A Coriolis meter is built on the assumption that the fluid in its tubes moves as one coherent mass, so its vibration and the fluid track together. When gas gets entrained in a liquid, or the liquid flashes to vapor inside the meter, that assumption breaks: the bubbles move independently of the liquid, decouple from the tube's motion, and the measurement degrades on both mass flow and density. The meter often fights to keep vibrating, which shows up as a spiking drive gain long before a human notices the flow number is wrong. This page explains the bubble effect, how it differs from slug flow, the diagnostics a modern meter exposes, and how cloud SCADA catches it early.
Coriolis Two-Phase Error in one line: Coriolis two-phase flow error is the loss of accuracy that occurs when gas is entrained in a liquid, or a liquid flashes to vapor, inside the meter's vibrating tubes. The bubbles move independently of the surrounding liquid and decouple from the tube's vibration, so the meter can no longer relate its motion cleanly to the fluid, and both mass flow and density readings degrade. The meter typically drives harder to maintain vibration, spiking its drive gain, which is the earliest and most reliable warning that bubble content is corrupting the measurement.
A Coriolis meter drives its tubes into a controlled vibration and infers mass flow from the phase relationship of that vibration, and it infers density from the resonant frequency of the vibrating fluid-filled tube. Both of these depend on the fluid and the tube moving together as a single coupled system, which is exactly what happens with a homogeneous single-phase liquid. The liquid fills the tube, vibrates with it, and its inertia and motion are faithfully reflected in what the sensors see. That coupling is the foundation of the whole measurement.
Entrained gas breaks the coupling. A bubble is far less dense and far more compressible than the surrounding liquid, so when the tube accelerates, the bubble does not follow the liquid's motion the way another parcel of liquid would, it slips and compresses instead. This is the bubble effect: the gas moves relative to the liquid within the vibrating tube, so the tube's motion no longer represents a single coherent mass. The density reading suffers directly because the mix of liquid and gas resonates differently than pure liquid, and the mass flow reading suffers because the phase relationship the meter relies on is disturbed by the gas moving independently.
Flashing liquid produces the same problem from a different origin. If pressure inside the meter drops below the fluid's vapor pressure, some liquid boils into vapor right there in the tubes, creating gas voids where there were none upstream. Whether the gas is entrained from the process or generated by flashing, the effect on the measurement is the same: two phases moving independently inside a device that assumes one. As the gas void fraction rises, accuracy on both mass and density falls off, and beyond some point the readings become unusable rather than merely biased.
It is worth distinguishing dispersed two-phase flow from slug flow, because they stress the meter differently even though both involve gas. In dispersed or bubbly two-phase flow, the gas is present as many small bubbles distributed through the liquid. This is the classic entrained-gas case, and its main effect is the decoupling described above, a continuous degradation of accuracy that scales with how much gas is present. The meter keeps flowing fluid and keeps measuring, just less accurately, and the drive gain rises as the meter works harder to sustain vibration through the compressible mixture.
Slug flow is the more violent cousin, where gas and liquid separate into alternating large pockets, so the meter sees a slug of liquid, then a pocket of gas, then liquid again. Each transition is an abrupt change in the mass and density of what is in the tubes, and the meter's drive and measurement have to cope with large swings rather than a steady mixture. The symptoms include a jumpy, erratic flow reading and drive gain that spikes as gas pockets pass through, and the meter can momentarily lose the ability to measure at all when a large gas slug fills the tubes.
For diagnosis, the distinction guides expectations. Dispersed two-phase flow tends to produce a persistently elevated drive gain and a density reading that reads low and unsteady, consistent with a continuous gas fraction. Slug flow tends to produce intermittent, dramatic excursions in gain, density, and flow that come and go as slugs pass. Both are two-phase conditions and both corrupt the measurement, but recognizing whether you have a steady bubble content or intermittent slugging points at different upstream causes, from a persistent gas source or flashing on the one hand to phase separation in the piping on the other.
Modern Coriolis meters expose the very diagnostics that reveal two-phase flow, chief among them drive gain. Drive gain is a measure of how much energy the meter has to inject to keep its tubes vibrating at the target amplitude. In clean single-phase liquid this is low and steady. When gas enters the tubes, the compressible, energy-absorbing bubbles damp the vibration, so the meter must drive harder to maintain amplitude, and drive gain rises. A high or spiking drive gain is therefore an early, direct indicator that the fluid is no longer single-phase, often visible before the flow number looks obviously wrong. Tube amplitude and the density reading itself reinforce the picture, since a density that suddenly reads lower than the known liquid density is a strong sign of gas voids.
The problem with relying on operators to catch this is that they usually see only the flow reading, and a jumpy flow number is easy to dismiss as process noise or a twitchy signal. By the time someone connects the erratic flow to a two-phase condition, the totalizer may already have accumulated bad volume, because the meter kept totalizing through the degraded period. The diagnostics that would have flagged the problem, drive gain, density excursion, tube amplitude, are present in the meter but not in front of the operator unless the system deliberately surfaces them.
This is where a cloud SCADA layer changes the outcome. When a platform like Merobix reads and trends the meter's drive gain and density alongside the flow, it can alarm on a rising drive gain or a density that drops out of its expected band, flagging bubble content before it corrupts the totalizer rather than after. Instead of an operator staring at a jumpy flow value and guessing, the system points directly at the cause, gas in the tubes, and can even mark the affected volume as suspect. Trending drive gain over time also exposes a slow-building problem, such as a process that is beginning to flash under changing pressure, so the two-phase condition is caught as a rising diagnostic trend rather than as a sudden custody dispute.
Entrained gas breaks the assumption that the fluid moves as one coherent mass. The bubbles are far less dense and more compressible than the liquid, so they slip and compress instead of following the liquid's motion inside the vibrating tubes, decoupling the vibration from the fluid. This degrades both the mass flow and density readings, and the meter has to drive harder to keep vibrating, which shows up as elevated drive gain.
Drive gain is how much energy the meter injects to keep its tubes vibrating at the target amplitude, and it stays low and steady in clean single-phase liquid. When gas enters the tubes, the compressible bubbles damp the vibration and absorb energy, so the meter must drive harder and the gain rises. A high or spiking drive gain is therefore an early, direct indicator of two-phase flow, often visible before the flow reading looks obviously wrong.
Dispersed two-phase flow means gas is present as many small bubbles distributed through the liquid, causing a steady degradation of accuracy and a persistently elevated drive gain. Slug flow means gas and liquid separate into alternating large pockets, so the meter sees abrupt swings between liquid and gas that produce jumpy readings and intermittent gain spikes. Both corrupt the measurement, but one is a continuous mixture and the other is intermittent slugging, which points at different upstream causes.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.