The flow rate through a channel or an orifice increases with the increase of the pressure difference ∆p between the two sections (or volumes), one upstream (feeding pressure vessel) and the other downstream (outlet receiver). There is a value of ∆p in correspondence to which the flow rate reaches a maximum value (critical flow rate) and stabilizes, even if ∆p increases (with a constant upstream pressure and a downstream pressure decrease). For a given channel with a given inlet pressure, the maximum static pressure at its outlet section, leading to the critical flow rate, is called “critical pressure”. Its value depends on the channel geometry and on the physical properties of the fluid upstream of the channel (inlet conditions).