Coriolis meters offer the highest accuracy in flow measurement and directly measure mass flow. Here is how they achieve this.
The Coriolis Effect
The Coriolis effect is an inertial force acting on objects in motion within a rotating reference frame. In a flow meter, tubes vibrate at their resonant frequency, and flowing fluid creates a twisting motion proportional to the mass flow rate.
How It Works
Vibrating Tubes
One or two tubes vibrate at their resonant frequency, typically between 80 and 1000 Hz. Tubes can be straight or curved in various configurations including U-tube and Omega shapes. A driver coil maintains the vibration.
Flow Creates Phase Shift
With no flow, the inlet and outlet of the tube vibrate in phase with each other. When fluid flows through the tube, the Coriolis force creates a phase difference between the inlet and outlet. Greater mass flow produces a greater phase shift.
Sensors Detect Phase
Two sensors positioned on the inlet and outlet sides measure the relative timing of the tube movement. The phase difference between these sensors is directly proportional to the mass flow rate.
Simultaneous Density Measurement
The resonant frequency of the tubes changes with fluid density. Denser fluid results in a lower frequency. This means Coriolis meters provide simultaneous density measurement alongside mass flow.
Advantages of Coriolis Flow Meters
- Direct mass measurement without density compensation
- Highest accuracy available, typically ±0.1% or better for liquids
- Measures mass flow, density, and temperature simultaneously
- Works with any fluid including gases, liquids, and slurries
- No straight-run requirements
- Bi-directional flow measurement
Limitations
- Higher purchase cost compared to other technologies
- Pressure drop across the meter
- Can be sensitive to vibration, though modern designs are improving
- Two-phase flow (gas bubbles in liquid) affects accuracy
- Large pipe sizes result in very expensive meters
Best Applications for Coriolis Meters
Coriolis meters excel in custody transfer for oil, gas, and chemicals. They are ideal for high-value fluids in pharmaceuticals and specialty chemicals, batching operations requiring precise mass measurement, and any application where the highest accuracy is required.
Accuracy Classes
For liquids, standard Coriolis meters achieve ±0.1% of rate. High-accuracy models can reach ±0.05% of rate. For gas measurement, accuracy is typically ±0.35% to ±0.5% of rate.
Cost Considerations
Coriolis meters are a significant investment. A 2-inch meter typically costs between $8,000 and $15,000, while a 6-inch meter can range from $25,000 to $50,000. However, for high-value fluids and custody transfer, the return on investment through improved measurement accuracy often justifies the cost.