The Working Principle of Coriolis Flowmeters

Critical process control applications require highly accurate measurement instruments. Coriolis flowmeters are one of the most accurate process measurement instruments and commonly applied in some of these applications.

Working of Coriolis Flowmeters

To help you understand the principle behind the Coriolis flowmeters, we will strive to make it as simple as possible.

Coriolis flowmeters works by shaking one or more tubes carrying the flowing fluid, then precisely measuring the frequency and phase of that shaking.

The back and forth shaking is driven by an electromagnetic coil, powered by an electronic amplifier circuit to shake the tube(s) at their mechanical resonant frequency.

Since this frequency depends on the mass of each tube, and the mass of the tubes depends on the density of the fluid filling the fixed volume of the tubes, the resonant frequency becomes an inverse indication of the fluid density whether or not the fluid is flowing through the tubes.

-Tube Frequency is inversely proportional to the Density

As the fluid begins to move through the tubes, the inertia of the moving fluid adds another dimension to the tube’s motion: the tubes begin to undulate i.e. twisting slightly instead of just shaking back and forth.

This twisting motion is directly proportional to the mass flow rate, and is internally measured by comparing the phase shift between motion at one point on the tube versus another point: the greater the undulation or twisting, the greater the phase shift between these two point’s vibrations.

-Tube twisting is directly proportional to Mass Flow rate

Temperature changes have the potential to interfere with density measurement that is why all Coriolis Flowmeters are equipped with RTD temperature sensors to continuously monitor the temperature of the vibrating tubes. The flowmeter’s microprocessor takes the tube’s temperature measurement and uses it to compensate for the resulting elasticity changes based on a prior modelling of the tube metal characteristics. This temperature measurement happens to be accessible as an auxiliary output signal, meaning that, Coriolis flowmeter may also work as a temperature transmitter in addition to measuring mass flow rate, and fluid density.

The ability of Coriolis flowmeter to measure three process variables i.e. Mass flow rate, Temperature and density makes it a very versatile instrument. This makes it easy to communicate in digital environment involving Foundation Fieldbus or Profibus Standard rather than the analog 4-20 mA signal. Fieldbus communication allows multiple variables to be transmitted by the device to the host system or on the same Fieldbus network.

The Advantages of Coriolis Flowmeters

Coriolis flowmeters are very accurate instruments, and reliable. They are completely immune to swirl and other fluid disturbances, hence they can easily be located anywhere in a piping system with no need for straight run pipe lengths upstream or downstream of the flowmeter. The ability of Coriolis flowmeter to measure true mass flow, along with their characteristic linearity and accuracy, makes them ideally suited for custody transfer applications, where the flow of fluid represents product being bought or sold.

You can also read: 

• The Ultimate Guide to Industrial Flow Instruments
• How Orifice Plates are used in Flow Measurement

Disadvantages of Coriolis Flowmeters

The main disadvantage of Coriolis flowmeters is the high cost compared to other flowmeters especially for large pipe sizes. They have also more limited in operating temperature than other types of flowmeters and may have difficulty measuring low-density fluid -gases and mixed-phase i.e. liquid/vapor flows.  The bent tubes used to sense process flow may also trap process fluid inside to the point where it becomes unacceptable for hygienic applications e.g. Food Processing, Pharmaceuticals. That is why; we have new Coriolis tube design to try to overcome some of these problems. Straight-tube Coriolis flowmeters are slightly better than U-shaped tubes however U-shaped tubes aren’t as stiff as straight tubes, and so straight tube Coriolis flowmeters tend to be less sensitive to low flow rates than U-tube designs.