Coriolis Flow Meter

Overview

A Coriolis flow meter measures mass flow directly by sensing Coriolis deflection of oscillating measuring tubes. When fluid flows through a tube vibrating perpendicular to the flow direction, Coriolis force deflects the tube in a direction proportional to mass flow. By detecting this tube deflection with precision sensors, the meter outputs mass flow independent of density, viscosity, or temperature, a fundamental advantage over inferential meters. Coriolis meters are the standard in custody-transfer, pharmaceutical, and food processing where mass accuracy and multi-phase handling matter more than cost.

The meter consists of a pair of Measuring Tube Assembly carrying the process fluid, a Drive Coil System exciting them at resonance, and a Pickoff Sensor Assembly detecting the phase shift between tube walls. The Transmitter Electronics converts the phase difference to mass and volumetric flow, outputting 4–20 mA or digital signal. A Flow Conditioner normalizes turbulent inlet streams, the Damping System stabilizes oscillation, and the Process Connection couples the meter into the line.

How it works

The drive coil oscillates both measuring tubes symmetrically at their mechanical resonance, typically 80–200 Hz. When the process fluid is stationary, both tubes swing in phase. But as flow enters, Coriolis acceleration acts perpendicular to both velocity and oscillation, deflecting the inlet tube in one direction and the outlet tube in the opposite direction. This phase lag is proportional to mass flow: no flow yields zero phase shift, higher flow yields larger shift.

The pickoff sensors are mounted on each tube. As the tubes oscillate out of phase, the pickoff magnets move through the pickoff coils, inducing voltage. The electronics measure the time difference (phase shift) between inlet and outlet signals: higher mass flow widens this delay. The transmitter's microcontroller solves for mass flow and, using a Pressure Sensor and Temperature Sensor, corrects for temperature effects and delivers density-compensated results.

The design is inherently immune to swirl, pulsation, or density change because it measures Coriolis force directly, not flow velocity. Twin tubes running in antiphase cancel external vibration, so the meter works on vibrating pipe. The Damping System uses viscous oil to suppress high-frequency noise while preserving the measurement signal. Response is fast, under 100 ms, making Coriolis meters suitable for closed-loop control in processes like extrusion or batch filling.