Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways. Positive-displacement flow meters accumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow. Other flow measurement methods rely on forces produced by the flowing stream as it overcomes a known constriction, to indirectly calculate flow. Flow may be measured by measuring the velocity of fluid over a known area. As noted in the preceding Dedication, the tendency to make flow measurement a highly theoretical and technical subject overlooks a basic tenet: Practical application of meters, metering principles, and metering instrumentation and related equipment is the real key to quality measurement. And that includes the regular maintenance by trained and experienced personnel with quality equipment required to keep flow measurement systems operating so as to achieve their full measurement potential.