William Hennessy, BMT Scientific Marine Services, Inc.

Flow sensors are used in many monitoring and control applications, to measure both air and liquid flows. There are many ways of defining flow (mass flow, volume flow, laminar flow, turbulent flow). Usually the amount of a substance flowing (mass flow) is the most important, and if the fluid s density is constant, a volume flow measurement is a useful substitute that is generally easier to perform. There are numerous reliable technologies and sensor types used for this purpose. Some technologies have been applied to both air and liquid flow measurements, as their principles of operation hold true in either application. Other technologies lend themselves to being airflow or liquid flow specific. In this chapter, we will discuss several of the most commonly used techniques for measuring both airflow and liquid flow. Complementary to flow measurement is level measurement. Used together, flow and level sensors answer the basic question of how much in laboratories and industries worldwide. Both measurement processes also share the distinction of being fairly complicated.