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Coriolis flowmeters: A subtle global effect with local applications, Part 1: The challenge

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The Coriolis effect, a subtle and often-misunderstood principle of physics, has been adapted for highly accurate mass flowmeter instrumentation.

The Coriolis effect is generally viewed, if at all, as a “large-canvas” effect. It is yet another consequence of the rotation of the Earth, with many interesting implications such as the rotational direction of air masses in each hemisphere (Figure 1), and curving the north-south path of projectiles and missiles. (It is also associated with swirling of water in a tub as it drains — but that’s another story, later). Its use in instrumentation is yet another example of how a subtle physics principle can be adapted to provide some very tangible benefits. This article will look at the effect and how it used to measure the mass flow of fluids in pipes and tubes.

Coriolis flowmeters
Fig 1: The Coriolis effect is the source of the clockwise and counterclockwise wind patterns in the Northern and Southern Hemispheres, respectively. (Image: NASA via The Verge)

Material flow (usually specified in kilograms per second or pounds per second) is second only to temperature as the most commonly measured variable in many industrial processes, lab experiments, and, increasingly, in medical studies. Although mass flow is roughly analogous to electrical current, it is more difficult to measure for two main reasons accurately. First, the material itself presents a challenge: It can be water, liquid, slurry, or gas, and corrosive, homogenous, or mixed. Second, changes in material characteristics due to internal and external factors affect measurement.

There are many different flow-meter designs in wide use in industrial and lab settings, and each has virtues and weaknesses. The reason is that while measuring flow is simple in concept, it is often difficult to do well in practice, and especially so in industrial applications. Among the many intrusive (contact) and non-intrusive (non-contact) methods are:

  • Magnetic flow meters, which are suitable only for conductive liquids. These meters use coils outside the pipe to generate a magnetic field in the pipe, which in turn induces a sensed voltage as the liquid flows through the field (Figure 2).
Fig 2: In the magnetic flowmeter, a magnetic field B is applied across the flowing conductive liquid, and the resultant induced voltage A is sensed. (Image: Analog Devices)
  • Ultrasonic flow meters, of which there are two basic types. One measures the transit times of an ultrasonic signal traveling with and against the fluid flow and then calculates the mass flow using the differences; the other measures the Doppler shift, which is a function of flowing fluid.
  • Vortex flow meters, which deliberately create an internal swirl or vortex, and the rotational rate of this vortex is proportional to the flow velocity.
  • Differential pressure flow meters, which measure the pressure drop as the fluid flows across laminar flow plates, and the pressure differential is proportional to the fluid flow.
  • Turbine meters, which use an in-stream propeller-like blade that rotates at speed proportional to the flow rate, while a tachometer or magnetic pickup senses the rotation (Figure 3).
Fig 3: In the turbine meters, the fluid flow turns an internal propeller, and the turns are sensed either by an external magnetic sensor as shown here, or an internal tachometer (Image: Crude Oil Daily)

Each of these has varying attributes with respect to accuracy, reliability, fluids it can handle, cost, impact on fluid flow, intrusiveness, and other factors. However, most of these flow meters really sense fluid velocity or volume, which must then be converted to the actual mass flow. Then, using the volume or velocity measurement to calculate the mass is simple, in principle: just the basic equation: density = mass/volume. Unfortunately, temperature, pressure, viscosity, and uniformity affect volume and can result in overall errors of 5% or more, depending on the material, flow rate, and other circumstances.

In the next part, we will look at the Coriolis principle as a basis for use in flow meters.

 

Related EE World Content

Other References

  1. History of Meteorology, “The Coriolis Effect: Four centuries of conflict between common sense and mathematics, Part I: A history to 1885”
  2. New World Encyclopedia, “Coriolis effect”
  3. “A Micromachined Coriolis-Force-Based Mass Flowmeter for Direct Mass Flow and Fluid Density Measurement,” 11th International Conference on Solid-State Sensors and Actuators (ICSSSA), 2001
  4. Analog Devices, “Electromagnetic Flow Meters: Design Considerations and Solutions”
  5. Bronkhorst High-Tech B.V. “The Importance of Mass Flow Measurement and the Relevance of Coriolis Technology”
  6. Control Engineering, “Are Coriolis flowmeters a universal technology?”
  7. Control Engineering, “Tutorial: Coriolis mass flowmeters”
  8. Chemical Engineering, “Patent granted on microfluidic device design”
  9. Chemical Engineering Progress “Coriolis: The Direct Approach to Mass Flow Measurement”
  10. Chemical Processing, “Flow Meter Design: Bend It Like Coriolis”
  11. Instrumentation and Control, “A history in Coriolis flowmeter technology – and the future…”
  12. com, “How a Coriolis Mass Flow Meter Works”
  13. Fierce Electronics, “A Review of Your Flow Sensing Options”
  14. Fierce Electronics, “An Overview of MEMS Inertial Sensing Technology”
  15. Flow Measurement and Instrumentation, “Flowmetering of non-Newtonian liquids”
  16. National Ocean Service, “Currents: The Coriolis Effect”
  17. University of Liverpool, “Experimental investigation into non-Newtonian fluid flow through gradual contraction geometries”
  18. Universal Flow Monitors, Inc. “Coriolis Mass Flowmeter Technology”
  19. Water Online, “3 Dirty Little Secrets About Coriolis Flow Meters”