Equalizing tanks

[Ingolf]

hey, a local gas supply company here has 6.4cu meter breathing gas tanks. I was going to pick one up so i am able to fill my own tanks out in remote locations when theres no filling stations around. If my math is right
[h=2]6.4 (cu meters) = 226.013867 cu feet[/h]
so on an 80 cu ft tank i would theoretically get almost 3 fills off of one of these tanks. But im pretty sure as the big tank fills the first 2 i would be losing pressure and i wouldnt get the total amount out. is there any way to calculate how much will still be in after i fill 2 and a half tanks?

also im just gonna get some hydraulic fittings made up for the high pressure, a coupling on each end with a pressure gauge on the incoming tank and a bleeder valve.

also they informed me they fill these bulk tanks at 2100 psi.

 

[SOMD Diver]

Then you wouldn't even get one fill. Since the pressure in your bank bottle is lower than the operating pressure of your 80cu ft tank. Just bring some extra bottles.

 

[Ingolf]

what? that doesnt make sense.

if i was using my 40 cu tank, i could theoretically get 226cu ft out of my storage tank, 226 / 40 = 5.5 theoretical fills. But more likely 3 fills and a bit.

and thats the whole point of doing this, is so i dont have to have 10 extra bottles. If i have 2 or 3 main tanks, i could fill a 40cu 9 or 10 times without having to drive back or buy 9 more 40 cu's

(i obviously wont be filling them to 3000 psi, 2100 is fine though as i wont be more then 10 or 15 feet down and maximum bottom time isnt the goal. Dock repair is what it involves so ill be on the surface more often then not)

 

[hypertech]

That's only if you can pump a vacuum on the storage tank and get all the gas out. But you can't.

It will fill the first tank to less than 2100 psi and each after that even lower.

 

[Ingolf]

actually i do have a vacuum pump so i could. pump is 6cfm how long would it need to be on

 

[kotik]

Unless your pump is rated for a few thousand psi, you're not going to get anything but injuries out of the deal. You would need a booster pump, Haskell is the name that has most brand recognition. They are $$.

 

[redacted]

actually i do have a vacuum pump so i could. pump is 6cfm how long would it need to be on

About as long as it takes for water to flow north in the Mississippi.

 

[chrpai]

Unless your pump is rated for a few thousand psi, you're not going to get anything but injuries out of the deal. You would need a booster pump, Haskell is the name that has most brand recognition. They are $$.

If you had a pump rated for a few thousand psi you wouldn't need these tanks! Boosters are meant for sucking expensive things like helium out of those tanks.

 

[JahJahwarrior]

What pressure will your vacuum pump pump to? You don't really want a traditional vacuum pump, we are dealing with input pressures of up to 2100psi (in this example) and output pressures from 3000-4000psi.

If you are asking questions like this then you probably are more than one weekend away from pulling it off safely. You need to grasp the relationship between pressure and volume, and how the gases equalize when connected.

Without a proper compressor or booster you won't get more than 2100 psi from the extra tank, with the amount falling a lot on subsequent dives.

The first dive, lets say you take the tank on your back to 100psi. You hook it up to the storage bottle you rented, and open all the valves on your whip, and two tanks. Gas moves back and forth until both bottles are at the same pressure. Lets say that pressure is 1900psi. You'll have taken about 60 cubic feet of gas out of the storage tank and put it in your scuba tank.

Next dive, you do the same thing, but now they equalize at 1600psi and you only put 45 cubic feet in your tank.

Third dive they are at 1200psi and you only put 30 cubic feet in your tank.

Of course those numbers are just wild guesses, but they illustrate the concept.

With a booster (a compressor which can accept an input pressure significantly higher than ambient and go all the way to 0) you can use the storage bottle at a lower pressure and fill the scuba tank to the proper fill. However, you will also need a third drive gas at a lower pressure but at a very high volume, to run the booster.

It might be cheaper to buy a small portable compressor, but then you have to educate yourself on not only the equalization stuff, but also filters and maintenance. For most "casual" divers it's simplest to buy a few extra scuba tanks. Also, consider the amount of space and weight that the fill setup will take up.

Remember, pressurized gases contain immense amounts of potential energy, and breathing gas impurities can be fatal. It's not rocket science, but it is very important to be careful, safe, and understand what you are doing.

 

[kotik]

If you had a pump rated for a few thousand psi you wouldn't need these tanks! Boosters are meant for sucking expensive things like helium out of those tanks.

Just pointing out that unless the pump is rated for a few thousand psi, it won't survive the pressures useful for scuba cylinders. It wouldn't survive the initial connection to a tank at 2100 psi, or be able to pump up the smaller tank.

A booster can be used to transfer any gas from one reservoir to another-- be it helium, oxygen (provided it's O2 clean), argon, or just plain clean, breathable air. Of course a compressor would be the more sensible route in this case, but if the OP's interested in pumping high pressure gas from one cylinder to another, a pointer to the kind of pump required might be in order.