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SAC rate

Discussion in 'Basic Scuba Discussions' started by R A Diver, Jul 21, 2009.

  1. R A Diver

    R A Diver Angel Fish

    # of Dives: 25 - 49
    Location: Sugar Grove, North Carolina
    As a newbie I'm just figuring out things. While on a trip to Florida this month discovered my sac rate was only 28. I was using air twice as fast as anyone else in our group. I'm a 58 year old male, exercise 4-5 x's weekly. Am overweight but have lost about 20% of my weight over the last four years. Diving was my reward for losing 10" off waist.
    Now I find out, after these years of training, I'm still sucking air at an impossible rate. Is there hope for me? I can't use treadmill due to a long ago running accident, but do ellipticals for 30 minutes several times a week, plus weight training. Am in better shape than any time in the last twelve years, but very discouraged following this most recent trip.
    Our dives were 55-90" in dept. Even using nitrox, on an air setting, the longest dive was only 35 minutes.
    Well, I ramble. Any help, suggestions, encouragment from the experienced.
    Robert, A Diver
  2. Teamcasa

    Teamcasa Sr. Moderator ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: Near Pasadena, CA
    First here is a link to calculate your SAC rate.
    SAC Calculator

    Second, don't fret it. Keep diving and learn to relax and your SAC will slowly improve.
  3. H2Andy

    H2Andy Blue Whale

    # of Dives: 200 - 499
    Location: NE Florida
    the more you dive and the more comfortable you get under water will help

    also, try to relax (that is key) and breathe slowly and measuredly: inhale, hold your breath (but keep your airway open) and exhale slowly

    try to be as calm and relaxed as you can, this will help enormously

    be still ... don't waste energy or effort
  4. bamamedic

    bamamedic Solo Diver

    # of Dives: 200 - 499
    Location: Somewhere between "hold my beer and watch this!" a
    A lot of it is just being a new diver. As you gain experience, you'll find that your SAC rate goes down quite significantly. New divers generally aren't as comfortable with their gear, tend to have more inefficient fin kicks, and do a lot of scullling with their hands...all things which can cause you to work harder underwater. As you become more comfortable in your new underwater environment, your SAC rate will improve.

    You also might give some thought as to how quickly you move through the water. If you tend to zip around, wanting to see everything possible on your dive, you'll burn through air much quicker. Try slowing down a bit....you won't breathe as fast if you're exerting yourself less.

    Another thing to consider is your trim. If you're vertical or near-vertical in the water and trying to move forward, you're presenting a lot of surface area, and thus resistance in the water. If you try to trim out so you're moving horizontally instead, you'll find that your fin kicks will move you forward much more efficiently.

    I wouldn't worry too much about the whole SAC rate thing...just relax and enjoy your dive!
  5. TSandM

    TSandM Missed and loved by many. Rest in Peace ScubaBoard Supporter

    How fast you use your gas is determined by two things: How much CO2 you are generating per minute, and how efficiently you use the gas you breathe to accomplish gas exchange in the lungs.

    To address the second idea first, I'm going to describe a little anatomy. Your respiratory system includes your mouth, larynx, trachea, large bronchi, small bronchioles and then the air sacs where gas exchange actually takes place. Until gas gets into the air sacs, it's just passing through -- it's not delivering any oxygen, or taking away any CO2. If the volume of air you breathe with each breathe is only the volume of your larynx, trachea and bronchi, you're moving a lot of air, but not exchanging any gas. That's why shallow, rapid breathing runs through your tank quickly (and also leaves you feeling short of breath). It's also why slow, deep breaths are routinely recommended by instructors.

    Tension and anxiety tend to make people take quick, shallow breaths, which are inefficient. Relaxation tends to allow people to slow their breathing . . . but the funny thing is that slowing your breathing also tends to bring relaxation with it. That's the essence of yoga breathing, or meditation.

    Assuming your breathing PATTERN is an efficient one, then you have to look at your CO2 production, which is a result of metabolic activity in cells. You have a certain basal metabolic rate, below which you really can't go. It's actually HIGHER in fitter people than it is in the unfit, so you'd think that getting fit would make your gas consumption worse. But at the same time that your basal metabolic rate increases, the amount of muscle effort you need to accomplish a given amount of work goes DOWN, and that's a much bigger influence. So fitness does pay off.

    But efficiency pays off even better. Every motion you make underwater, you pay for with some gas used. Therefore, the less motion, the longer your gas lasts. You can reduce motion by becoming horizontal, so that all your kicking effort succeeds in propelling you forward. If you are tilted at a 45 degree angle to the bottom, each time you kick, you drive yourself upward. To compensate, you have to keep your buoyancy negative, so you will have an equal tendency to sink. At that point, you are expending energy for a net displacement of zero! Very inefficient, and a very common new diver error.

