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Coriolis Flow Meter Principle & Phase Shift Guide
Quick Answer
A Coriolis mass flow meter measures flow directly by detecting the phase shift between two vibrating tubes. The phase shift is proportional to mass flow rate, not volume. This gives you true mass readings even when fluid density, temperature, or viscosity change during the process.
What Actually Happens Inside a Coriolis Meter
Every Coriolis meter has one or two flow tubes inside the sensor body. An exciter coil keeps these tubes oscillating at their natural frequency. When fluid is static, both tubes swing in phase. The moment fluid starts moving through the tubes, Coriolis forces twist the oscillation pattern. The inlet side of the tube bends slightly differently from the outlet side. This small distortion creates a time lag, a phase shift, between two sensor pickoff points.
We measure that delay in microseconds. The bigger the mass flow, the larger the phase shift. It is a linear relationship. No need to compensate for pressure or temperature swings to get a correct mass flow value. That is the core advantage over volumetric meters like turbine or vortex types.
Phase Shift Is the Key Measurement Signal
In practice, the meter does two things at once. It tracks the tube frequency to get density directly. And it tracks the phase shift to get mass flow. The onboard transmitter converts phase shift into a 4-20 mA HART signal, a pulse, or a digital Modbus output. On a typical Silver Instruments CMF series meter, the phase shift resolution is in the range of nanoseconds. That is what allows a DN15 meter to measure a few kg/h of syrup with 0.1% accuracy.
Many engineers new to Coriolis meters ask if vibration from a pump muddles the phase shift reading. Most modern meters use digital signal processing to filter out external noise. Our CMF transmitters apply a Fast Fourier Transform (FFT) and notch filters specific to power line frequencies like 50 Hz or 60 Hz. Because a pump running at 2900 RPM near the meter can inject mechanical noise, the DSP cleanly separates drive frequency from disturbance.
Why Phase Shift Beats Pulse Counting for Batch Control
On a typical loading rack at a chemical plant in Thailand, operators care about totalized mass transferred into a tank truck. A Coriolis meter gives them a direct mass pulse, not a volume pulse that later needs density correction. We have seen a paint manufacturer in Vietnam swap out an old positive displacement meter for a Coriolis meter on their resin line. The old setup required constant manual sampling to adjust for viscosity drift during the day. With the Coriolis meter, the phase shift signal stays stable regardless of whether the resin is thin at 40°C or thick at 15°C in the morning. Their batch accuracy moved from roughly ±1.5 percent to ±0.15 percent. That kind of number speaks directly to the plant manager.
Installation Factors That Disturb Phase Shift Readings
External stress is the enemy of a Coriolis meter. If a pipe hanger is too rigid and transmits thermal expansion force into the sensor body, the meter zero point drifts. Then the phase shift baseline is no longer true zero. We recommend a proper pipe support on both sides, roughly 10 to 12 pipe diameters away from the flanges. For a DN50 Coriolis meter on a hot oil loop, leave enough flexibility so the pipe can breathe. Also, avoid hanging the meter from the tubes themselves. A small alignment error during welding creates axial stress you cannot see, but the meter feels it as a zero offset
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Two-phase flow, gas bubbles in a liquid, kills the signal. The tubes lose their stable oscillation and the transmitter cannot extract a clean phase shift. If your process cannot avoid occasional gas slugs, install an air eliminator or a small degassing vessel upstream. We have seen this on edible oil filling lines in the Middle East where nitrogen blanketing introduces micro bubbles.
How the Transmitter Extracts Mass Flow and Density from the Same Sensor
The oscillation frequency of the tubes drops when the fluid is denser. A thick molasses at 20°C makes the tubes oscillate slower than a solvent like acetone. The transmitter measures that frequency shift in real time and computes density. So one sensor gives you mass flow, density, and temperature from a single Pt100 element. You get three outputs for the price of one instrument. For a biodiesel plant measuring methanol-to-oil ratio, having live density helps the control system verify blend accuracy without a separate density analyzer.
Mass flow is derived from the phase shift. Density is derived from the frequency. Temperature is a simple resistance measurement. All three parameters are available over a single Modbus RTU link. This cuts wiring cost and simplifies the control panel.
Silver Instruments CMF Series: Typical Specs for Your Sizing Request
Our CMF series covers DN02 up to DN200 in common line sizes. Standard wetted parts are 316L stainless steel, with Hastelloy C22 available for acids and seawater applications. Process temperature range is -50°C to 200°C. The transmitter outputs 4-20 mA HART plus pulse and RS485 Modbus. ATEX Zone 1 certification is available for gas and dust environments. For a typical diesel custody transfer skid in Oceania, we often supply DN40 meters with integrated Pt100 and NMI-approved firmware.
FAQ
Which process fluids work with a Coriolis meter based on phase shift?
Clean liquids, slurries with up to 30 percent solids, viscous oils up to 100,000 cP, liquid CO2, and many food products like chocolate or cream. Gas measurement is possible but requires high-density gas or higher flow velocity to generate enough Coriolis force.
How do I know if my pipe vibration will affect the phase shift reading?
If the external vibration frequency matches the tube drive frequency (typically 80 Hz to 1100 Hz depending on size), the digital filter may struggle. In most cases, low-frequency pump vibration below 50 Hz is easy to suppress. Share your pump speed and pipe layout with us, and we will simulate it.
What is the smallest mass flow a DN15 Coriolis meter can measure reliably?
With our CMF-015 model, the minimum measurable flow is around 0.08 kg/min for water-like liquids. Turndown ratio is 20:1 or better. For very low flows, like additive injection, we suggest a DN06 or DN02 meter.
Can I mount a Coriolis meter vertically?
Yes, vertical installation works well and helps drain liquids or sweep out gas bubbles. Flow direction should be upward for liquids to keep tubes full. This orientation also reduces solids settling in slurry applications.
How do I get a price for a Coriolis mass flow meter matched to my process?
Send us your fluid name, operating pressure in bar, temperature in °C, pipe size in DN, and the normal and maximum flow rate in kg/h or L/min. Add communication output preference and any hazardous area requirement. Write to [email protected] and we reply with a quotation and dimensional drawing within one working day.
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