I’m moving gas from an HP100 to an AL19 pony. We’ll round the 19cf pony up to 20cf for simplicity. So I would expect that the HP100 would lose 20/100 or 20% of its volume of air when I transfer to the pony bottle which is 3000*.20 or 600 psi. So should expect the two cylinders to equalize at 2400 psi?
My eyes tend to glaze over when I see written equations, so for a practical matter in this case I would just watch a gauge and try to fill the Al19 to it’s rated pressure. If you have a full HP100 (3442) you should be able to get the AL19 to close to 3000PSI.
If you want to understand what’s actually going on with the pressures/volumes and have trouble with equations like I do, you can pretty close by figuring it out; start by understanding that a cubic foot of gas (volume) is represented by a quite different amount of pressure in each tank. For the HP100, you have 100cft at 3442 PSI, so divide 100 by 3442 and you get each PSI is .029 cft. For the AL19, you have 19 cft at 3000 PSI, so each PSI is .0063 cft. Or, in the HP100 each cubic foot is 34.42 PSI, where in the AL19 each cft is roughly 158 PSI.
Assuming you start with a completely full HP 100 and a completely empty AL19, if you are transferring 19 cft from the HP 100, that is about 654 PSI in the HP 100. That would leave you with 2788 in the HP 100. But that can’t happen because the AL19 would then be at 3000. You can’t just split the difference (2894) because the tanks are different sizes, and that means they fill at far different rates. This is where the equations come in handy because all that can be factored in. But, since we’re just wasting time trying to reason it out…you can get pretty close by estimating. We know 19cft won’t work, let’s try 18. That drops the 100 to 2822 PSI. 18 cft in the AL19 is 2844 PSI. That’s pretty close.
These figures are not exactly accurate for a variety of reasons that someone more knowledgeable than I am could explain. One factor is that we’re talking about PSIG, as opposed to absolute PSI, and that the tanks are not ‘empty’ at 0 PSIG, they each have air at atmospheric pressure which equals the liquid volume of the tanks.
All of this assumes equal temperature, which I’m sure you realize is not going to be the case as you’re filling.
I have no idea if this long winded post is helpful, and I’m sure that if I’ve made a mistake, some kind soul will correct me.