Fills off of a Cascade of two Scuba tanks: is it worth the $?

[matteusz]

So I asked once before and got some wonderful info on how many fills I would get off one scuba tank. pneumagger gave us this:

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.

SO now I am thinking of getting a second tank and a cascade to connect them. Can anyone do a little more math or share their experience with me on what you can fill with two tanks instead of one? Your help is greatly appreciated.
Mat

[Madmarx]

Well no math here but...
I use 2 tanks (not hooked together) and that seems to work rather well.
I use the first tank for the bulk and the second tank to top it off.
This was a suggestion from the scuba shop that fills my tanks.
If hooking them together would be evan better I'd be interested in hearing how that should be done.

[DevilMan]

The place I play don't have a compressor so they use scuba tanks. Have like 5 of them. Not connected. You start with one and move down the line going from the lowest pressure to the highest.... Yeah it's a bit of a bother but it works.

DM

[matteusz]

The place I play don't have a compressor so they use scuba tanks. Have like 5 of them. Not connected. You start with one and move down the line going from the lowest pressure to the highest.... Yeah it's a bit of a bother but it works.

DM

I talked to a scuba shop this morning and they said all I do by hooking them together is increase the volume. Which makes sense. When you fill down a line of tanks like that you get the same effect as with the tanks hooked together (as I understand it). So no the cascade is not likely to be worth it.

What about a larger tank? Anybody get a big one (like 100+ CF and have your local shop fill it?)
Mat

[Raven001]

We used to use three scuba tanks per person. Bulk fill from the first and top up from the second. When that got low, top up from the third. Eventually though, we purchased a 4500 psi 50lb steel tank. It ended up cheaper in the long run and as long as we only topped up to 3000 psi, it worked well for us but you need a truck to move it around.

[Mechanic79]

Get one large 120 cu ft 3500 psi scuba tank
get 2 80 or 100 cu ft 3000 psi tanks

That should last you quite some time.

Start filling off one of the 2 80's. once it drops about 500 psi (2500) then use all three in a line.

[matteusz]

OK so I have the option to buy a 321 CF 3500 PSi tank for 120$ and it will cost 60$ to have it filled. It would cost me 80$ for the fill adapter.

Or I can buy three 80 cf scuba 3000 psi tanks for 450$ and it will cost 25$ to get them filled. Fill adapter would be 3 of the yoke types for 150$ roughly.

Which one will give me cheaper fills per use with my smaller tanks. So how much will it cost me to fill my smaller tanks from each one and is it worth the higher fill price or the higher purchase price?

If someone does some impressive math on this (or comes up with a great and workable alternative) I will give them a discount on a Logic frame or some free AGD stickers in the mail to make it worth your while.
mat

[athomas]

Don't fasten the tanks together. It will increase your capacity, but it will mean both tanks will be empty (low pressure) at the same time.

I cascade my tanks. I use one to fill and one to top off. The topper tank allows me to maintain a higher pressure in my HPA tanks. When the first tank gets low, I get it filled and it becomes the top-off tank and the original top-off tank, becomes the starting fill tank. This maximizes the amount of usable fills you get from your tanks.

I only pay $5 at the local scuba shop for my 3000 psi fills too.

[MANN]

Don't fasten the tanks together. It will increase your capacity, but it will mean both tanks will be empty (low pressure) at the same time.

I cascade my tanks. I use one to fill and one to top off. The topper tank allows me to maintain a higher pressure in my HPA tanks. When the first tank gets low, I get it filled and it becomes the top-off tank and the original top-off tank, becomes the starting fill tank. This maximizes the amount of usable fills you get from your tanks.

I only pay $5 at the local scuba shop for my 3000 psi fills too.

QFT

I can supply air for 4emags, 2 classic RTs, 3 tippmann A5s for an entire day with each marker shooting ~ 1500 rounds. I use 3 scubas, and always take enough home to play/test/fix any time I want. I actually only fill 2 each time I play, and use the highest pressure tank from the previous game.

My fills are also only 3.00ea So each time I play I pay 6 bucks, and get 5 bucks from everyone who gets "all day air". Scubas are defiently worth it IMO.