    Use your fins, not your hands. Hands are great for swimming on the surface, without fins, because your feet aren't very efficient propellers. But fins are, and that's what you should be using underwater. Flailing wildly with the hands uses a lot of muscle effort and produces very little net propulsion, so people who swim with their hands tend to suck gas.

    Master your buoyancy. Although the volume of gas going into your BC or drysuit is relatively small, if you are putting it in and letting it out and putting it back in and letting it back out . . . after a half hour, your BC has breathed a lot of your gas. To master buoyancy, you have to start with proper weighting, because being significantly overweighted will make you unstable in the water column, and result in a lot of yo-yoing that wastes BC gas and ALSO makes you breathe harder. So reducing your weight to the proper amount will, in the long run, make your gas last longer.

    And finally, move slower! One of the major strategies of sea life is camouflage, so if you move quickly, you miss many animals you might otherwise find. Unless you have a specific purpose for rapid movement, like spearfishing (and spearfishermen are NEVER going to win any awards for low SAC rates!) slow swimming will result in a much more productive and interesting dive.

    Finally, recognize that body size and muscle mass will have a detectable effect on gas consumption. My favorite dive buddy is 6' tall and very strong. He's an absolutely beautiful diver -- quiet, relaxed, balanced and efficient in the water -- but he will never equal my SAC rate, because I'm a little old lady. If you habitually dive with people who are much smaller than you are, then buying bigger tanks may be your best answer.
    101recon likes this.
  6. DA Aquamaster

    DA Aquamaster Directional Toast ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: NC
    Lynn covers it well. One thing I'll add is that the large muscles in your trunk and upper thighs burn a lot of O2 and create a lot of CO2. (Generally you can plan on generating 0.8 L of CO2 for every 1.0 L of 02 used, so the less muscle movement or tension you have the better.)

    In that regard proper balance and trim are important. If you tend to tip forward or back ward or rooll to one side or the other when you relax and are not moving in the water, you will be using those large muscle groups, possibily combined with hand sculling or extra finning, just to maintain stability. So if you find that you do not remain stable when motionless in the water, focus on correcting the underlying buoyancy and trim problems.

    Also, once you achieve that you will find it also helps you maintain a horizontal position in the water and to take full advantage of a long glide after each kick cycle. Invariably the divers who move the fastest through the water over any distance are those who are the smoothest in terms of technique and have efficient glides after their kicks. Water is very draggy and streamlining, proper trim and efficiency will do a lot more than brute power as doubling the drag requires 4 times the power to maintain the same speed.
  7. dkktsunami

    dkktsunami Solo Diver

    # of Dives: 1,000 - 2,499
    Location: Georgia coast
    P.S. In addition to all of the above, the fact that you were using nitrox has nothing to do with your SAC rate. Lung volume and gas usage remain the same regardless of what you are breathing.
  8. claymore

    claymore Manta Ray

    # of Dives: 200 - 499
    Location: Minneapolis Minnesota
    One thing that has not been mentioned is the type of deep breathing a person is doing.

    If the person is not breathing with the diaphragm but expanding the chest they are not getting good gas exchange.

    When I started diving I was a Hoover because I was breathing fast and not using my diaphragm
    to slow down my breathing and get better gas exchange.

    Over the last year I have made a concerted effort to retrain my body to breath with my diaphragm instead of my chest as it is a much more efficient form of breathing and is the way that we are breathing when we are born and our bodies get lazy as we grow up.
  9. David_57

    David_57 Solo Diver

    # of Dives: 500 - 999
    Location: Montreal, Canada
    in addition to the afformentioned dive conditions can have a great affect on your SAC rate for example drifting with a current in Cozumel my SAC rate is around .5, diving up here in Canada in the St Lawrence wearing a drysuit, double tanks, finning against current in 5ft to 10 ft of vis I have been over .8
  10. Doc Harry

    Doc Harry Solo Diver

    # of Dives: 500 - 999
    Location: Appalachia

    What are the units that are using for your SAC?

    Most people have a SAC between 0.3 and 1.2 cubic feet per minute.

    BTW, breathing Nitrox isn't going to affect your SAC. Nitrox doesn't make your cylinder last longer. Nitrox extends your no-decompression limits because you are breathing less nitrogen. If you are running out of gas before your run out of no-decompression time, then there is no definite advantage to using Nitrox.

    (Some claim that breathing Nitrox using air no-decompression limits increases safety, and that makes sense, but statistically-speaking this claim has not been proven.)

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