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Jeff, Your recommendation is on target. However, it completely leaves Greg out of the loop as he is completely lacking when it comes to having a clue! At 09:17 PM 8/5/00 , Jeff wrote: >All you need is a pressure gauge and a clue. Scales are for the DOT. >stfu, >Jeff > > >gzambeck1 wrote: > > > Bill you need to start looking at equipment that can do the mixing by > weight and > > analyzers that have the accuracy that is needed. You might have some > experience > > to talk about then. > > > > Greg Z > > > > Bill Bott wrote: > > > > > Brian, > > > > > > Like so many other dumb ass ideas this has been tossed out for > > > consideration before. the fact of the matter is there so many problems > > > with this idea I'm not sure where to begin but the bottom line is you can > > > get a more accurate mix with the pressure gauge you are using on your > > > rig. If a CDN $100 ( US $67 ) scale was the way to go every mixing > station > > > in the world would be using it. The fact of the matter is it just > does not > > > work like that in the real world. > > > > > > Try this and see what happens: > > > > > > Set the tank on the scale with the fill whip attached and the tank valve > > > closed. Then pressurize the whip without opening the tank > valve. How much > > > does the weight as read on the scale change?? Why??? Do it a couple of > > > times completely breaking the assembly down each time. Are your results > > > consistent??? > > > > > > I'll help you out with this so you don't have to buy the scale to try > this > > > at home. When you charge the system the weight of the tank will > change as > > > read by the scale because the pressure of the gas in the hose is causing > > > the hose to stiffen and straight out. It is the same principal that > > > causes your analog pressure gauge to work. That part is > > > predictable. However, every time you break the system down to fill > another > > > tank you get things back together just a little differently and the > > > stresses that cause the hose to flex act in a different direction and or > > > to a varying degree. Some times there will be no change other time the > > > "weight" will increase as the stresses push down on the tank and other > > > times the "weight" will decrease. The amount of force is related to the > > > pressure in the hose and the direction of the force is related to the > shape > > > which the hose is, for lack of a better term, bent. > > > > > > This is NOT a blender friendly situation. But if you chouse not to > take my > > > word for it spend CDN $100 for the scale and give it a try. You > will soon > > > learn why that method is used only under lab conditions and why some > > > blenders chouse a US $50 ( CDN $75 that exchange rate sucks ) gauge > over a > > > scale. There was on shop I know of that actually used a digital SPG > > > attached to the other post when filling doubles. I don't know if they > > > still do this but they always managed to give me the gas I asked for > ( + or > > > - .5% or better). I was spoiled with a $500 digital gauge and a blenders > > > dream for a fill station ( 2 haskels 1 for air and 1 for gasses) You > could > > > mix any gas to 4500+psi and hit the FO2 within + or - 0.1 - 0.2% every > > > time and frankly the O2 analyzer is not any more accurate than that! > > > > > > Forget the scale, save your money and do it right or forget about > doing it > > > at all. > > > > > > At 06:05 PM 8/2/00 , Brian Greenberg wrote: > > > >First off, I'm not suggesting this is better than what's being done > now, nor > > > >that it is problem free. This is something that I have been thinking of > > > >for a while now, and seems to make sense. I thought I'd bring it up > here, > > > >and let The Experts(tm) rip it up and add their comments. > > > > > > > >Caveat Emptor: I am not a technical diver, and I don't play one on > the net. > > > > > > > >Mixed gas is, from what I have seen, traditionally mixed by > pressure. To do > > > >this well, one needs a fairly accurate (and fairly expensive) gauge, > and one > > > >also needs to worry about compressibility of the various component > gases, > > > >and the speed at which you fill them, which will affect the > temperature of > > > >the mix gas in the tank, and the resulting percentages of the mixture. > > > >Overall, there are several variables that can make it tricky (though far > > > >from rocket science) to do accurately. > > > > > > > >But why mix by pressure at all? Why not mix by weight? > > > > > > > >I can get at the local office supply store, for CDN$100, a postal > scale that > > > >will measure to 0.1 pound accuracy, with a max weight of 200 pounds. A > > > >more accurate scale is probably fairly easy to find. If we assume > that a > > > >standard 80 holds 6 pounds of air, then a twin 95 set will hold > 14.25 pounds > > > >of air. If those tanks are filled to 2250psi, the the "guage error" > of the > > > >scale is: > > > > > > > > (error)x (pressure) / range == (.1) x (2250) / 14.25 == 15.8 > > > > > > > >Which is +/- about 15psi, which is more accurate than a digital > gauge, at > > > >+/- around 50psi. > > > > > > > >This of course changes for smaller tanks. A 40cuft stage, for > example, holds > > > >about 3 pounds of air at 3000psi, resulting in an error of +/- > 100psi, but > > > >this is still quite good. > > > > > > > > From here, calculating mix percentages is easy. For a 50N/25He/25O > mix in > > > >the aforementioned twin 95s. > > > > > > > >190 cuft of gas / .79 cuft per mol of gas == 240 mols of gas to fill > > > >the tank. That means, we need: > > > > > > > > 120 mols of Nitrogen == 120 * 28 grams = 7.4 pounds of N2 > > > > 60 mols of Helium == 60 * 4 grams = .5 pounds of He > > > > 60 mols of Oxygen == 60 * 32 grams = 4.2 pounds of O2 > > > > > > > >This works out to 9.6 pounds of Air (7.4 pounds of N2 + 2.2 pounds > of O2, > > > >which is 79% and 21% *by* *molecular count*) blown onto 2 pounds of > O2, and > > > >half a pound of He. > > > > > > > >Similarly, if you had 1200psi of this gas left in the tank, then the > > > >fractions are: > > > > Total weight of gas == 6.5 (This assumes the tank is full > at 2250. > > > > normally, you'd just > subtract the > > > > empty weight of the tank > from the > > > > current weight) > > > > 6.5 pounds = 2950 grams > > > > Number of mols remaining = 2950 / (%N2 * 28 + %O2 * 32 + > %He * 4) > > > > = 2950 / (.5 * 28 + .25 * 32 + > .25 * 4) > > > > = 2950 / 23 > > > > = 128 mols > > > > > > > > Nitrogen == 128 * .5 * 28 = 4.0 pounds > > > > Oxygen == 128 *.25 * 32 = 2.3 pounds > > > > He == 128 * .25 * 4 = .3 pounds. > > > > > > > >It would take a little re-learning, and you would still want to > analyse your > > > >gas before diving it, but it seems to make a lot of sense to > me... As I see > > > >it, the pros are: > > > > > > > > 1) Cheaper equipment needed > > > > 2) Gas temperature is irrelevant > > > > 3) Smaller range of measurement results in smaller "gauge > error" > > > > (ie: you're measuring between 80-100 pounds, not 200 - > > > > 2500 psi > > > > > > > >Cons: > > > > > > > > 1) % percent of gas == %psi of tank pressure in old > system, % gas > > > > is *NOT* % weight in this system. May be confusing. > > > > 2) % error increases as tank volume decreases > > > > > > > > > > > >Comments? > > > > > > > >Brian Greenberg > > > >Armchair diver :) > > > >-- > > > >grnbrg@cc*.um*.ca* > > > >-- > > > >+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > +++++ > > > >+ grnbrg@cc*.um*.ca* + NETDOC Developer, Libraries > Electronic + > > > >+ PGP public key available. + Technologies and > Services + > > > >+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > +++++ > > > >-- > > > >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > > > >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > > > > > > Bill (aquadart) Bott > > > > > > -- > > > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > > > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > > > > -- > > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. Bill (aquadart) Bott -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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