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. The number of parcels that pass through the chamber determines the media volume. The rate of revolution or reciprocation determines the flow rate. There are two basic types of positive displacement flow meters. Sensor-only systems or transducers are switch-like devices that provide outputs for processors, controllers, or data acquisition systems. Complete sensor systems provide additional capabilities such as an integral display and/or user interface. For both types of positive displacement flow meters, performance specifications include operating pressure, temperature range, maximum material density, connection size, and percent accuracy. Suppliers indicate whether devices are designed to move fluid or gas.

Metering Technologies

There are several metering technologies for positive displacement flow meters. Gear meters have two rotating gears with synchronized, close-fitting teeth. Oval, spur and helical gears are often used because shaft rotation can be monitored to obtain specific flow rates. Vortex meters measure the frequency with which vortices are shed from a bluff body placed in the flow stream. Typically, the frequency is proportional to the material velocity. Nutating disc meters use media pressure to rock a disc in a circulating path without causing the disc to rotate about its own axis. A pin that extends from the disc is connected to a counter that monitors the disc’s rocking motions. Meters that measure incremental volumes of flow with a piston are also available. 

Electrical Outputs

Positive displacement flow meters differ in terms of electrical outputs. Analog current levels such as 4 – 20 mA are suitable for sending signals over long distances. Feedback is used to provide the appropriate current regardless of line noise and impedance. Analog voltages are simple, usually linear functions of the measurement. Modulated analog output signals are encoded, but still analog in nature. Examples include sine wave, pulse wave, amplitude modulation (AM), and frequency modulation (FM) signals. Several types of digital outputs are available. RS232, RS422, and RS485 are common serial, digital protocols. Popular parallel protocols include the general-purpose interface bus (GPIB), a standard which is also known as IEEE 488. Positive displacement flow meters with outputs that change the state of a switch or alarm are also available.

Selection

Selecting positive displacement flow meters requires an analysis of available features. Sanitary devices are designed for medical or food processing applications. Flow meters that can accommodate mixed phase materials such as steam or liquids with suspended solids (slurries) are also available. Some positive displacement flow meters contain built-in audible or visible alarms. Others are programmable and can be adjusted electronically for different materials, ranges, and outputs. Devices with recorder or totalizer functions are used to plot or chart flow history, or to provide information about total flow for a given unit of time. Multi-insertion or averaging flow meters determine the flow rate by taking flow rate measurements across several points in the flow path. Devices with controller function are commonly available.


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