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Coriolis Flow Meter Problems: Addressing Phase Lag Errors and Fluid Entrained Air
Quick Answer: Phase lag errors on a Coriolis flow meter usually come from low flow, pipe strain, zero drift, or two-phase flow. Entrained air is the most common cause of unstable mass flow and density readings in chemical, food, and wastewater lines. Fix the process first: raise back pressure, remove air slugs, check the zero, and inspect the sensor mounting before you replace electronics.
Phase Lag Errors Are Not Always a Sensor Fault
A phase lag error means the transmitter sees a time shift between the two measuring tubes that does not match the real flow. The phase shift is small at low flow. On a DN25 meter running below 30 kg/h, normal phase shift can be less than a few microseconds. Any pipe vibration, thermal stress, or zero offset can be enough to corrupt that small signal.
We have seen this on customer sites in Malaysia, Vietnam, and the Philippines. A paint manufacturer in Southeast Asia ran a DN25 Coriolis meter on a 2 cP solvent line. The meter read 12 percent high whenever a diaphragm pump started. The issue was not the electronics. Air was pulled into the suction line through a loose fitting. Once the fitting was sealed, the phase reading settled.
What Entrained Air Does to the Coriolis Signal
Coriolis meters measure mass flow by vibrating one or two tubes. The liquid inside the tube is part of the vibrating mass. Gas bubbles do not couple to the tube the same way liquid does. They reduce effective density, dampen tube motion, and force the drive loop to work harder. The result is higher drive gain, unstable phase shift, and density that drops below the expected value.
Here is the thing: small bubbles and slug flow behave differently. Micro-bubbles often cause noisy flow and density signals. A large slug can cause the transmitter to switch to a two-phase alarm or hold the last output. Do not assume the sensor is defective. Check the process first. A water line at 2.5 bar g may keep air dissolved. A pump suction line at 0.4 bar absolute can release dissolved air and create false flow pulses.
Field Checks That Separate Phase Lag from Air
First, stop flow and close isolation valves with the meter full. Check the zero. A zero drift above 0.02 percent of rate on a DN25 or DN40 meter usually points to pipe strain or mounting stress. Second, compare the density reading with the known density of your fluid. If density drops from 1050 kg/m3 to 980 kg/m3 when a pump starts, the difference is air or cavitation. Third, check drive gain. Clean liquid operation often sits between 10 and 20 percent. Drive gain above 60 percent is a warning sign.
Most engineers skip this part. They call the supplier first. But three minutes of checking zero, density, and drive gain can save a shutdown. Also check the PT100 temperature sensor reading. A wrong temperature input can shift the phase and density calculation. The transmitter may still output values even when the PT100 is drifting. Compare the RTD reading with a local thermometer at the pipe surface.
Process and Installation Corrections
Install the meter in a location that keeps the line full. For liquids with occasional air, mount vertical upward flow if possible. Gas rises with the liquid. This keeps bubbles moving through the sensor instead of collecting inside the tubes. Use an air release valve upstream before the meter, not directly at the sensor. Keep straight pipe runs short. Pipe supports must be on both sides of the sensor to avoid external stress.
Raise downstream back pressure. For water-like liquids, 2 to 4 bar g downstream is enough to keep many micro-bubbles in solution. For viscous resins or syrups, 1 to 2 bar g may work. A back pressure valve downstream of the meter is common in food and beverage batching. It helps keep the tu
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Recommended Configuration for Silver Instruments Coriolis Meters
For small pilot injection lines, a DN15 Silver Instruments Coriolis meter covers 0 to 120 kg/h. For standard liquid chemicals, select a Silver Instruments Coriolis mass flow meter with 316L stainless steel wetted parts and 24 VDC electronics. A DN25 sensor handles 0 to 1000 kg/h for water-like liquids. A DN50 sensor handles roughly 1000 to 60000 kg/h. A DN80 sensor covers higher flow in water, wastewater, and fuel oil lines. Specify 4-20 mA HART output, pulse output for totalizing, and RS485 Modbus RTU if you need digital density and temperature values.
For entrained air applications, specify a meter with two-phase diagnostic output and a density alarm. Set the low flow cutoff at 0.5 to 1 percent of full scale. Increase damping only after you fix the air source. Damping will smooth the display but it does not correct the live mass flow. We usually start with 0.5 seconds damping and increase to 2 seconds only for batch lines with pump noise. Electronics are available for general purpose and ATEX Zone 1 or IECEx Zone 1 installations. Specify the hazardous area certificate at order stage.
When to Recalibrate or Replace
If the meter is still unstable after process changes, check the zero again at operating temperature. A zero shift that changes with temperature may point to internal stress or a damaged sensor. If drive gain is high with no air and a clean fluid, the tube may be coated or corroded. In chemical lines, polymer buildup on the tube wall adds mass and damping. This shifts density and phase lag. Cleaning the sensor or selecting a straight-tube design with high shear can help.
Replacement is the last step. Before replacing, send the transmitter log data to Silver Instruments. A few minutes of raw phase, drive gain, density, and temperature data usually shows whether the problem is process, electrical, or mechanical.
Five Most Common Questions
1. Why does a Coriolis meter read high when air bubbles pass through?
Air bubbles change the tube damping and phase relation. The transmitter may interpret this as a momentary high mass flow pulse. Raising back pressure or moving the meter to a vertical upward run reduces the error.
2. What drive gain level is too high for a Coriolis meter?
Above 60 percent is high for clean liquid service. Above 80 percent often means slug flow, heavy coating, or sensor damage. Check density and zero before assuming a fault.
3. Can damping fix entrained air errors?
No. Damping only smooths the output signal. It does not remove the measurement error from variable density and phase shift. Use damping only after the process issue is corrected.
4. What back pressure should a Coriolis meter have?
For water-like liquids, 2 to 4 bar g downstream is common. For hot liquids or solvents with high vapor pressure, calculate or test the minimum back pressure above the vapor pressure plus a safety margin.
5. Does Silver Instruments supply Coriolis meters with density and HART output?
Yes. Silver Instruments Coriolis flow meters provide mass flow, density, temperature, 4-20 mA HART, pulse, and Modbus RTU outputs. Wetted parts include 316L stainless steel with Hastelloy C22 optional for aggressive chemicals.
Send your application data: pressure in bar, temperature in °C, pipe size in DN, flow range in kg/h, fluid viscosity in cP, and density if known. Silver Instruments will recommend a Coriolis meter with the right flange rating, tube material, and output. Contact Silver Instruments: Tel: +86-25-68650347, WhatsApp: +86-25-52155837, WeChat: +86 15365082610.
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