A positive displacement flow meter is a type of flow meter that measures the flow rate of fluids by dividing the fluid into specific volumetric units and counting the number of these units to determine the total flow. It operates based on the principle that the fluid displaces or fills a known volume with each cycle or revolution of the metering mechanism. Here’s a breakdown of the working principle of a positive displacement flow meter:
- Metering Mechanism: A positive displacement flow meter consists of a metering chamber or a pair of rotating or reciprocating elements, such as gears, lobes, vanes, or pistons. These elements create chambers or compartments that trap and hold a fixed volume of fluid.
- Fluid Flow: The fluid whose flow rate is being measured enters the metering chamber or compartments, and as the flow progresses, it causes the metering mechanism to rotate, reciprocate, or otherwise move.
- Volume Displacement: With each cycle or revolution of the metering mechanism, a fixed volume of fluid is displaced or drawn into the chamber or compartments. This displacement is independent of the flow velocity or variations in fluid properties.
- Counting Mechanism: The positive displacement flow meter is equipped with a counting mechanism that keeps track of the number of displacements or cycles made by the metering mechanism. Each displacement represents a known volume of fluid.
- Flow Rate Calculation: By measuring and accumulating the number of displacements over a specific time period, the flow meter determines the flow rate of the fluid. The volume of fluid displaced per cycle multiplied by the number of cycles provides the totalized flow volume.
- Display and Output: The calculated flow rate or totalized flow volume can be displayed on the flow meter’s local display or transmitted as an output signal, such as a pulse, analog (e.g., 4-20 mA), or digital (e.g., Modbus) signal, for further processing, recording, or integration into control systems.
Positive displacement flow meters are known for their high accuracy and repeatability. They are suitable for measuring a wide range of fluid viscosities, including viscous liquids and oils. However, they can be sensitive to the presence of particulates or debris in the fluid and may require regular maintenance or cleaning to ensure proper operation. Positive displacement flow meters are commonly used in applications such as custody transfer, batching, blending, lubrication systems, and fuel consumption monitoring in various industries, including oil and gas, chemical processing, food and beverage, and pharmaceuticals.