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Scuba Forum / General / February 2004SAC vs. RMV
How exactly are these related...? Is is possible to derive one from the other? The books I have are frustratingly unclear. I can get my SAC easily enough from my air-integrated computer, averaged over a long series of dives, so I consider it to be pretty accurate. But some gas calculations require RMV, and the only method I've seen for determining that is sort of a one-time thing (well, you could always sit around in your living room for two hours sucking on a tank 20 times, but....) Any good sources of info you folks can suggest? Thanks; --Toren P.S. I'll be on Cayman Brac first couple of weeks of March. If any of the non-screwballs on rec.scuba will be on the island at that time, let me know and let's hook up for some shore diving. I'm building a pretty good list of GPS coordinates for pristine sites.... > I can get my SAC easily enough from my air-integrated computer, > averaged over a long series of dives, so I consider it to be pretty > accurate. No, you can't.  Signature"We're going to rush the hijackers." -Jeremy Glick, aboard United Airlines flight 93, September 11, 2001 > How exactly are these related...? Is is possible to derive one from > the other? [quoted text clipped - 10 lines] > P.S. I'll be on Cayman Brac first couple of weeks of March. If any of > the non-screwballs on rec.scuba <snip> Do those exist? >> P.S. I'll be on Cayman Brac first couple of weeks of March. If any of >> the non-screwballs on rec.scuba > ><snip> > >Do those exist? I have a dream.... --Toren Toren Smith <mrt0bak@NOmindSPAMspring.com> pounded away at his keyboard resulting in: :How exactly are these related...? Is is possible to derive one from :the other? [quoted text clipped - 6 lines] :times, but....) :Any good sources of info you folks can suggest? Just out of curiousity, how do you intend to use the information? Dan Bracuk If at first you don't succeed, you run the risk of failure. The Best of rec.scuba http://www.pathcom.com/~bracuk/RecScuba/ >Toren Smith <mrt0bak@NOmindSPAMspring.com> pounded away at his >keyboard resulting in: [quoted text clipped - 12 lines] > >Dan Bracuk Calculating gas supplies for deep (deco) diving. In fact, as noted elsewhere, I've already done the calculations successfully (confirmed by actual dives) numerous times, which somehow increased my confusion. But I'm all cleared up on it now. Or just narced. --Toren > Calculating gas supplies for deep (deco) diving. Perfect. > But I'm all cleared up on it now. Good thing. Good luck, and leave us your Old Lady's phone number. There's only one way to comfort a new widow, and we have some vultures here who will close right in. Signature"I know we're going to die. There's three of us who are going to do something about it." -Tom Burnett, aboard United Airlines flight 93, September 11, 2001 Toren Smith <recscuba@spamex.com> pounded away at his keyboard resulting in: :Calculating gas supplies for deep (deco) diving. Must be for that shore diving in Cayman Brac you mentioned earlier. Not my cup of tea, but, to each his own. Dan Bracuk If at first you don't succeed, you run the risk of failure. The Best of rec.scuba http://www.pathcom.com/~bracuk/RecScuba/ > you could always > sit around in your living room for two hours sucking on a tank Dunno if you realise it, but you're getting pretty close... Cheers David M > Dunno if you realise it, but you're getting pretty close... Except, of course, that he thinks he's joking. Signature"A bunch of us went down to Gettysburg. Some of us didn't come back. If you weren't there, you'll never understand." -Unknown Infantryman > How exactly are these related...? Is is possible to derive one from > the other? A google search would provide this answer about 300 times. ************************************************************* US Navy Diving Manual, Chapter 3 3-4.5.1 Respiratory Cycle. The respiratory cycle is one complete breath consisting of an inspiration and exhalation, including any pause between the movements. 