The Howl-Mann equation is a model of the “Helium Back-Pressure Method”, formally known as MIL- STD-883L Method 1014. Test Condition A2, which admits a calculation of the standard leak rate L, from the measured leak rate R, for the potentially leaky enclosure. This equation was first derived by D A Howl and C A Mann in their 1965 paper on back-pressuring techniques for leak testing. The model assumes a molecular flow regime, in which the flow rate of a gas species into, or out of the enclosure is proportional to the difference in partial pressures of that same gas species on the interior and exterior of the enclosure. L is the standard leak rate of air, which is defined as the flow rate of air through the leak under conditions of one atmosphere of air on one side of the leak, and a vacuum on the opposing side. Po refers to the aforementioned pressure of “one atmosphere”, and is thus, by definition, always equal to one.