Numerical simulations were used to study the molecular flow transmission probability through two tubes of different diameters connected in series. It is well known that Oatley’s simple area ratio correction to the exit loss is inexact due to beaming of the gas from one tube to the next. Many months of computer time on a PC‐AT with 80287 numerical processing unit and a VAX‐780 were required to obtain sufficiently accurate data on numerous different diameter ratios in order to allow for high‐accuracy curve fitting to various potential correction equations for interpolation. The method of analysis used was similar to one by Santeler for two tubes of the same diameter in series. It was found that Oatley’s area correction has a maximum error of 5%. A curve‐fit equation is provided that reduces this error to 0.6%.

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