Trans-filling and 'true' tank volumes

[lowviz]

I need to know exactly how much gas will be added to my dive tank if I attach a sacrificial dive tank to it with a trans-fill whip. The problem revolves around not knowing the true internal volume of either cylinder. So here is how I'm looking at it:

The difference in pressure between the two tanks gets converted to cubic feet of gas using the sacrificial tank's 'tank factor'. I'll call this available gas for this discussion. Now, it is obvious that all of this gas will not make it into the dive tank because the pressure in the dive tank will rise, thus holding a portion of the available gas back in the sacrificial tank. My plan is to ratio the available gas volume between the two tanks using the ratio of the tank factors for both tanks. This should let me know the gas volume that is actually transferred, making the pressue increase (of my dive tank) and its new oxygen content easy to determine. Anyone know the right way to calculate this?

[Everything gets checked with my O2 analyzer after it equalizes.]

 

[spectrum]

Just to be clear.... You are dealing with mixes other than air and want to determine final pressure and mix?

 

[TSandM]

It's a pretty simple simultaneous equation in two unknowns:

x*TF1 = y*TF2
SP1 + x = SP2 - y

Where x is pressure rise in the low tank in 100's of psi
y is pressure fall in fuller tank
TF1 is tank factor for low tank; TF2 is tank factor for full tank
SP1 is starting pressure in low tank; SP2 is starting pressure in full tank

 

[elan]

It seems to me you contradict yourself, in the beginning you say that the volume of either tank is unknown, then you suggest to use the second tank tank factor and after that you say you are going to use the tank factor ratios, for me it looks like enough data to calculate the tanks' volumes.

 

[rjack321]

Don't work in CF, make nitrox using the percentage of O2 needed relative to service pressure.

I.e. EAN is 14% O2 topped with air to service pressure.

 

[TSandM]

Are you trying to create a given mix, or just transfill from one tank containing a given mix to another? Some of the answers appear to assume the former; I assumed the latter.

 

[lowviz]

Apologies all, away for a while...

Just to be clear.... You are dealing with mixes other than air and want to determine final pressure and mix?

I wrote a simulator that will allow me to add a rich mix to a fully or partially depleted tank of some known mix then boost it with either air or some other nitrox mix. It is a two step process that runs simultaneously so you can adjust anything and immediately see what happens to the final mix.

It's a pretty simple simultaneous equation in two unknowns:

x*TF1 = y*TF2
SP1 + x = SP2 - y

Where x is pressure rise in the low tank in 100's of psi
y is pressure fall in fuller tank
TF1 is tank factor for low tank; TF2 is tank factor for full tank
SP1 is starting pressure in low tank; SP2 is starting pressure in full tank

Thanks, your two equation, two unknown set is what I'm assuming. Had to think about it though.

It seems to me you contradict yourself, in the beginning you say that the volume of either tank is unknown, then you suggest to use the second tank tank factor and after that you say you are going to use the tank factor ratios, for me it looks like enough data to calculate the tanks' volumes.

I should have said that the true volume that you would find by filling it with water is unknown.

Are you trying to create a given mix, or just transfill from one tank containing a given mix to another? Some of the answers appear to assume the former; I assumed the latter.

Perceptive, it is actually both. I'm trying to get the highest possible final pressue of the target mix using only the mixes and pressures I happen to have on hand.

 

[lowviz]

Don't work in CF, make nitrox using the percentage of O2 needed relative to service pressure.

I.e. EAN is 14% O2 topped with air to service pressure.

I don't have a compressor. This is just for the odd times that I can't get a fill in time for an unplanned dive opportunity. -not worth the expense of a compressor. The idea is to boost my tank with whatever is lying around in the most effective way possble. -see attached image, first trans-fill done, working on second.

(Thanks again Lynne, both methods agree. This is coded with my 'around the barn' method, -still checking it.)

 

[TSandM]

You're welcome. We use our transfill whip a LOT (although not to create mixes, just to top off tanks) so I can pretty much do these calculations in my head. I honestly don't know how anybody who dives a lot lives without a transfill whip! (And BTW, I got mine from a SB member -- ppo2diver builds a mean whip at a reasonable price.)

 

[rjack321]

I don't have a compressor. This is just for the odd times that I can't get a fill in time for an unplanned dive opportunity. -not worth the expense of a compressor. The idea is to boost my tank with whatever is lying around in the most effective way possble. -see attached image, first trans-fill done, working on second.

(Thanks again Lynne, both methods agree. This is coded with my 'around the barn' method, -still checking it.)

Ok but you can skip the math. Just use the highest pressure supply tank in the first place