I am needing to calculate the fill time of a pressure vessel. I know the following parameters regarding the vessel: Volume, Temperature, Properties of the fluid media, inlet pipe diameter, flow rate of the inlet, pressure of the inlet. I tried calculating the mass flow rate of the inlet then dividing the capacity of the tank (lbm) by the mass flow to find a time of fill. However, this number was extremely small (unrealistically small). I know as the vessel fills the fill rate is going to go down until S.S. is finally reached. However do I have to implement a D.E. to calculate the time for the vessel to fill?