How many fills...

[subbeh]

...will an 80 cubic foot 3300 PSI scuba tank put into my 88 ci 3000 PSI tank? Yes I do realize that after the first fill or two I would not get 3000 PSI into the tank. How many decent fills could I get?

[Cliffio]

if its a 3300 psi tank it would be dangerous to run it straight into a 3k tank

after enough times of you ramming in an etra 300 psi that tank isnt gonna be able to hold it all, just a thought

[subbeh]

Good point... well say it's a 3000 PSI Tank... how many fills?

[MikeCouves]

Actually I am pretty sure tanks don't blow when they are just a little over 3000 psi. Apparently you can go over somewhat. Depends what burst disc(s) you have. Better check with someone that knows alot more about it than me.

[subbeh]

With what I've heard of accidents happening, I won't risk the extra 300 psi. I Would go with a 3000 PSI Tank instead, definatly.

[Cliffio]

Originally posted by MikeCouves
Actually I am pretty sure tanks don't blow when they are just a little over 3000 psi. Apparently you can go over somewhat. Depends what burst disc(s) you have. Better check with someone that knows alot more about it than me.

just because you can do it doesnt make it safe
i dont care how many psi it is over, if the tank is 3k then fill the thing to 3k not 3k+1

[FatMan]

I've posted on this before :rolleyes:

Anyway, if you're going to do fills from a scuba tank you want to get a 3300 psi tank. The main reason is that you don't HAVE to fill it all the way to 3300 - even so you will want to fill over 3000 psi because the pressure will drop a good bit as the tank cools. I never seem to end up with 3000 psi in our 3300 psi tanks by the time we get to the field.

That said, The number of fills depends not only the size of the tank and pressure, but on how low your gun's bottle gets before you refill it. If you refill after every game that will be different than running it bone dry and then filling. I have never tried to see how many empty bottles I could get to say 2000 psi from a tank. I know we have gotten 15-20 top-offs from a tank before. Suffice it to say that one guy can easily play all day with his own scuba tank. Two guys should be able to also. Three, it depends. Four or more you won't be going all day. Our club has three and we call arrive at the field with a full bottle (fill off N2) and manage to support 10 people or so for a day of play.

I suppose if there is REAL interest we could try it out some time and see what we get. Actual empirical numbers. Or you could do the math.

Play safe and have fun,
FatMan

[FatMan]

Hey, just for grins I worked up a quick spreadsheet that plots what you should expect filling from a scuba tank - ignoring temp issues and such.

If you assume an 80cf 3300 psi tank and you fill 60ci bottles that are flat empty (0 psi) you would expect the first 10 fills to look like this in psi:

2956
2648
2371
2124
1903
1704
1526
1367
1225
1097

Note that even through the tank is 3300 psi, the first fill is still under 3000 psi. For a more realistic scenario, where the tank starts at 3150 psi (assume it has cooled) and the bottle has 500 psi in it before each fill you get:

2874
2626
2404
2206
2028
1868
1726
1598
1483
1381

If you want to try other situations, I'll let you have the spreadsheet.

Play safe and have fun,
FatMan

[subbeh]

Originally posted by FatMan
Hey, just for grins I worked up a quick spreadsheet that plots what you should expect filling from a scuba tank - ignoring temp issues and such.

If you assume an 80cf 3300 psi tank and you fill 60ci bottles that are flat empty (0 psi) you would expect the first 10 fills to look like this in psi:

2956
2648
2371
2124
1903
1704
1526
1367
1225
1097

Note that even through the tank is 3300 psi, the first fill is still under 3000 psi. For a more realistic scenario, where the tank starts at 3150 psi (assume it has cooled) and the bottle has 500 psi in it before each fill you get:

2874
2626
2404
2206
2028
1868
1726
1598
1483
1381

If you want to try other situations, I'll let you have the spreadsheet.

Play safe and have fun,
FatMan

Thanks much for the effort! I Think it would be well worth my time to get an 80 CF Tank because I have to drive 30 miles to fill my 88 ci 3000 psi.

[media]

Well, I used correct math on this subject before, but I used bad numbers. A 68ci tank holds 68ci inches of water but around 11cf of compressed air. Scuba tanks use the cf of air they hold for reference while paintball tanks are commonly refered to by the water volume number.

simple formula to estimate fills:

(pressure scuba after fill)*(volume scuba) + (pressure pb tank)*(volume pb tank(11cf for 68ci)) = (original scuba pressure)*(scuba volume)

since the pressure in pb tank and scuba will be equal when done filling just use the same variable for both.

x(80) + x(11) = 3300(80)

x(91) = 264000

x = 2901psi

no worries about over filling there.

for your next fill multiply 2901 times 80 then divide by 91
and so on.

[Tufiremn]

Just FYI, There is no danger in filling a 3000psi tank off one that has 3300 psi. For one the burst disk is set much higher than 3000psi, and that tank can actually take somewhere in the range of 4000+psi safely. There is a safety factor calculated into it. Second, you won't fill the 3000psi over that mark, unless you just leave the valve open till the two tanks equalize. Look at a 4500psi fill station. The pressure in the fill tanks can be as high as 6000psi, but these same tanks can be used to fill both 4500, and 3000 psi tanks. It's all about when you close the valve. BTW, Try to fill the HPA tank slowly, as filling it fast will heat the air, and you will end up with less air once it cools down.

Another idea is to use 2 or more scuba tanks as a cascade system. Start filling off the tank with the lowest pressure that is still higher than what is in your HPA tank. Once it stops filling move to the tank with the next higher pressure and so on.

[FatMan]

Originally posted by media

simple formula to estimate fills:

(pressure scuba after fill)*(volume scuba) + (pressure pb tank)*(volume pb tank(11cf for 68ci)) = (original scuba pressure)*(scuba volume)


Well, I looked at it like this:

Vt is volume of the scuba tank
Vb is volume of the gun bottle
Pt is the original pressure of the tank
Pb is the original pressure of the bottle
Pf is the final pressure of the tank and the bottle.

Then you have

PtVt + PbVb = Pf(Vt+Vb)

Thus Pf = (PtVt + PbVb) / (Vt + Vb)

The volume of a bottle is 68ci. A scuba tank rated at 80cf 3300psi has a volume such that when you compress 80cf of atmopheric pressure air until it reaches 3300psi, it fits in the tank. I threw together a quick formula to compute the actual volume of the tank in ci from the pressure and air capacity.

FatMan