Fills out of a 80 CF scuba?

[matteusz]

I can do the math but don't find it reliable in the non perfect world. I am wondering if some people can clue me in on what kind of fills you get off a scuba tank filled to 3000 psi. I am wondering if a cascade setup is worth it (or a HPA setup for that matter).

So basically if someone knows. About how many tanks say 68 3000's do get of one tank, two tanks in a cascade and three tanks?

I would really appreciate the info. Thanks, Mat

[JRingold]

I'm going to go with around 14, 28 and 42 if you don't lose much in bleed off. I could be wrong, and my volume mathematics could be off.

I usually refill the scuba after about 10 fills currently, but I also have two 92cu.in tanks in addition to the 68's, and I don't use it unless I'm trying to fix something at home, so that lasts me about an entire season.

[Pneumagger]

PV=nRT
Assume nRT = constant
Assume V(s) = internal volume of scuba
Assume P(1) = Pressure in scuba before fill
Assume P(2) = Pressure in scuba and pb tank after fill
Assume V(t) = internal volume of paintball tank
State 1 = value before fill
State 2 = Value after fill

-----------------------

You can see the tank pressure drops with each tank fill and you will have to determine an "acceptable" minimum level of fill before you consider it in need of refill. With the above assumptions, P(1)*V(s) = P(2)*V(2) quickly simplifies to for your case:

P(1)*V(s) = P(2)*(V(s)+V(t))

Tabulated out, assuming a 3000psi filled 80CF cuba filling a 68/3000 tank, while taking dropping scuba pressure into account we see that...

Fills....Scuba Pressure...Tank Pressure
0.......3000.000000.....0
1.......2729.083665.....2729.083665
2.......2482.632551.....2482.632551
3.......2258.437314.....2258.437314
4.......2054.488127.....2054.488127
5.......1868.956663.....1868.956663
6.......1700.17970.......1700.1797
7.......1546.644216.....1546.644216
8.......1406.973822.....1406.973822
9.......1279.916425.....1279.916425
10.....1164.333003.....1164.333003
11.....1059.187393.....1059.187393
12.....963.5370043.....963.5370043

After 11 fills, the bulk tank fills the 12th tank at under 964psi and only filled the first tank to 2729psi. Factoring in quick or frequent filling (allowing the bulk tank to cool off) which is more representative of an adiabatic situation, the fill efficiency will worsen. So I'd say your looking at about 10-11 fills while maintaining at least 1000psi.

[hvacman250]

With my two tanks, the above post is right on the money. Actually, a little worse then that, but close.

[matteusz]

PV=nRT
Assume nRT = constant
Assume V(s) = internal volume of scuba
Assume P(1) = Pressure in scuba before fill
Assume P(2) = Pressure in scuba and pb tank after fill
Assume V(t) = internal volume of paintball tank
State 1 = value before fill
State 2 = Value after fill

-----------------------

You can see the tank pressure drops with each tank fill and you will have to determine an "acceptable" minimum level of fill before you consider it in need of refill. With the above assumptions, P(1)*V(s) = P(2)*V(2) quickly simplifies to for your case:

P(1)*V(s) = P(2)*(V(s)+V(t))

Tabulated out, assuming a 3000psi filled 80CF cuba filling a 68/3000 tank, while taking dropping scuba pressure into account we see that...

Fills....Scuba Pressure...Tank Pressure
0.......3000.000000.....0
1.......2729.083665.....2729.083665
2.......2482.632551.....2482.632551
3.......2258.437314.....2258.437314
4.......2054.488127.....2054.488127
5.......1868.956663.....1868.956663
6.......1700.17970.......1700.1797
7.......1546.644216.....1546.644216
8.......1406.973822.....1406.973822
9.......1279.916425.....1279.916425
10.....1164.333003.....1164.333003
11.....1059.187393.....1059.187393
12.....963.5370043.....963.5370043

After 11 fills, the bulk tank fills the 12th tank at under 964psi and only filled the first tank to 2729psi. Factoring in quick or frequent filling (allowing the bulk tank to cool off) which is more representative of an adiabatic situation, the fill efficiency will worsen. So I'd say your looking at about 10-11 fills while maintaining at least 1000psi.

