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Scuba Forum / UK Scuba / August 2004Air consumption
Hi, Just come back from my first 'proper' sea dive, and I can get through a 10l tank in 30-40 mins. One of the guides however used (he claimed) a 7l tank and did two 50 minute dives with the same tank. Watching him underwater, he did seem to exhale very seldom. Is this sort of consumption remarkable or achievable? How much does correcting bouyancy using your lung capacity waste air? As a novice I find myself wasting air, breathing in and out when I don't need it for the oxygen, - just to correct my position. Also, as I understand it, a tank at 300 bar is holding 50% more air than one at 200bar, what is the maximum pressure you can fill a tank? Should you get upset if you pay for a fill and its only 190bar rather than say 240 . Cheers Wayne >Hi, > >Just come back from my first 'proper' sea dive, and I can get through a 10l >tank in 30-40 mins. I've seen someone drain a 15l tank in les time that that on a shallow dive... >One of the guides however used (he claimed) a 7l tank and did two 50 minute >dives with the same tank. >Watching him underwater, he did seem to exhale very seldom. > >Is this sort of consumption remarkable or achievable? Without knowing the depth, it's hard to judge the true breathing rate. >How much does correcting bouyancy using your lung capacity waste air? As a >novice I find myself wasting air, breathing in and out when I don't need it >for the oxygen, - just to correct my position. Don't wory about your air consumprion rate. Just go diving. The more diving you do the better it will come as you become more relaxed. The key to relaxation is buoyancy control via your BC. The more you go diving, the more you'll get a feel for fine tuning this and then your breathing will become less of an issue, except when in very shallow water. >Also, as I understand it, a tank at 300 bar is holding 50% more air than one >at 200bar, what is the maximum pressure you can fill a tank? It depends on the tanks! Tanks are typically rated to 232 bar, some older ones are 207 bar and some stupidly heavy tanks are 300 bar... >Should you get upset if you pay for a fill and its only 190bar rather than >say 240 . Yes, but no-one seems to care enough to make a big enough fuss about it. If you want your tank filled slowly then you'll get it to full pressure, most people don't want to wait for them to cool down and be topped up... Gordon >Should you get upset if you pay for a fill and its only 190bar rather than >say 240 . No because if you do, you will spend a lot of time upset ;-) Laz  Signature~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A foolproof method for sculpting an Elephant: First, get a huge block of marble. Then, chip away everything that doesn't look like an Elephant. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Change "nospam" to "ntlworld" to reply. >>Should you get upset if you pay for a fill and its only 190bar rather than >>say 240 . > > No because if you do, you will spend a lot of time upset ;-) Wouldnt complain if a cyl was a couple of bar down, but 190 would be a very short fill. A full cylinder should be 232 bar - so a 240bar would usually be an slight overfull or a warm cylinder that will cool down. Not correct as it assumes free air (someone who understands ideal gas laws can correct the maths!) A 12ltr filled to 190bar hold 2280lts of free air, a 12ltr of 232bar holds 2784ltr. So a fill is short by 504ltr of air., or the same air as a 9.8ltr cyl to 232 bar. > >>Should you get upset if you pay for a fill and its only 190bar rather than > >>say 240 . [quoted text clipped - 11 lines] > 2784ltr. So a fill is short by 504ltr of air., or the same air as a 9.8ltr > cyl to 232 bar. Der! where did you learn arithmetic? A 9.8L cylinder at 232 BAR holds 9.8 x 232 = 2273.6 Litres NOT 504 Litres, ideal gas laws or not! 504 Litres is equivalent to a 2.17L cylinder at 232BAR not a 9.8Litre cylinder at that pressure! If you are pi*sed off with dive shops giving short fills, incorrect nitrox mixes, overnight waits, stroppy service and overcharging, then do what I did & buy your own compressor! BTW, If you do want a compressor I have a lovely little 3.5CFM portable compressor for sale in excellent condition (with both petrol & electric motors). > "Pete" <pete@hotmail.com> wrote in message >>A 12ltr filled to 190bar hold 2280lts of free air, a 12ltr of 232bar holds >>2784ltr. So a fill is short by 504ltr of air., or the same air as a 9.8ltr [quoted text clipped - 4 lines] > A 9.8L cylinder at 232 BAR holds 9.8 x 232 = 2273.6 Litres NOT 504 Litres, > ideal gas laws or not! I think in your haste you may have misinterpretted the OP's suggestion. He makes the point that a 12 l at 190 bar has the same free air (2280 l) as a 9.8 l at 232 bar (2273.6 l), and that that is somewhat different (in fact 504 l short) of what could be obtained from a 12 l at 232bar (2784 l). When the OP suggests that this is the wrong way of calculating, he's correct: Free Air = volume x pressure is only true for an ideal gas - a Van der Waals (or even more complex) treatment will give a different amount of free air for a given pressure. > 504 Litres is equivalent to a 2.17L cylinder at 232BAR not a 9.8Litre > cylinder at that pressure! Indeed, also equivalent to the difference between a 12 l at 232 bar and a 12 l at 190 bar. Interesting that 9.8 l + 2.17 l ~ 12 l, which I think brings us neatly back to the point the OP made... Just my tuppenceworth, Ben Signature-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Ben Panter, Edinburgh, UK email is false. Use my name (no spaces) at bigfoot which is a com. > Just my tuppenceworth, > > Ben Ben, OK I may be a bit pedantic, however, I've seen so many badly worded and calculated 'answers' on this NG before I thought I should clarify. As far as gas laws are concerned, for mixes of Nitrox (not TriMix), at the pressures quoted, there is little difference between Ideal & Real gas calculations We are not dealing with the issues of Helium pressurisation or gases nearing liquefaction pressures. The only issues are those of the reduction in pressure due to the gas cooling, easily resolved by most cylinder filling centres by the logical expedience of 'topping off' when the cylinders have cooled down. Tony. >OK I may be a bit pedantic, however, I've seen so many badly worded and >calculated 'answers' on this NG before I thought I should clarify. > >As far as gas laws are concerned, for mixes of Nitrox (not TriMix), at the >pressures quoted, there is little difference between Ideal & Real gas >calculations Gasses below 230 bar mostly have a straight line for compressibilty, but I think you will find that gasses pumped over 230 bar cease to have a straight line. It is significant enough at 300 bar to put your available gas calculations out by quite a bit in the wrong direction. You will have less gas than you think. Pete . > >OK I may be a bit pedantic, however, I've seen so many badly worded and > >calculated 'answers' on this NG before I thought I should clarify. [quoted text clipped - 8 lines] > available gas calculations out by quite a bit in the wrong direction. > You will have less gas than you think. Another good reason never to buy 300bar cylinders (apart from thir negative buoyancy and not always getting full fills). > Pete . > Just come back from my first 'proper' sea dive, and I can get through a > 10l tank in 30-40 mins. One of the guides however used (he claimed) a 7l > tank and did two 50 minute dives with the same tank...Is this sort of > consumption remarkable or achievable? Without knowing what depth you were at it's impossible to say, but when I was still teaching people it was quite common for me to take two or sometimes three students in on a single tank of gas. The breathing rate between an experienced and a first time diver can really be that high. It's nothing to worry about, you're breathing rate will decrease with time. Nothing is going to help except going diving a lot... > Also, as I understand it, a tank at 300 bar is holding 50% more air than > one at 200bar... No, a tank pressurised to 300bar does NOT hold 50% more gas than one filled to 200bar, see http://www.babilim.co.uk/pages/gas_laws.html for more information. Al. > > Just come back from my first 'proper' sea dive, and I can get through a > > 10l tank in 30-40 mins. One of the guides however used (he claimed) a 7l [quoted text clipped - 5 lines] > sometimes three students in on a single tank of gas. The breathing rate > between an experienced and a first time diver can really be that high. First dive was to 18m, second to 14m > It's nothing to worry about, you're breathing rate will decrease with > time. Nothing is going to help except going diving a lot... [quoted text clipped - 7 lines] > > Al. That page really good, thanks! it was quite common for me to take two or > sometimes three students in on a single tank of gas. I bet there was a fight for the octopus > > ...it was quite common for me to take two or sometimes three students > > in on a single tank of gas. > > I bet there was a fight for the octopus *shake head in mild bemusement* Al. > > > ...it was quite common for me to take two or sometimes three students > > > in on a single tank of gas. [quoted text clipped - 4 lines] > > Al. You liked that one didnt you Al, go on admit it. > > > > ...it was quite common for me to take two or sometimes three students > > > > in on a single tank of gas. [quoted text clipped - 6 lines] > > You liked that one didnt you Al, go on admit it. I did :-) |
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