What Is a Coriolis Meter?
A Coriolis meter is a mass flow meter that measures flow directly - not by inferring it from velocity or pressure drop, but by sensing the tiny twisting force that mass in motion exerts on a vibrating tube. As a bonus it measures fluid density at the same time. Its high accuracy and direct mass reading make it a favorite for custody transfer of crude, NGLs, and other liquids.
Coriolis Meter in one line: A Coriolis meter measures mass flow directly by vibrating a tube and detecting the phase shift, or twist, that flowing mass induces in it; the vibration frequency also gives fluid density, so it reports mass, density, and inferred volume together.
How a Coriolis Meter Works
The meter continuously vibrates one or two flow tubes at their resonant frequency. When fluid flows through a vibrating tube, the Coriolis effect makes the inlet and outlet halves of the tube twist slightly out of phase with each other - the faster the mass flow, the greater the phase shift. Sensors at each end measure that phase difference, which is directly proportional to mass flow rate. No flow profile assumptions, no pressure/temperature compensation for the flow measurement itself.
Separately, the natural resonant frequency of the vibrating tube depends on the total mass of tube plus fluid, so the meter derives fluid density from the vibration frequency. Knowing mass flow and density, it can also compute volumetric flow. This is why a Coriolis meter is so useful: one device reports mass flow, density, and volume, and density is exactly what a crude LACT unit or NGL measurement point needs to characterize the fluid.
Why Coriolis Meters Are Used for Custody Transfer
Direct mass measurement sidesteps the biggest error sources in inferential meters. Because flow does not depend on the velocity profile, a Coriolis meter is far less sensitive to upstream piping and needs little or no straight run, and because it reads mass, it is immune to the density swings from pressure and temperature that complicate volumetric metering. It handles a very wide turndown with high accuracy, which is why it is common in LACT units, NGL and product custody, and chemical injection.
The trade-offs are cost, size and pressure drop on large lines, and sensitivity to entrained gas or two-phase flow, which disturbs the tube vibration. In service, the meter is verified or proved to confirm its calibration for custody use. A Coriolis transmitter reports mass flow, density, temperature, volume, and drive/diagnostic data - almost always over Modbus - so a cloud SCADA platform can poll those values to trend production, watch density as a quality indicator, and alarm on the tube-drive diagnostics that reveal entrained gas or a coating problem.
Installation Details That Decide Accuracy
A Coriolis meter forgives poor upstream piping but not poor mounting. The tubes must stay completely full of the phase being measured: for liquid service that means orienting the meter so gas cannot collect in the tubes and the meter cannot sit at a high point that traps vapor, and for gas service it means avoiding low points where condensate pools. The manufacturer's orientation guidance for the specific fluid is the controlling document here, because the right answer differs between liquid, gas, and slurry duty.
Mechanical isolation matters just as much. The measurement is a vibration measurement, so external vibration at frequencies near the tube drive, pipe stress from misaligned flanges, and pulsation from nearby positive-displacement pumps can all couple into the reading. Support the adjacent piping per the installation manual, keep the meter away from strong vibration sources where the layout allows, and pipe in block valves and a bypass. The block valves are not a luxury - they are what makes it possible to establish a true zero-flow condition later without shutting in the well or the transfer.
Commissioning: The Zero and the First Proof
Zeroing is the commissioning step that bakes in accuracy or error for the life of the installation. The meter must be flooded with process fluid, blocked in so flow is genuinely zero, and thermally stable at operating temperature before the zero is taken; a zero captured while a block valve weeps flow through the meter records that leak as a permanent offset. The mechanics and the pitfalls are covered in how to zero a Coriolis meter, and the reasons a good zero degrades over time - temperature shifts, density changes, mounting stress - are the subject of the Coriolis zero drift reference.
For custody duty, commissioning ends with proving. The meter is run against a prover or master meter to establish a meter factor, and that factor - not the factory calibration alone - is what the ticket calculations use. From then on the factor is re-established at the frequency the contract or measurement procedure requires, and trending it is the health check: a factor that wanders tells you something changed in the meter or the fluid before any single proof looks alarming. Checking the meter's live density against a lab sample of the same fluid is a cheap, independent sanity check on the same trip.
Reading the Meter's Own Diagnostics
A Coriolis transmitter continuously reports how hard it is working to keep the tubes vibrating, and that drive gain signal is the most useful diagnostic in the device. Rising drive gain means the vibration is being damped, and the usual culprit is gas in a liquid stream.
| Symptom | Likely cause | First check |
|---|---|---|
| Drive gain climbing | Entrained gas or coating | Upstream separation and density stability |
| Density shift with steady process | Tube coating, erosion, or corrosion | Compare meter density against a lab sample |
| Flow indicated at confirmed no-flow | Zero offset | Verify block valves seal, then re-zero per the manual |
| Erratic reading at very low rate | Operating near the low-flow cutoff | Review cutoff setting against the operating range |
| Noisy flow signal | External vibration or pulsation coupling in | Mounting, pipe supports, nearby rotating equipment |
Gas is the diagnosis to take most seriously, because slugs and bubbles do not just add noise - they disturb the tube dynamics enough to bias the measurement, as explained in the Coriolis two-phase flow error reference. On a lease, climbing drive gain at the LACT meter is often the first evidence that a separator upstream is carrying gas under with the oil, which makes the meter's diagnostics a process instrument in their own right, worth trending in SCADA alongside the flow and density values.
Frequently Asked Questions
What does a Coriolis meter actually measure?
It measures mass flow rate directly, from the phase shift a flowing mass induces in a vibrating tube, and it measures fluid density from the tube's resonant frequency. From those two it can also report volumetric flow and, with temperature, additional derived values - all from a single meter.
Why is a Coriolis meter good for custody transfer?
Because it reads mass directly, it avoids the velocity-profile and density-correction errors that limit inferential meters, is insensitive to upstream piping, handles wide turndown accurately, and simultaneously reports density for fluid characterization. That combination makes it well suited to LACT units and liquid custody points, where it is proved for legal-for-trade accuracy.
What are the limitations of a Coriolis meter?
It is more expensive than an orifice or turbine meter, gets bulky and adds pressure drop on large lines, and its accuracy suffers with entrained gas or two-phase flow, which disrupts the tube vibration. For clean single-phase liquid measurement, though, its accuracy and built-in density reading usually justify the cost.
Why does a Coriolis meter show a small flow when the line is blocked in?
That is a zero offset. Either the zero was taken under imperfect conditions, conditions have drifted since it was taken - temperature, density, or mounting stress - or a block valve is passing and the indicated flow is real leakage. Confirm the valves actually seal first, then re-zero following the manufacturer's procedure with the meter flooded and thermally stable.
How often does a custody Coriolis meter need proving?
At whatever frequency the sales contract or governing measurement procedure specifies - it is a contractual and site-specific matter, not a fixed rule. What is universal is the practice: establish a meter factor at commissioning, re-prove on the agreed schedule, and trend the factor, because a drifting factor is the earliest warning that the meter or the fluid has changed.
Sources and verification
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
- Modbus Application Protocol Specification - Modbus Organization
Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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