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Coriolis mass flow

A Coriolis meter measures mass and density directly — the pickoff phase shift for mass, the tube resonant frequency for density — so it is the gold standard until something disturbs the tubes. Six meters, four reads — mass, density, drive gain, zero — including the two faults that both move the density: two-phase flow drops it and spikes the drive gain, while coating raises it with a calm gain.

By Francisco Padilla Interactive Technician level

Before you start — job brief

A Coriolis meter measures mass flow and density directly: it times the phase shift between two pickoffs on a pair of vibrating tubes for mass, and reads the tubes' resonant frequency for density. That makes it the gold standard — unaffected by pressure, temperature or flow profile. But the tubes have to be vibrating cleanly. Gas, coating, poor mounting or a lazy zero each corrupt the reading in their own signature way. Read the meter; don't trust it blind.

  • Mass comes from the pickoff phase shift; density comes from the tube resonant frequency
  • Watch the drive gain — it spikes when anything (gas, bubbles) damps the tubes
  • Density is a second, independent tell: gas drops it, coating raises it
  • Zero at true no-flow; a lazy zero or a too-high low-flow cutoff hides real low flows

Why a Coriolis meter is the gold standard

Almost every other flowmeter measures something you have to convert into flow — a differential pressure, a velocity, a frequency of vortices — and each of those conversions leans on the fluid's density, its temperature, its pressure, and the shape of the velocity profile. A Coriolis meter skips the conversion. It vibrates a pair of tubes and watches how the flowing mass twists them: the phase shift between two pickoff sensors is directly proportional to the mass flow rate, and the tubes' resonant frequency gives the fluid's density outright. Mass and density fall straight out of the physics, unaffected by pressure, temperature or flow profile. That is why it is trusted for custody transfer and hard-to-measure fluids — and why, when it does read wrong, the cause is almost always something disturbing the tubes rather than the measurement principle.

The drive gain is the tube's pulse

A drive coil at the apex keeps the tubes oscillating at their resonant frequency, and the transmitter reports how hard it has to push as the drive gain. On a healthy meter that is a low, steady few percent. The moment anything absorbs the tubes' energy, the drive has to push harder and the gain climbs — so the drive gain is the single most useful diagnostic on the meter. Watch it the way you would watch a bearing temperature: a low flat number is health, and a spike means something has changed inside the tubes.

Field note — the two faults that both move the density

Two conditions push the density reading off, and they are opposites. Two-phase flow — entrained gas or flashing liquid — lets the fluid slip inside the tubes and absorb vibration: the density reads low (a gas-liquid mix is lighter) and the drive gain spikes as the coil fights the damping. Coating — buildup on the tube wall — does the reverse: it adds mass and stiffness, so the density reads high and the calibration shifts, while the drive gain stays normal because a stiffer tube is not a damped one. Same symptom, opposite readings: a low density with a high gain is bubbles; a high density with a calm gain is coating. Read the density direction and the drive gain together and the two never blur.

Two-phase flow is the number-one problem

Gas in the tubes is far and away the most common Coriolis complaint. Bubbles decouple the fluid from the tube wall, so the mass reading goes erratic, the density drops, and the drive gain climbs — often to the point where the meter alarms or stalls. The fix is almost never in the meter; it is in the process. Keep the fluid a single liquid phase: hold enough back-pressure to stop it flashing across valves, orient and locate the meter so gas cannot collect in the tubes, and chase down the gas source, whether that is a leaking pump seal drawing air, a vortexing tank, or a partially flashing line. A drive gain that spikes with a sagging density is your bubble alarm — believe it.

Zero it, and don't blank real flow

Two low-flow faults look alike until you read them closely. A Coriolis must be zeroed at true no-flow after installation and after any real temperature change; skip it and the meter carries a small offset that, as a percentage, is worst at low flow — it reads a little high when the line is barely moving. Separately, every meter has a low-flow cutoff that blanks tiny readings so a drifting zero doesn't totalize noise. Set that cutoff too high and it hides real low flow, forcing an exact zero while product is genuinely moving. The tell is in the number: a zero-drift reads a small nonzero value sitting above true; a too-high cutoff reads a hard, exact zero at a flow you can prove is there. One needs a re-zero; the other needs the cutoff opened up.

The discipline

Four reads name every fault on this meter. Compare the mass to an independent check; compare the density to the known product; watch the drive gain for the damping that betrays gas; and confirm the zero is stable and the cutoff isn't blanking real flow. A high density with a calm drive is coating; a low density with a spiked drive is bubbles; a nonzero offset at low flow is a missed zero; a hard zero at real flow is a cutoff set too high; noise on everything at once is the mounting. Read all four against their expected values and the meter — the most trustworthy one in the plant — tells you exactly what is wrong with it.