3-4.5.6 Respiratory Minute Volume. The respiratory minute volume (RMV) is the total amount of air moved in or out of the lungs in a minute. The respiratory minute volume is calculated by multiplying the tidal volume by the rate. RMV varies greatly with the body's activity. It is about 6 to 10 liters per minute at complete rest and may be over 100 liters per minute during severe work. ************************************************************** Go to 33 feet, mark your time and pressure on a slate. Stay still for ten minutes at the same depth. Mark how much pressure you used. Now, mark the time and the pressure again, and swim normally for ten minutes at 33 feet. Lets say: 95 cu ft bottle @ 2400 PSI 95/2400 = .0395 cuft per PSI Lets again assume you dropped 200 psi in ten minutes at rest .0395 X 200 = 7.9 cuft used at 2 atm. 7.9/2 = 3.95 cuft used at 1 atm 3.95/10 = .395 cuft per minute SAC at rest, at 1 atm. Say you used 400 psi during the swim. 400 X .0395 = 15.8/2 = 7.9/10 = .79 cuft per minute working SAC at 1 atm. Take your working SAC, multiply by the depth in ATM, and you get the amount of gas you need for that portion of the dive. You would use the at rest SAC to determine deco gas requirements. And, of course, all this changes from day to day, dive to dive. Scott "Scott" <scottk@localaxes.com> pounded away at his keyboard resulting in: :And, of course, all this changes from day to day, dive to dive. Makes it seem so unworthwhile. Dan Bracuk If at first you don't succeed, you run the risk of failure. The Best of rec.scuba http://www.pathcom.com/~bracuk/RecScuba/ > "Scott" <scottk@localaxes.com> pounded away at his keyboard resulting > in: > > :And, of course, all this changes from day to day, dive to dive. > > Makes it seem so unworthwhile. It's a window or envelope, the boundaries of which are defined by each diver, each dive. Creates that ambiguity in which the lawyers operate. >> "Scott" <scottk@localaxes.com> pounded away at his keyboard resulting >> in: [quoted text clipped - 7 lines] > >Creates that ambiguity in which the lawyers operate. Scott, you know Dan gets paid to be contrary :) --- Rich http://richlockyer.tripod.com/ >A google search would provide this answer about 300 times. Well, it didn't. Apparently my Google is broken. I'll hit it with a hammer. >************************************************************* > [quoted text clipped - 14 lines] > >************************************************************** This I knew. >Go to 33 feet, mark your time and pressure on a slate. Stay still for ten >minutes at the same depth. Mark how much pressure you used. [quoted text clipped - 18 lines] > >400 X .0395 = 15.8/2 = 7.9/10 = .79 cuft per minute working SAC at 1 atm. Actually, my Cobra does all that fun calculating for me. So I know my SAC as averaged over more than 100 dives, warm water and cold. (I'm a big guy, 6'3" and 230, so it ranges from 0.5 to 0.7 depending on the circumstances of the dive.) But I'd read things that said "RMV is not SAC so don't use SAC to estimate your gas requirements" and then gone on to do calculations similar what you did above, but calling it RMV instead. So bafflement ensued, because the numbers were all coming out right for me, as proven over several test dives. From what you've written it seems to me that if you already know your SAC, knowing your RMV is not very useful (unless you were having custom counterlungs made or something). >Take your working SAC, multiply by the depth in ATM, and you get the amount >of gas you need for that portion of the dive. [quoted text clipped - 4 lines] > >Scott What you have written all makes sense, hence my previous confusion. My attempts at determining gas consumption in advance using SAC and such calculations have been pretty damn reliable (within the bounds of your comment that it all changes day to day--too true) for both dive and deco. So somehow I've been getting the calculations right. But in the interests of being a better educated and less confused diver I came to rec.scuba to be enlightened, just in case I was missing something. It seems I was "overthinking" the problem and the answer was so simple I had looked right past it. Thanks for the help! --Toren > So I know my SAC as averaged over more than 100 dives, warm water and > cold. No, you don't. Signature"I know we're going to die. There's three of us who are going to do something about it." -Tom Burnett, aboard United Airlines flight 93, September 11, 2001 > Thanks for the help! You're welcome. Goodbye. Signature"A bunch of us went down to Gettysburg. Some of us didn't come back. If you weren't there, you'll never understand." -Unknown Infantryman |
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