filling a small tank from a big tank

[remynajm]

Hello everyone, I'm a little new to the concept of tanks and how air pressure operates. I do have a 1L/4500psi tank and i am looking forward to buy a bigger one around 5L/4500psi.
My plan was when i go hunting, i ll take both tanks full with me, i ll keep the big one in the car and the small one in my backpack to refill the rifle once it's empty. when the small tank gets empty, i ll refill it from the big one. But my biggest concern was the pressure, i ve read somewhere that when you fill a tank from another, both tanks will try to equalise in pressure , which comes down to my question, if the 5L bottle gauge went down to 3500psi, will i still be able to fill the small one to 4500psi? Because in my head i m thinking that a 5L bottle should fill the 1L bottle 5 times but I guess that's not how pressure works. Can someone please clarify this point to me if a big bottle will fill my small one 5 times or i ll have a decrease in pressure everytime i refill the small one from the big one?

 

[PasadenaMike]

Youll never get 4500 in the little tank. Maybe very close if your 5 liter is at 4500 but just once then after that you’ll get a bunch of partial fills. Lower pressure than the last every fill.

 

[rc4fun]

You'll need to decide at what pressure you will refill the small bottle and what is the lowest pressure to make the fill usefully to you.
With those numbers this will give you an over estimate of number of refills.
Each time you refill the max pressure will be less. Still usefull just less shots each time before the next refill, until you get to the point both bottles are to low to be useful. The only way to get a 4500 refill each time is with a compressor.

https://www.pyramydair.com/widgets/tank-calculator.php

 

[Motorhead]

Big tank goes lower as small tank goes higher ... Being tanks are different sizes the ratio will be skewed based on the volume of each.

 

[AlanMcD]

It's just simple algebra, really - with ratios, as Scott said, being the key result . . . , but you have to remember that the pressure in your 5L tank will be dropping as the pressure in the 1L tank goes up. The pressures in the two tanks will equalize as they fill, and it is important to know that the smaller tank won't be empty when you go to refill it - just reduced and partially full (but too low to continue to fill your gun). The ratio will be roughly 5:1 in the pressure change (not perfect due to the air not behaving as an ideal gas, but close enough for what you want to know).

Based on what you said, the basic equation is:

Pressure in 5L tank minus "X" = Pressure in 1L tank plus 5 times "X"

If we solve for X we find that X = (pressure in 5L tank - pressure in 1L tank) / 6

As an example, suppose you shoot down the small tank to 3000 psi before the first fill, and then you hook up the two tanks and let the pressure equalize (that is the highest you can get the 1L tank without a compressor).

X = (4500 - 3000) / 6 = 1500 / 6 = 250 psi

So when you are done with that fill, your two tanks will be equal at 4250 psi . . . (big tank = 4500 - 1x250, and the small tank = 3000 + 5x250 )

Hope that helps!

 

[Spartan]

...if the 5L bottle gauge went down to 3500psi, will i still be able to fill the small one to 4500psi? Because in my head i m thinking that a 5L bottle should fill the 1L bottle 5 times but I guess that's not how pressure works. Can someone please clarify this point to me if a big bottle will fill my small one 5 times or i ll have a decrease in pressure everytime i refill the small one from the big one?

No. As mentioned, the air in the tanks is stored energy and unless you get a compressor back in the mix, all the big tank can do is transfer some of it's 4500 psi air into the 1L tank which is equalizing the pressure across both tanks and therefore resulting in a pressure below the starting point for both tanks (assuming the 1L tank was below a typical working pressure, say 2000psi, or even empty). There is no situation where a tank that is at 3500psi can fill another tank above that starting pressure when equalizing.

But much of this may not matter since 4500 psi is ~310bar and most (but not all) airguns operate at fill pressures between 200-250bar. So if your large tank is filled to 4500 psi, you can get several fills (equalizations) to the smaller 1L tank that will stay above that 200-250 bar mark. Especially if you are refilling the 1L tank when it is already at 150bar or so. Keep in mind you should only have to fill the 1L tank from empty one time. Each other fill, there will be residual pressure from when it dropped just below your guns fill pressure, so really you are just topping it off each time (150bar to 300bar, not 0bar to 300bar). Of course, you will need to occasionally empty the tank for service or inspection but this should not happen often at all...really only once every 5 years unless there is a problem.

Now, if you are using guns that have 300bar fill pressures, this will certainly limit the number of practical fills for the 1L tank. And this is just one of the reasons I avoid the 300bar guns.

 

[rodm1]

You may need a Altaros booster.

 

[xiong_fong]

Not to hack or steal thread. This is wat im wondering too. But from 45min tank at 3000psi. And to fill a gun, (not tank) to 3000psi/250 bar. How many fills can i expect to get. I ask because i tried to fill a hatsan jet2 (250bar) with a .45l bottle (250 bar) And it didnt even fill it due to same max pressure. I was hoping at least 1 fill. But didnt.

 

[AlanMcD]

Not to hack or steal thread. This is wat im wondering too. But from 45min tank at 3000psi. And to fill a gun, (not tank) to 3000psi/250 bar. How many fills can i expect to get. I ask because i tried to fill a hatsan jet2 (250bar) with a .45l bottle (250 bar) And it didnt even fill it due to same max pressure. I was hoping at least 1 fill. But didnt.

Well, first off, 3000 psi is only 207 bar, so if you are trying to fill a 250 bar gun from that you are way short to start with . . . 250 bar is 3625 psi (~14.5 psi per bar). You say you have a 45 minute tank, and those are almost always fill to a max of 4500 psi / 310 bar. But if your tank is not full and only at 3000 psi, then you can't fill even to that level when you add a reservoir to it that is below that level.

Air pressure will always equalize at a level that is between the high and low pressure levels of the two tanks that are connected together, and will do so based on the formulas I described to the OP in reply #5. Read it a few times if needed, and try to work it out based on what you have yourself - you need to know the volume and pressure of the two tanks involved, but it is easy to figure out once you understand it . . . . and you can do the math in either PSI or Bar, as long as your units are the same for both tanks.