Abstract: A moveable fluid barrier (16) is driven from an upstream position to a downstream position through a measuring cylinder (18) connected fluidically in series with the flowmeter (22) under test to induce fluid flow therethrough at a flow rate related to the movement of the barrier. The response of the flowmeter is sensed during a plurality of time intervals as the barrier is driven through the conduit from the upstream position toward the downstream position in the course of a test run. The movement of the fluid barrier is also sensed (28) during these intervals. From the flowmeter response and the fluid barrier movement, a K-factor sample is determined for each interval. The K-factor samples are compared (12) with each other to selected a valid K-factor data point for the particular test run. Specifically, the criterion for determining a valid K-factor data point is the variation between K-factor samples under comparison.