# Pressure change inside a tank(s)

#### Pengwuino

Gold Member
Ok i was wondering something, lets say you have tank A and tank B that are identical. Tank A is pretty much empty and tank B is full of some gas at 200psi. Lets say you hook them up with a valve but the valve is closed. After the valve opens up and they are allowed to equalize, what will the pressure be overall?

Gut instinct says 100psi... but i dunno :)

Also, does anyone know if something exists that regulates pressure. Say you have a tank at 40psi and dont want ti to go any higher, is there something you could attach that opens up and releases the gas until it comes back down to 40psi in the event it goes beyond that?

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#### quasi426

If you neglect that volume of the valve it should be 100psi.

#### LeonhardEuler

Gold Member
The total pressure will be 100 psi if you neglect the volume of the valve and the gas is ideal. This is a result of Boyle's Law: PV=Constant. Double the volume[itex]\rightarrow[/tex]halve the pressure.

As for your second question, there exist devices that give digital measurements of pressure and there are devices that can control a valve electronically, so I've got to imagine there is a device like the one you describe.

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#### Pengwuino

Gold Member
Ah yes, gut instinct was correct again!

How bout that pressure regulator i was talken about? I was tinkering with the (probably non-feasible) idea of using LN as a propellant in a cannon but obviously, putting LN into a sealed tank without a safety mechanism is just asking for losing a limb or two.

#### zoobyshoe

Pengwuino said:
Also, does anyone know if something exists that regulates pressure. Say you have a tank at 40psi and dont want ti to go any higher, is there something you could attach that opens up and releases the gas until it comes back down to 40psi in the event it goes beyond that?
gas pressure relief valve - Google Search

#### Pengwuino

Gold Member
sweeeeeeeeet, thanks :D

#### LeonhardEuler

Gold Member
I just realized something else to keep in mind for the pressure to be 100 psi: the process must be isothermal. This is not that big a deal because a perfect gas expanding into a vacuum does no work, and the the internal energy of a perfect gas is a function only of temperature, so in order for energy to be conserved, the temperature will remain constant even if the process is adiabatic. This could pose a problem, however, if you have the gas stored at a very high temperature, much higher than that of the empty tank. This would cause a temperature drop, which would lower the pressure,

#### FredGarvin

Pengwuino said:
How bout that pressure regulator i was talken about? I was tinkering with the (probably non-feasible) idea of using LN as a propellant in a cannon but obviously, putting LN into a sealed tank without a safety mechanism is just asking for losing a limb or two.
Yes it would be since LN2 will flash off in a heart beat if it's not properly insulated and your pressure will skyrocket in an instant. BTW...not all pressure relief valves are created equal. If you do venture into this you had better make darned sure that the relief is sized properly or it won't do you any good.

A relief can be as simple as a check valve that has a high cracking pressure that is equal to the desired pressure you want in the tank. Once that pressure is reached, the check valve unseats and will start to bleed off the excess pressure. Of course there are some drawbacks, but it is easy.

Also at this pressure, technically you are dealing with pressure vessels.

Any particular reason why you would want to complicate things with using LN2? Why not simply compressed air?

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#### Pengwuino

Gold Member
haha, simply to complicate it :)

I was just thinken if you used liquid nitrogen, you could fire off more rounds (obviously we're insane people going for the gatling gun/cannon type of thing :P) because the tanks in a sense, constantly re-pressurizing.

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