Handbook of Chemical Hazard Analysis, Appendix B Click on PDF icon, wait and then scroll down to page 394 of 520 PDF pages. 1 2 Risk Management Program Guidance For Offsite Consequence Analysis, U.S. EPA publication EPA-550-B-99-009, April 1999. Guidance for Offsite Consequence Analysis ↑ Cunningham (1951) first drew attention to the fact that choked flow will not occur across a standard, thin, square-edged orifice.[11] The mass flow rate through the orifice continues to increase as the downstream pressure is lowered to a perfect vacuum, though the mass flow rate increases slowly as the downstream pressure is reduced below the critical pressure.[12] ↑ Cunningham, R.G., "Orifice Meters with Supercritical Compressible Flow", Trans.