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*
> > >--
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> >
> > Bill (aquadart) Bott
> >
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