The Coriolis effect, a subtle and often-misunderstood principle of physics, has been adapted for highly accurate mass flowmeter instrumentation.
The application of the Coriolis force offered a new way to solve the long-standing problem of measuring the flow of material. In 1977, Micro Motion Inc introduced the first standard industrial flow meter based on the effect. Because the measurement depends only on mass, such an instrument is a true mass flow meter and is unaffected by the issues that corrupt other flow-meter designs. (Micro Motion is now part of Emerson Electric Co.) Since Micro Motion’s introduction, many other vendors have also developed flow meters based on the Coriolis effect and refined them with new twists. A properly installed Coriolis flow meter can provide readings with errors of less than 0.1% and over a wide dynamic range; handling readings over such a range is a problem for most other flow-meter designs.
A practical Coriolis flow meter is more complicated than a conceptual one, of course. The fluid to be measured passes through the vibrating tubes and accelerates as it flows toward the maximum vibration point and slows down as it leaves that point, causing the tube to twist. The amount of twisting is directly proportional to mass flow. Position sensors detect tube positions. Most implementations use a pair of loops, called sensor tubes, which vibrate in opposite directions (Figure 1). This design nulls out many of the error sources, just as a differential circuit in electronics cancels many common-mode errors.
The flow meter measures the angle of twist between the two sensor tubes. Some designs have a single sensor tube and compare the distorted twist of the tube when fluid is flowing with the no-flow position. Typical dual-tube flow-meter designs have a loop-vibration amplitude of a few millimeters at 75 to 100 Hz and cost several thousand dollars.
An important advantage of the Coriolis-based design is that there are no internal obstructions in the flow path, such as sensor, propeller, or pressure-drop plate. This helps the meter to avoid problems due to clogging, internal wear, and pressure drop.
Still, the bent sensor loop causes some pressure drop and turbulence, so some Coriolis designs use an offset, straight pair of sensor tubes. Another potential problem is that the fluid being measured may have trapped air or other gases (known as entrained gases) that affect the fluid mass and thus the flow rate. This two-phase situation occurs when the flow is not continuous but starts and stops in batch processes. Many vendors now offer Coriolis flow meters which use advanced algorithms and processors (DSPs or FPGAs) to compensate for any erratic vibration of the flow tubes.
The Coriolis flow meter has an advantage in another difficult fluid situation. Newtonian fluids such as water and air, are “well-behaved,” with viscosity and flow characteristics that are constant. In contrast, non-Newtonian fluids such as some polymers and clays, for example, have viscosities that are functions of the fluid shear rate. The mass flow of non-Newtonian fluids can be difficult to measure accurately. Still, the Coriolis design can provide very good results even with this situation or even under these circumstances.
While many end-users love Coriolis flowmeters, price is often an issue. Coriolis flowmeters are relatively expensive, with an average selling price of Coriolis flowmeters are between $4,000 and $6,000. Some suppliers have introduced low-cost Coriolis flowmeters in the $3,000 and below range. Performance specifications for the lower-cost flowmeters are not at the same level as those of the higher-priced meters. However, these lower-cost meters can help satisfy the needs of users who want the essential benefits of Coriolis technology but prefer not to pay the higher price.
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