rstofer
Contributor
Then too, the ST72 isn't empty! It probably has 1 ATM of air already in it. It's a small difference but it is real.
Richard
Richard
transfill question
- Thread starter DaleC
- Start date
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OP
DaleC
Contributor
I'm bubblewrapping some right now!
If I'm strangely silent on the matter it is because I am still trying to wrap my head around those calculations. I have a good brain cell but it takes a while for everything to slip through the tunnel.
halocline
Contributor
Assuming that the two tanks would equalize at 1500 PSI is not correct. Imagine filling a 6 cft bottle from an AL80 at 3000 PSI. The two tanks equalized would be slightly under 3000.
The way I would do this is to find out the total volume of both tanks. The AL80 has 77cft @3000 PSI. The LP 72 has 71.2 @2475. If I'm doing it correctly, that's roughly .37 cft for the AL80 and .42cft for the LP72 at 1 atm. So, total volume .79cft, which means the original air should equalize at around 1405PSI, which leaves around 36 in the AL80 and 41 in the LP72. There's your 77cft.
I'm not sure if I did this exactly correctly, I've never been that good at math. If someone can point out my mistakes, I'd be appreciative.
The way I would do this is to find out the total volume of both tanks. The AL80 has 77cft @3000 PSI. The LP 72 has 71.2 @2475. If I'm doing it correctly, that's roughly .37 cft for the AL80 and .42cft for the LP72 at 1 atm. So, total volume .79cft, which means the original air should equalize at around 1405PSI, which leaves around 36 in the AL80 and 41 in the LP72. There's your 77cft.
I'm not sure if I did this exactly correctly, I've never been that good at math. If someone can point out my mistakes, I'd be appreciative.
That's pretty close to what Spimon and I came up with. Spimon was a little low because he didn't take into account the 10% overfill on the 72. My numbers were a little off because I couldn't find the exact tank volumes when I looked them up. One way or another, you need the tank volumes.
halocline
Contributor
Spimon
Contributor
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It seems we're all working with slightly different figures. For the original poster's sake, I've made a MS Excel conversion tool to calculate what you wanted to know. You can feed in whatever information you like/have.
However, don't take it to be gospel, as the manufacturer probably hasn't given you particularly accurate information anyway. 77 cubic feet might be 76.6 or 77.4. But it'll give you a 'close enough' approximation. My guess is you just want to know if you'll get a dive out of this fill, or not - it's unlikey in the SCUBA world that you'd rely on results being highly accurate. The spreadsheet is reasonably accurate, relative to the input information. It does not take into account heat transfer, but if you leave the tanks long enough that they return to ambient temperature, that won't really make a difference anyway.
The tool I made is in the 'tools' section of my website, called Tank Equalisation Calculator. The gas constants are just there to accommodate other gasses, so just leave it at 0.287 for air. For your purpose, leave the temperature at 24.5C (room temp), but it may be of interest to you to change the temperature and see what happens to the pressures
The site is Simon Yachting - YA/RYA Skipper , as in my signature or directly at http://www.simonyachting.110mb.com/tools/Tank_Equalisation.xls
Hope it helps
However, don't take it to be gospel, as the manufacturer probably hasn't given you particularly accurate information anyway. 77 cubic feet might be 76.6 or 77.4. But it'll give you a 'close enough' approximation. My guess is you just want to know if you'll get a dive out of this fill, or not - it's unlikey in the SCUBA world that you'd rely on results being highly accurate. The spreadsheet is reasonably accurate, relative to the input information. It does not take into account heat transfer, but if you leave the tanks long enough that they return to ambient temperature, that won't really make a difference anyway.
The tool I made is in the 'tools' section of my website, called Tank Equalisation Calculator. The gas constants are just there to accommodate other gasses, so just leave it at 0.287 for air. For your purpose, leave the temperature at 24.5C (room temp), but it may be of interest to you to change the temperature and see what happens to the pressures
The site is Simon Yachting - YA/RYA Skipper , as in my signature or directly at http://www.simonyachting.110mb.com/tools/Tank_Equalisation.xls
Hope it helps
Last edited:
OP
DaleC
Contributor
Thanks all!
