure of tank, the starting tank PSI, ending tank PSI, average depth, and total dive time.

    Using this method of "average" SAC/RMV calculation won't account for a lot of variables from dive to dive, but over time you should see a trend of SAC/RMV improvement until you attain whatever your "comfortable" SAC/RMV rate is.
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    What would you do? ScubaBoard has a "What if...?" series geared for beginner divers.

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    Will_Evo's Avatar
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    Bubbletrouble-

    Well in our spreadsheet/database we are tracking our SAC per dive. The cell automatically calculates your SAC and corrects it to surface pressure using your change in pressure during the dive, average depth and time. Now of course this is good info to have by itself, and great for comparing dive vs dive or trending, but you can't really quantify those numbers into any workable goal.

    For example my average SAC right now is 26.7psi/min(Remember we are correcting these values to the surface so they can be compared absent depth). Say I do want to improve that number, how low is perfect and what is an acceptable goal? How much do I breathe normally? How can I know by how much I am improving if I don't know the end all be all of psi/min? Thats where this lazy SAC would come in handy, and on our spreadsheet will automatically populate into a percentage or efficiency allowing a quick glance at your air management on a dive as compared to just sitting watching tv or other dives.

    -Evo

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    Bubbletrubble's Avatar
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    Ah. OK. Don't know what I was thinking when I wrote that last post.

    I have to agree with Charlie99. Normalizing to a barcalounger/lazy rate really isn't useful. Environmental conditions (current, surge, water temp.) or even the dive plan in and of itself (need to move quite a distance underwater?) can cause movement away from the lazy rate.

    Goal-setting can be a very useful tool to help focus efforts on improvement. However, I wouldn't recommend setting isolated "breathing goals." That approach will invariably lead to skip-breathing and carbon dioxide retention.

    By the way, you might want to start recording the service pressure and capacity (cubic feet) of the tank you are using for each dive. 26.7 psi/min with an AL80 is not the same consumption rate as 26.7 psi/min with a HP130. Many people will convert psi/min to cubic feet/min so that their gas consumption rates can be compared when using different tanks.

    Sorry I couldn't be more helpful...
    Last edited by Bubbletrubble; April 16th, 2011 at 09:09 AM. Reason: added info
    Insta-Gator likes this.
    Ear Equalization problems? Check out Dr. Kay's Ear Lecture for Divers.

    What would you do? ScubaBoard has a "What if...?" series geared for beginner divers.

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    Insta-Gator's Avatar
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    http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

    You probably already have these, but just in case. And I agree with Charlie99 and Bubbletrubble.
    Efficiency: the ratio of the useful work done by a machine, engine, device, etc., to the energy supplied to it, often expressed as a percentage.
    If you're sitting in a Lazy-boy you aren't really doing any measurable work, therefore you aren't being very efficient.
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    Charlie99's Avatar
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    I find that my SAC on the surface is much higher than my "surface air consumption".

    I find it much more relaxing to be below the surface than bouncing around at the surface.

    Will Evo, the math for the corrections for depth aren't all that complicated and really will get you more reliable, useful information than doing weird air consumption measurements on dry land or floating on the surface.

    If you are going to put your knowledge of SAC to any useful purpose, you need to be able to do the rough calculations in your head. So your SAC is 26.7psi/min (in other words, about 25 or 30). For that to be of much value you need to be able to calculate that at 30', you will be about 50psi/min or 500psi/10 minute. Being able to do that calculation in your head will make it so you have a rough idea of how long you can stay at a given depth with the gas you have.

    Your consumption rates will vary enough from dive to dive, that measuring down to the last percent doesn't add much value, nor does calculating down to the last percent. So it's OK to use crude approximations like 15' = 150% of SAC (actual point is 1/2 of 33') and 30' is twice (actual is 33') and 3 times is 65 or 70' depth.

    Almost all of my shore diving is in that 30' to 70' range, where actual consumption rate is between 2 and 3 times my SAC. So the only other point I need to remember is that air consumption is 2.5 times SAC at 50'. I find it pointless during the typical dive to bother with calculations any more precise than 15' vs 30-35' vs 50' vs 65-70'. For the 100' and 132' (4 times and 5 times SAC) dives, I will have done a plan before getting in the water.

    Another tidbit of info that is important is what your SAC is during normal sort of swimming speed. If you are 25psi/min at rest, then at 1knot (100' per minute) swim rate you will probably be 30 or 35psi/min (or perhaps more useful is to call it 300 or 350psi/10 minutes per ata).

    As some others have mentioned, the psi/min rate is dependent upon the type of tank. The conversion factor for an AL80 is 3000psi/77.4cu ft = 38.75psi/cu foot, or for the sort of rounding that is appropriate for SAC, 40psi = 1 cu ft. So 30psi/min = 0.75cfm.

    Some people calculate using the reciprocal of this and call it a tank factor. 40psi/cu ft is 0.025cu ft/psi, or more usefully, 2.5 cu ft per 100psi.

    They above may seem to be a random jumble of facts, but in reality, the above is just about everything that I've found to be useful rather than simply theoretical or for intellectual curiousity.

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