Introduction: Entrained gas turns a clean liquid measurement into a moving mixture of liquid and bubbles, and the meter must decide whether it can keep tracking.
When a pump pulls in air, a reactor releases dissolved gas, or a tank outlet begins to draw a vortex, the liquid arriving at a Coriolis meter is no longer a neat single-phase stream. The tube still vibrates, the drive coil still pushes, and the pickups still listen, but the signal becomes noisy and uneven. Plant teams often see this first as a dropout alarm or a sudden jump in drive gain. The useful question is not whether bubbles affect the measurement; they do. The useful question is how an entrained gas management system keeps the meter online while the mixture moves through the tubes. That is the difference between a flowmeter that quits during aeration and one that keeps providing a continuous process signal.
A Coriolis meter measures mass flow by vibrating a tube and observing how the tube twists as fluid moves through it. The drive coil maintains the vibration, and the pickup sensors compare the motion at two points. From that comparison, the transmitter calculates phase shift, tube frequency, and eventually mass flow and density. This mechanism works best when the tube contains a liquid with stable density and predictable damping. Entrained bubbles break that assumption. They lower the average mixture density, change the speed of sound, and create compressible pockets inside the tube. Instead of one stable fluid mass, the tube now carries a moving, uneven mixture. The biggest mechanical effect is damping. A bubble is not a rigid liquid particle. It can absorb and release energy as the tube moves, and a train of bubbles can make the tube oscillation feel heavier or softer depending on the local gas fraction. The drive system responds by asking for more power, which appears as higher drive gain. If the gas fraction rises quickly, the tube can lose amplitude, and the pickup signals can become too weak or too distorted for a clean phase measurement. In intermittent aeration, the meter may see slugs of gas followed by liquid, then more gas. Each slug changes damping and density within seconds. Without control logic designed for that behavior, the transmitter may flag a measurement error, freeze the output, or drop out until the flow becomes single-phase again.
Entrained gas management, often shortened to EGM, approaches the problem as a control and signal challenge rather than a simple filtering task. The goal is to keep the tube vibrating at a usable amplitude and keep the pickup signals coherent enough for the transmitter to track phase. KROHNE OPTIMASS 6400 includes EGM™ entrained gas management technology, and the product family supports mass flow, density, volume, and temperature measurement, with features depending on the selected device configuration. The value of EGM appears when the liquid carries bubbles or when aeration comes and goes, because the meter is expected to maintain operation instead of treating every bubble as a reason to stop.
Drive gain is the amount of energy the transmitter sends to the drive coil to keep the tube oscillating at its target amplitude. In clean liquid, drive gain usually sits in a modest range. As bubbles enter the tube, damping increases and the tube needs more energy to maintain the same vibration. A conventional drive loop may reach its limit and lose control, especially when gas arrives in slugs. EGM-type control adjusts the drive strategy so the tube keeps moving through those damping changes. Higher drive gain is not automatically a fault; it is a visible response to a harder measurement condition. The important behavior is that the control loop stays stable while the mixture passes, rather than oscillating between full power and loss of amplitude. That stability gives the signal-processing stage a chance to work.
Phase tracking is the heart of Coriolis measurement. The transmitter compares pickup signals to find the small time difference caused by tube twist. Bubbles distort those signals because the tube's motion is no longer uniform and the fluid mass is not evenly distributed. Signal processing in an EGM system looks for the valid phase relationship inside a noisy signal, using the tube frequency and drive behavior as references. It can maintain a continuous measurement through short gas pockets and intermittent aeration, provided the flow remains within the operating range of the meter and the device is configured for the application. The output may become more variable as gas fraction changes, but the meter stays online and keeps producing a process value instead of dropping to a fault state.
Continuous measurement is valuable because industrial processes rarely stop when a bubble appears. A reactor feed, a dosing line, or a transfer pump may need a flow signal for control, totalization, or alarm logic even while the liquid is aerated. EGM helps the meter remain in service during those moments. That continuity supports better process visibility, fewer nuisance trips, and more stable control than a meter that repeatedly drops out. For an instrumentation learner, the key distinction is that staying online is not the same as delivering zero error. The meter is still measuring a two-phase mixture, and the physics of that mixture can shift the reading. Gas fraction, bubble size, flow velocity, installation orientation, pressure, and fluid properties all influence how much deviation appears in mass flow and density. EGM keeps the measurement running; it does not erase every multiphase effect at every gas ratio. This distinction matters when the measurement is used for different purposes. A process control loop can often tolerate a continuous signal with some added uncertainty because the alternative is no signal at all. A custody transfer or billing application may need a different level of verification, because even a small deviation can matter over a large volume. In practice, the right expectation is that EGM provides continuity and keeps the instrument operating through entrained gas and intermittent aeration, while the final accuracy depends on the actual process conditions and the selected configuration. Engineers should treat the continuous value as a useful operating measurement and review the application requirements before using it as a high-accuracy reference during heavy aeration. That is a practical engineering judgment, not a reason to dismiss the technology.
Entrained gas affects a Coriolis meter at the mechanical and signal levels. Bubbles change damping, push drive gain upward, and disturb the pickup signals that carry phase information. EGM-type technology keeps the tube vibrating and helps the transmitter maintain phase tracking so the meter can stay online during aeration. KROHNE OPTIMASS 6400 is one example of a Coriolis platform that includes EGM™ technology, along with multi-parameter measurement and configuration-dependent features. The realistic benefit is continuous operation in aerated liquids, not a claim of zero error at every gas ratio. Engineers who understand that difference can choose the right measurement strategy, set sensible alarms, and keep the process visible even when the liquid is not perfectly clean.
A：Bubbles interrupt the measurement because they change the damping and density of the fluid inside the vibrating tube. The drive system needs more energy to maintain amplitude, and the pickup signals become distorted as gas pockets pass through. If the transmitter cannot track a stable phase difference, it may freeze the output or report a measurement fault until the flow returns to a more liquid-like state.
A：EGM-type technology keeps the tube vibrating and helps the signal processing maintain phase tracking during aeration. It adjusts the drive response to damping changes from bubbles and looks for valid phase information inside a noisy signal. The result is continuous measurement through entrained gas or intermittent aeration, so the meter can keep supporting control and monitoring instead of dropping out.
A：No. EGM is designed to maintain continuous operation when the liquid carries bubbles, not to make a two-phase mixture behave like a clean single-phase fluid. The meter can stay online and provide a useful process signal, but the absolute error still depends on gas fraction, flow pattern, installation, fluid properties, and device configuration.
Coriolis Mass Flow Meter Working Principle - Coriolis Flowmeter
Standards for Automation - ISA
