<div class="htmlview paragraph">The parameters governing the leakage rate across metal-to-metal and metal gasketed seals are discussed qualitatively and quantitatively based on experimental evidence. The sensitivity of the sealing phenomenon to the surface finish of the mating components is illustrated. The results of several experiments in which flat annular sealing surfaces of both 347 stainless steel and 2024 aluminum were mated together, either directly or with intermediate flat annular gaskets of indium, lead, aluminum, copper, or nickel are presented.</div> <div class="htmlview paragraph">It is concluded in this paper that, in order to ignore the surface finish as a parameter in the design of a fluid connector (or the quality control of the surface finish), the normally applied stress to effect sealing must be approximately 2.75 times the yield strength of the weaker material in the system. For surfaces controlled, both in asperity magnitude and direction, the required stress level may be considerably less. For extremely fine finishes having no measurable radially directed asperities, repeated sealing is possible for stress levels less than the yield strength (even with the accumulated damage from their assembly operations).</div>