I take it back I am too rusty to do the math and would have had to ask one of my engineer buddies for that formula. pnuemagger thanks man that is great stuff. Sounds like it is close to what people are experiencing too.

I think I will come over to the HPA side of life after I move. I can buy a new scuba and I get the first 20 fills free. Considering a fill costs 7-10$ that is close to getting the tank for free. I think I will buy one and then once I run out of fills get another one and a cascade. So worth it as my gun ran so nicely at D-Day on hpa.

[Spider-TW]

With my two tanks, the above post is right on the money. Actually, a little worse then that, but close.

I thought the losses from the hose might come out to something, but they were insignificant (~0.1 cu. in. of actual volume per foot of hose).

When doing pneu's calculation, you have to make sure you use the same volume units since the 80 cu. ft. is in standard cubic feet of air and the 68 cu. in. is actual volume (which you fill with some bigger number of standard cu in under pressure). With that, I came up with about 60 psi shorter at 10 fills.

It may just be the difference in order of my calculations. However, the Carleton specs list 68 cu in. as the minimum volume of the cylinder. The difference between using 68 and 70 cu. in. volume will get you a 30 psi difference (lower with 70 cu in) by the 10th fill anyway.

I used to have a CO2 fill station long ago. Looking at this, I am very glad my local field has reasonable air.

[hmudd13]

When I go out to play, I'll get one full fill, and then top off all day.
Keep in mind that every top off gets lower.
Good for atleast a case of paint. When I get home I'll top off all my other tanks (usually to around 1000 lbs) and go get it filled again.

[Spider-TW]

With decent 4500 fills (at least over 4000) I can get through a case with two fills on a 68. But I know what it is like not having good field air.

I wonder how much those air booster pumps cost that some fields use on their cylinder banks...

[matteusz]

With decent 4500 fills (at least over 4000) I can get through a case with two fills on a 68. But I know what it is like not having good field air.

I wonder how much those air booster pumps cost that some fields use on their cylinder banks...

Enough to make you cry. Unless you have a standing line of customers it is not worth investing in anything more than scuba tanks and a cascade (and even that is just an assurance that you will get to spend more time and money playing paintball because you have air!). If co2 were so problematic (even on classic mags it eventually gets old) I would just stick with that. Cheap bulk tanks, cheap fill station, cheap cheap cheap. Anything that regularly available has got to be part of what got paintball going hmmm maybe that is why every classic from the 90's has a massive expansion chamber on it.

[athomas]

Scubas are great. I use 2. I have one (tank A) that I start my fills with and the 2nd (tank B) to top it off. When tank A gets low, I get it filled and use it as the top off tank and use tank B to start the fills. I keep cycling them like this. I always get good fills.

The above numbers listd by Pneumagger are sort of correct. I don't think they take into account that the 2nd, 3rd, etc fills are into a tank that is not completely empty. 1/3 of the tank is already full. Therefore, the number and values of the subsequent fills are actually higher than listed.

example:
starting tank psi = 0
you fill the tank with 1000psi left

fill#..psi
1.....2726
2.....2568
3.....2425
4.....2295
5.....2177
6.....2070
7.....1972
8.....1883
9.....1803
10...1729
11...1663
12...1602
13...1547
14...1497
15...1452


example#2
refill at 1500psi

starting tank psi=0
1.....2726
2.....2614
3.....2512
4.....2420
5.....2336
6.....2260
7.....2190
8.....2127
9.....2070
10.....2018
11.....1971
12.....1928
13.....1889
14.....1853
15.....1821


In reality, you probably will never want to use any fill that is less than 2000psi as it doesn't give you much capacity on the field.

[maglover728]

This should be stickied.

[Pneumagger]

In reality, you probably will never want to use any fill that is less than 2000psi as it doesn't give you much capacity on the field.

Depends on what you shoot

Also, you are correct on the empty tank assumption.