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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'.
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