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Flow measurement is the practice of determining the amount of material flowing through a specified system, usually a pipe or duct. This typically is accomplished with an inline flow meter. Flow measurement is used extensively in applications ranging from chemical plants to commercial air conditioning systems. The type of flow meter required for any particular application will depend on the properties of the measured substance.
The field of flow measurement is incredibly diverse and far-reaching, and it is one of the central subjects of transport phenomena studies. Measuring the flow rate of a substance can be as simple as measuring the change in level of a process tank or as complicated as measuring the inducted voltage across a conductive fluid moving in a magnetic field. Although flow measurement usually refers to material in pipes or ducts, it also can refer to the flow of solids or the flow of material through systems such as traffic grids or riverbeds.
Directly measuring the flow rate of a gas or liquid requires the use of a flow meter. There are many types of flow meters, including orifice meters, turbine meters, Venturi meters, ultrasonic meters and electromagnetic flow meters, to name a few. In most chemical process industries, orifice meters are the most popular type of flow meters because of their simplicity and low cost.
Orifice meters and Venturi meters operate on Bernoulli's principle, or more specifically, the relationship between the flow rate of the material and the pressure drop that the material experiences across the meter. Turbine meters correlate the number of rotations that an internal turbine makes in a set amount of time to the flow of the fluid passing over the blades. Electromagnetic flow meters utilize Faraday's law to correlate fluid velocity with induced voltage over a magnetic field in the fluid.
All of these methods of flow measurement might be subject to limitations, depending on the nature of the measured fluid. For example, an electromagnetic flow meter will work only on an electrically conductive fluid. Meters that have moving parts, such as turbine meters, are not recommended for abrasive or corrosive materials. Unusual systems might require unique or hybrid flow meters and calculations.
Calculated flow measurements can be improved with the use of correction or compensation factors. These factors are often used to compensate for temperatures, pressures and molecular weights that differ significantly from the design basis of the fluid flow. Compensation factors are often applied in the calculations of the distributed control system (DCS) in a chemical process setting.