Recently, current standards governing the evaluation of prosthetic heart valve designs have come under scrutiny. One important issue in the interpretation of data obtained from in vitro tests is that different pulse duplicators will produce different results for nominally the same set of conditions. This paper explores physical sources for disagreement in pressure and flow measurements between pulse duplicators, and suggests ways to account for them. By considering these physical phenomena, standards can be developed for testing chambers that improve similarity between systems. This should not compromise innovation in the design of new pulse duplicators to address additional concerns besides pressure and flow characteristics of the valve.