The reasoning behind my question is that I have a 72 with a 1/2" valve that nobody wants to fill even though it passes hydro. For the few times I want to do a vintage dive with it I'm thinking of transfilling from 2 or 3 80's or a 130. Where there's a will; there's a way.
Besides the + rating to get it to 72 cuft. I see my mistake in assuming both would even out at 1500psi. I knew something was amiss but couldn't put my finger on it.
I'm off to look at that tool now Simon.
The reasoning behind my question is that I have a 72 with a 1/2" valve that nobody wants to fill even though it passes hydro. For the few times I want to do a vintage dive with it I'm thinking of transfilling from 2 or 3 80's or a 130. Where there's a will; there's a way.
Besides the + rating to get it to 72 cuft. I see my mistake in assuming both would even out at 1500psi. I knew something was amiss but couldn't put my finger on it.
I'm off to look at that tool now Simon.
Spimon
Contributor
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Hmm. Might take a little more than 2 or 3 goes at it. Might be worth while if you dive very often and just take a little each time you get a tank filled. Remember, each time you equalise, less mass is transferred so by the end, you're not losing much from the donor tank.
With an 80@3000psi:
1st: 1336 psi
2nd: 2077 psi
3rd: 2487 psi
4th: 2716 psi
5th: 2842 psi
6th: 2912 psi
7th: 2951 psi
8th: 2973 psi
With a 130@3000psi:
1st: 1718 psi
2nd: 2449 psi
3rd: 2758 psi
4th: 2890 psi
5th: 2946 psi
6th: 2970 psi
7th: 2980 psi
8th: 2984 psi
Not a great deal of difference between using an 80 and a 130. There still isn't enough difference in capacity for it to be dramatically different. Pressure still plays more of a role than volume, and as the pressure difference decreases, so does the mass transfer.
Even with 2x130's (if you have the facilities to set them up on a doubles manifold and put the fillling whip on that) will start off quicker, but it still takes a long of goes before that final bit to get to the high 2900's. And you're then paying for twice the refills, anyway.
With a 2x130@3000psi:
1st: 2182 psi
2nd: 2770 psi
3rd: 2929 psi
4th: 2971 psi
5th: 2983 psi
6th: 2986 psi
If you can get your hands on a 300 BAR (~4000psi) 80cu.ft, it'd be done after only 2 goes.
1st: 1978 psi
2nd: 3057 psi.
I have no experience with the 300 bar tanks so not sure if it's possible for you to get your hands on them (or how many places can fill them), but if you want to do this regularly, it might be worth investigating. It's really the pressure difference that you need.
With an 80@3000psi:
1st: 1336 psi
2nd: 2077 psi
3rd: 2487 psi
4th: 2716 psi
5th: 2842 psi
6th: 2912 psi
7th: 2951 psi
8th: 2973 psi
With a 130@3000psi:
1st: 1718 psi
2nd: 2449 psi
3rd: 2758 psi
4th: 2890 psi
5th: 2946 psi
6th: 2970 psi
7th: 2980 psi
8th: 2984 psi
Not a great deal of difference between using an 80 and a 130. There still isn't enough difference in capacity for it to be dramatically different. Pressure still plays more of a role than volume, and as the pressure difference decreases, so does the mass transfer.
Even with 2x130's (if you have the facilities to set them up on a doubles manifold and put the fillling whip on that) will start off quicker, but it still takes a long of goes before that final bit to get to the high 2900's. And you're then paying for twice the refills, anyway.
With a 2x130@3000psi:
1st: 2182 psi
2nd: 2770 psi
3rd: 2929 psi
4th: 2971 psi
5th: 2983 psi
6th: 2986 psi
If you can get your hands on a 300 BAR (~4000psi) 80cu.ft, it'd be done after only 2 goes.
1st: 1978 psi
2nd: 3057 psi.
I have no experience with the 300 bar tanks so not sure if it's possible for you to get your hands on them (or how many places can fill them), but if you want to do this regularly, it might be worth investigating. It's really the pressure difference that you need.
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