A Coriolis meter is usually bought for mass flow, and the density reading arrives as a by-product. Understanding where that density number comes from explains both why it is so good on clean liquids and why it collapses on aerated ones.
Two measurements from one vibrating tube
The sensor is one or more tubes driven into oscillation at their natural frequency. Two independent quantities are extracted:
- Mass flow comes from the Coriolis effect. Fluid moving through a vibrating tube resists the change in its direction of motion, twisting the tube. The resulting phase difference between the inlet and outlet halves is proportional to mass flow rate — directly, with no need to know density, pressure or viscosity.
- Density comes from the resonant frequency itself. The tube and its contents form a mass-spring system; denser contents mean more mass, and more mass means a lower natural frequency. Track the frequency and you have density.
The relationship is close to ρ = A/f² + B, where f is the measured resonant frequency and A and B are calibration constants for that specific tube geometry. This is why a Coriolis density reading is inherently traceable to a frequency measurement — and frequency is one of the things electronics measure exceptionally well. Hence the accuracy class: the PS7200 is specified at ±0.001 g/cm³ with ±0.1 kg/m³ repeatability.
What it is genuinely best at
Clean, single-phase, homogeneous liquids where you want density and mass flow from one process penetration: refined-product transfer, solvent blending, alcohol and sugar solutions, chemical dosing. Because both measurements come from the same tube, they are inherently consistent — you are not reconciling two instruments with two calibration histories.
The limits, stated plainly
- Two-phase flow. Entrained gas decouples the fluid from the tube wall, and the frequency no longer reports the bulk density. Errors are large and, more dangerously, not obviously wrong-looking. This is the single most common cause of a Coriolis meter being blamed for a problem it is merely reporting.
- Abrasive slurry. The tube wall is the sensing element. Erosion changes tube stiffness, which changes the calibration constants — the meter drifts rather than fails, which is worse. Mining slurry, FGD gypsum slurry and tailings are outside sensible service.
- Pressure drop. The flow is routed through narrow bent tubes. On viscous fluids or large lines this is a real hydraulic cost, and the pump has to pay it.
- Cost and size. Price scales steeply with line size. Above roughly DN100 the economics usually push toward another principle.
- Zero stability. Zeroing must be done under process conditions with genuinely no flow. A sloppy zero is the most common installation error.
Coriolis versus tuning fork
Both are vibration-principle instruments, and both recover density from a frequency shift, which is why they are so often cross-shopped. The difference is what vibrates. In a Coriolis meter the fluid passes through the vibrating element; in a tuning fork the vibrating element is immersed in the fluid. That single structural difference drives everything else — pressure drop, line-size economics, solids tolerance, and whether mass flow is available at all. The PS7400 vs PS7210 comparison works through it dimension by dimension.
Installation notes that actually matter
- Keep the tubes full. A drainable orientation on a fluid that gasses out is a self-inflicted wound.
- Isolate from pipe vibration. Pumps and valves near the meter inject energy at frequencies the sensor cares about.
- Mount so that gas cannot collect — flow upward through the meter where the fluid may carry gas.
- Do not skip the process-condition zero, and record it. A zero done cold on a hot service is not a zero.