# Gas Pressure vs Temperature moved between containers?

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## Main Question or Discussion Point

I have a question regarding an occurrence with Nitrogen gas and two separate containers. I'm trying to explain why when I fill my steel pressure vessels to ~180psi on an avalauncher from a larger 2000psi steel tank, the pressure slowly drops to ~175psi after the fill. I believe this to be temperatures equalizing after the fill but am unsure the properties at play here. Typically filling a tank causes pressure and thus heat. Pressure escaping from a container to a lower pressure releases heat, correct? In my situation, what causes one to over ride the other?

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Chestermiller
Mentor
The cooling takes place at constant volume and constant number of moles. What does the ideal gas law tell you about such a situation? In my judgment, the cooling to lower temperature is the cause, and the lower pressure is the effect.

The cooling takes place at constant volume and constant number of moles. What does the ideal gas law tell you about such a situation? In my judgment, the cooling to lower temperature is the cause, and the lower pressure is the effect.
forgive my ignorance but are you saying that when the nitrogen leaves the larger tank it will cool and then immediately heat as it compresses into the new tank?

Chestermiller
Mentor
Depending on the volume and initial pressure in the larger tank, the gas in the larger tank will cool as it expels gas. The amount of cooling will typically be small for a very large high-pressure tank. The gas entering through the inlet valve of the smaller tank is about the same temperature as it was when it was being expelled from the larger tank. But the newly entering gas to the smaller tank compresses the gas already inside the tank, and causes its temperature to increase. So immediately after the smaller tank is filled, its temperature of the gas in the smaller tank is higher than it was when it was originally in the larger tank. Next, the gas in the smaller tank will cool as it re-equilibrates with the surroundings temperature. This will cause its pressure to drop a little.

Depending on the volume and initial pressure in the larger tank, the gas in the larger tank will cool as it expels gas. The amount of cooling will typically be small for a very large high-pressure tank. The gas entering through the inlet valve of the smaller tank is about the same temperature as it was when it was being expelled from the larger tank. But the newly entering gas to the smaller tank compresses the gas already inside the tank, and causes its temperature to increase. So immediately after the smaller tank is filled, its temperature of the gas in the smaller tank is higher than it was when it was originally in the larger tank. Next, the gas in the smaller tank will cool as it re-equilibrates with the surroundings temperature. This will cause its pressure to drop a little.
thank you so much for the thorough answer. is there a particular "law" associated with this?

Chestermiller
Mentor
thank you so much for the thorough answer. is there a particular "law" associated with this?
We can quantify this using the specific geometry and conditions of the system. The main part of the calculation involves application of the open system (control volume) version of the first law of thermodynamics.