Help with tank sizing (math)

[Fats]

I'm not a diver (though I did once in Mexico and loved it), but I need advice about buying a used 91cu' aluminum tank with a fresh hydro for High Pressure Airgun use. How many fills of a 100cu" airgun tank at 2900psi can I get from that 91' scuba tank filled to 3000? (It's been too long since I had physics).

Now, as I think about it I know that there are 1728 cubic inches in one cubic foot so 17 fills would use up one foot, and that would maybe reduce the pressure by 1.1%, no? So now we have how much pressure in the scuba tank? 3000 minus 330? If I lose another 330 after a second round of 17 I'm done. No more useful air I think. So, did I answer my own question? 34 useful fills (maybe 40)

 

[tbone1004]

something like that, but it's very inefficient since you'd be wasting all of the gas left below 2900psi and the shops will charge you full price to top off the 100psi that you used so it would be a colossal waste of money

 

[JackD342]

First, 1.1% of 3000 is 33, not 330. Now, I'm pretty good with math, but I know squat about airguns.
First question - is that 100 ci tank specifying the interior volume of the cylinder? Or is it the volume of compressed air that it holds at 2900 PSI, the way that scuba tanks are measured? For the moment I will assume it is the latter.

If so, here is my math, although I'm sure someone else who has done this will weigh in and confirm or correct.

1. A 91 cf tank that moves from 3000 psi to 2900 psi, will supply 1/30 of 91cf, or 3.033cf. Converted to 1728 ci per cf is 5246ci.
2. 5246 ci will provide 52 fills of 100ci at 2900 psi.

Of course you can continue to fill far longer than that, just not to your full 2900 psi, so each subsequent fill lasts a little shorter... less long... whatever...

But the more I think about it, I suspect it is far more likely your airgun tank holds 100ci with zero pressure. (is this the same as a paintball tank, just called something different?) In that case, I think you would find that the first transfill will equalize at about 2650 PSI in each tank, if the airgun tank starts at empty, so not even one at 2900. If it started at 1500, your first transfill would level off at about 2825.

In practice, you would be continually topping off the airgun tank, maybe once it hits 1000 psi? So you get a series of continually diminishing partial fills.

I'll be curious to see what someone closer to the subject has to say.

 

[Bob DBF]

I'd just like to know more about a 91 cuft Al tank, never ran across one.


Bob

 

[noj3333]

Maybe look into a HP tank... more usable air due to the higher starting PSI???

**Not sure if that'll help at all... didn't put much thought into it...**

 

[Fats]

First, 1.1% of 3000 is 33, not 330. Now, I'm pretty good with math, but I know squat about airguns.
First question - is that 100 ci tank specifying the interior volume of the cylinder? Or is it the volume of compressed air that it holds at 2900 PSI, the way that scuba tanks are measured? For the moment I will assume it is the latter.

If so, here is my math, although I'm sure someone else who has done this will weigh in and confirm or correct.

1. A 91 cf tank that moves from 3000 psi to 2900 psi, will supply 1/30 of 91cf, or 3.033cf. Converted to 1728 ci per cf is 5246ci.
2. 5246 ci will provide 52 fills of 100ci at 2900 psi.

Of course you can continue to fill far longer than that, just not to your full 2900 psi, so each subsequent fill lasts a little shorter... less long... whatever...

But the more I think about it, I suspect it is far more likely your airgun tank holds 100ci with zero pressure. (is this the same as a paintball tank, just called something different?) In that case, I think you would find that the first transfill will equalize at about 2650 PSI in each tank, if the airgun tank starts at empty, so not even one at 2900. If it started at 1500, your first transfill would level off at about 2825.

In practice, you would be continually topping off the airgun tank, maybe once it hits 1000 psi? So you get a series of continually diminishing partial fills.

I'll be curious to see what someone closer to the subject has to say.

Thanks Jack!

Your second supposition is correct. The "tank" is the hollow tube just below the barrel of the air rifle. It's volume is said to be 100 cc. --Yes, I thought inches, but really its cubic centimenters, so about 68cu inches empty. This particular gun shoots about 18 pellets before the power starts to drop and you are correct about being able to keep shooting after that, but I need the power of the first 18 so I'll probably be just drawing off the bit used for 18-36 pellets.

Now, actually, the optimum fill pressure for the gun is 2700 psi. And yes, since I will have around 100 bar left in the gun, so I would get easily 52 plus 1/3 = about 70 fills per 100psi of tank pressure therefore a sure 200 fills before having to go back to get a refill, correct?

 

[Fats]

I'd just like to know more about a 91 cuft Al tank, never ran across one.


Bob

It's a monster tank Bob; noticeably bigger than a normal aluminum 80.

 

[Fats]

Maybe look into a HP tank... more usable air due to the higher starting PSI???

**Not sure if that'll help at all... didn't put much thought into it...**

Yeah, I'm looking into SCBA tanks, but they seem to be outrageously expensive even used. I see there are Chinese ones on ebay at about $300 new, but then you have to buy a valve and hose and I haven't figured out whether the fill valves are compatible with the metric thread of the Chinese ones.

 

[JackD342]

Thanks Jack!

Your second supposition is correct. The "tank" is the hollow tube just below the barrel of the air rifle. It's volume is said to be 100 cc. --Yes, I thought inches, but really its cubic centimenters, so about 68cu inches empty. This particular gun shoots about 18 pellets before the power starts to drop and you are correct about being able to keep shooting after that, but I need the power of the first 18 so I'll probably be just drawing off the bit used for 18-36 pellets.

Now, actually, the optimum fill pressure for the gun is 2700 psi. And yes, since I will have around 100 bar left in the gun, so I would get easily 52 plus 1/3 = about 70 fills per 100psi of tank pressure therefore a sure 200 fills before having to go back to get a refill, correct?

Let's define terms before any more math. First a check that you are using bar correctly. You are saying that at about 1500 psi is when you need to top up the gun pressure, right? Also, about that tank size. How did you get 91, just a guess? is it definitely a scuba cylinder and not something else? Why not include a couple photos of the shoulder markings. Might be a 100cf, 3300psi tank.

 

[Fats]

Well, the guy selling it has a little shop in his basement with some used gear and a huge compressor. He told me it was 91cu feet and it is much larger than the 80 and 65 sitting next to it. I didn't think to take a photo. It did have a hydro stamp 8-18 and he said if I paid for a VIS sticker he would put one on. I know, sketchy. I'm looking into alternatives. Plus it is kinda pricey. He did have an aluminum tap valve for $30 and a good brass one for a hundred over the tank price.
It is a Scuba tank, Catalina I think. 1999. Definitely limited to 3000psi. I asked if it could go 10% over. He said no. and yes 100 bar is about 1500 give or take since I'm at 800 altitude.