Ultrasonic flow meters use sound waves to determine flow rates. They can be either Doppler Effect meters or Time-of-Flight meters. Doppler Effect meters measure the frequency shifts caused by fluid flow. This frequency shift is proportional to the fluid's velocity. In order for Doppler flow meters to work properly with a liquid, particles must be present (measuring "pure" liquids is not possible). Time-of-Flight meters use the speed of the signal traveling between two transducers. This signal increases or decreases with the direction of transmission and the velocity of the fluid being measured. When used with liquids, both types of ultrasonic flow meters work best with a less than 10% by volume content of suspended solids or air gaps.
Selecting Ultrasonic Flow Meters
Selecting ultrasonic flow meters requires an understanding of the requirements for the particular application. Measuring the flow of liquids fluids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit and taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results. Suppliers of ultrasonic flow meters can assist potential clients with product selection.
Important parameters to consider when specifying ultrasonic flow meters include flow rate range, operating pressure, fluid temperature, and accuracy. The flow rate range is the flow in distance/time. The operating pressure is the maximum head pressure of the process media the meter can withstand. The maximum temperature of the media that can be monitored is usually dependent on construction and liner materials. Accuracy is measured as a percentage. Pipe diameter is also important to consider when selecting ultrasonic flow meters.
Mounting options for ultrasonic flow meters include insertion types, in-line flanged, in-line threaded, and clamp-on. Insertion flow meters are inserted perpendicular to flow path. They usually require a threaded hole in the process pipe or other means of access. In-line flanged flow meters are inserted parallel to the flow path, usually inserted between two pieces of existing flanged process pipes. In-line threaded flow meters are also inserted parallel to the flow path, but threaded into two existing process pipes. I-line clamp flow meters are ultrasonic flow meters that are inserted parallel to the flow path, and then clamped between two existing process pipes.
Ultrasonic flow meters differ in terms of output types, interface options, and user interface options. Output types for ultrasonic flow meters are analog voltage, analog current, frequency or pulse, and switch. Interface options for ultrasonic flow meters include serial and parallel interfaces. User interface options that are available for ultrasonic flow meters include analog front panels, digital front panels, and computer controls.
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Flow transmitters provide electrical outputs that are proportional to flow inputs. They use flow meters to measure the flow of liquids and gases.
Paddlewheel Flow Meters
Paddlewheel flow meters have a paddle wheel that is perpendicular to the flow path. The rotor axis is positioned to limit contact between the paddles and the flowing media.
Positive Displacement Flow Meters
Positive displacement flow meters measure the volume or flow rate of a moving fluid or gas by dividing the media into fixed, metered volumes. These devices consist of a chamber that obstructs the media flow and a rotating or reciprocating mechanism that allows the passage of fixed-volume amounts.
Rotameters are typically tapered glass tubes positioned vertically in the fluid flow. A float within the tube rides upward in relation to the amount of flow.
Turbine Flow Meters
Turbine flow meters measure the rate of flow in a pipe or process line via a rotor that spins as the media passes through its blades.
Volumetric Liquid Flow Switches
Volumetric liquid flow switches measure the flow or quantity of a moving fluid in terms of a unit of volume per time, such as liters per minute. These flow sensing devices have a switch output.
Vortex Flow Meters
Vortex flow meters are flow sensors that detect the frequency of vortices shed by a bluff body placed in a flow stream. The frequency of the vortices is proportional to the flow velocity.