When gas stored under pressure flows through an opening, the velocity through that opening may be choked (i.e., it has attained a maximum) or it may be non-choked.1, 2, 5, 6 Choked velocity, which is also referred to as sonic velocity, occurs when the ratio of the absolute source pressure to the absolute downstream ambient pressure is equal to or greater than [ ( k + 1 ) / 2 ] k / ( k - 1 ) , where k is the specific heat ratio of the discharged gas (sometimes referred to as the adiabatic index). When the gas velocity is choked, the equation for the mass flow rate in SI metric units is: Notes: