Why does changing the atmospheric pressure property of a hydropneumatic tank not seem to effect the transient simulation?
The "Atmospheric Pressure" property of a hydropneumatic tank only applies to the initial conditions. Meaning in an EPS simulation, you can use a custom atmospheric pressure value based on your location, and it will be used when the gas volume compresses and expands. (the gas law works with absolute pressure) However, during a transient simulation, the atmospheric pressure is always assumed to be 1 atm or 10.33 m.
Modeling Reference - Hydropneumatic Tanks