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Coriolis Mass Flow Meter Primary Factors Affecting Measurement Performance & Solutions

2020-08-12
1. Installation Stress
During the installation of the mass flow meter, if the sensor flange of the flow meter is not aligned with the central axis of the pipeline (that is, the sensor flange is not parallel to the pipeline flange) or the pipeline temperature changes, the stress generated by the pipeline will cause pressure, torque and pulling force act on the measuring tube of the mass flow meter; which cause asymmetry or deformation of the detection probe, leading to zero drift and measurement error.
Solution:
(1) Strictly follow the specifications when installing the flow meter.
(2) After the flow meter is installed, call up the “zero adjustment menu” and record the factory zero preset value. After the zero adjustment is completed, observe the zero value at this time. If the difference between the two values is large (the two values must be in one Order of magnitude), it means that the installation stress is large and should be reinstalled.
2. Environmental Vibration and Electromagnetic Interference
When the mass flow meter is working normally, the measuring tube is in a state of vibration and is very sensitive to external vibration. If there are other vibration sources on the same supporting platform or nearby areas, the vibration frequency of the vibration source will affect each other with the working vibration frequency of the mass flow meter measuring tube, causing abnormal vibration and zero drift of the flow meter, causing measurement errors. It will cause the flow meter to not work; at the same time, because the sensor vibrates the measuring tube through the excitation coil, if there is a large magnetic field interference near the flow meter, it will also have a greater impact on the measurement results.
Solution: With the continuous improvement of mass flow meter production technology and technology, for example, the application of DSP digital signal processing technology and MVD technology of Micro Motion, compared with previous analog equipment, the front end The digital processing greatly reduces the signal noise and optimizes the measurement signal. The flow meter with the above functions should be considered as limited as possible when selecting the instrument. However, this does not fundamentally eliminate the interference. Therefore, the mass flow meter should be designed and installed away from large transformers, motors and other devices that generate large magnetic fields to prevent interference with their excitation magnetic fields. 
When vibration interference cannot be avoided, isolation measures such as a flexible pipe connection with the vibration tube and a vibration isolation support frame are adopted to isolate the flow meter from the vibration interference source.
3. The Influence of Measuring Medium Pressure
When the operating pressure differs greatly from the verification pressure, the change of the measuring medium pressure will affect the tightness of the measuring tube and the degree of buden effect, destroy the symmetry of the measuring tube, and cause the sensor flow and density measurement sensitivity to change, which is cannot be ignored to accuracy measurement. 
Solution: We can eliminate or reduce this effect by performing pressure compensation and pressure zero adjustment on the mass flow meter. There are two ways to configure pressure compensation:
(1) If the operating pressure is a known fixed value, you can input an external pressure value on the mass flow meter transmitter to compensate.
(2) If the operating pressure changes significantly, the mass flow meter transmitter can be configured to poll an external pressure measurement device, and the real-time dynamic pressure value can be obtained through the external pressure measurement device for compensation. Note: When configuring pressure compensation, the flow verification pressure must be provided.
4. Two-phase Flow Problem
Because the current flow meter manufacturing technology can only accurately measure single-phase flow, in the actual measurement process, when the working conditions change, the liquid medium will vaporize and form a two-phase flow, which affects normal measurement.
Solution: Improve the working conditions of the fluid medium, so that the bubbles in the process fluid are distributed as evenly as possible to meet the requirements of the flow meter for normal measurement. The specific solutions are as follows:
(1) Straight pipe laying. The vortex caused by the elbow in the pipeline will cause air bubbles to enter the sensor tube unevenly, causing measurement errors.
(2) Increase the flow rate. The purpose of increasing the flow rate is to make the bubbles in the two-phase flow pass through the measuring tube at the same speed as when they enter the measuring tube, so as to offset the buoyancy of the bubbles and the effect of low-viscosity fluids (bubbles in low-viscosity fluids are not easy to disperse and tend to gather into large masses); When using Micro Motion flow meters, it is recommended that the flow rate is not less than 1/5 of the full scale.
(3) Choose to install in a vertical pipeline, with upward flow direction. At low flow rates, bubbles will gather in the upper half of the measuring tube; the buoyancy of the bubbles and the flowing medium can easily discharge the bubbles evenly after the vertical pipe is laid.
(4) Use a rectifier to help distribute the bubbles in the fluid, and the effect is better when used with a getter.
5. The Influence of Measuring Medium Density and Viscosity
The change in the density of the measured medium will directly affect the flow measurement system, so that the balance of the flow sensor will change, causing zero offset; and the viscosity of the medium will change the damping characteristics of the system, leading to zero offset. 
Solution: Try to use a single or several medium with little difference in density.
6. Measuring Tube Corrosion
In the use of mass flow meter, due to the effects of fluid corrosion, external stress, entry of foreign matter etc., directly causing some damage to the measuring tube, which affects the performance of the measurement tube and lead to inaccurate measurement. 
Solution: It is recommended to install a corresponding filter at the front of the flow meter to prevent foreign matter from entering; minimize the installation stress during installation.
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