how to plot the average pressure over each period?

Tsui

Hello, My model is quite simple. A closed cavity initially with an atmosphere pressure,inside of which is the air. And at the bottom of the cavity, there is an inlet where the H2 could get in.

This case was treated as transient. As the pressure will augment along with the entring of H2, we are interested in the average pressure of the whole field at each period. (the time duration is 6 second)

My question is that how to solve the average pressure? and after solved it, how to plot it by time?

Thanks a lot

Tsui