The Principles Underlying a Coriolis Flow Meter

Dual Coriolis Flow Meters

The Principles Underlying a Coriolis Flow Meter

The industrial production uses lots of complicated measurement instrumentation to monitor and control the manufacture and quality of many materials. But what if we told you that some of the most advanced tools rely solely on the basic principles of physics? A Coriolis mass flow meter is one such instrument that has a lot of physics going on inside it but it’s not as complicated as you think. Coriolis mass flow meters are a staple in many production plants, whether food and beverage, chemicals, oil and gas, even research. So how do you go from a sine wave to accurate measurement of liquid mass flow and density?

Fluid in a Coriolis Meter

A process fluid can be just about anything — turpentine, sugar solution, petroleum, milk, even heavy water – and it’s often very necessary to measure just how much of it is flowing at any one time. That is where Coriolis meters come in. A Coriolis flowmeter primarily measures mass flowMass flow rate is the mass of a fluid that passes through a fixed point and place (such as a pipe or tube) over a given amount of time. In addition to mass flow, Coriolis meters can also be used to measure the flowing Density of a fluid.

A Coriolis flow meter sensor is typically constructed with two internal measurement tubes. Liquid (or gas) flowing in the process is split at the entrance to the Coriolis sensor and flows 50:50 through the two tubes, reuniting again at the exit of the flow meter. These two parallel tubes are formed into a curve inside the meter body. When the flow meter is powered up, these tubes are oscillated back and forth against each other using a drive coil(s). When process liquid flows through the Coriolis sensor, the force of the liquid flowing in the tube opposes this sideways movement (the oscillation) of the tubes, causing the tubes to twist or distort.  This is where the physics kicks in – Newton’s Laws of Motion can be used to describe the distortion of the meter tubes. Understanding how this distortion is caused and accurately measuring it allows us to calculate the mass flow of the process fluid within the meter. The twist or distortion is often described as the Coriolis effect.

Sine Waves and Coriolis Flow Pickoffs

So, how to measure the distortion in the tubes?  Magnet and coil assemblies called “pickoffs” are often used to provide feedback to the flow meter electronics. Pickup coil and magnet sets are mounted on the tubes one set towards the fluid entry point and one towards the fluid exit with the magnet on one tube and coil on the other so they are in close proximity.  When the meter starts oscillating, the coil moves through the magnetic field around the magnet and generates a small sine wave output – more physics! With the meter powered up and oscillating and with no fluid flowing in the tubes, the sine wave pattern of the two pickups are in phase, or synchronous; when drawn on a graph using the same timeline, their zero crossover points are at the same point on the graph’s axis. When there is fluid flowing through the tubes, however, the relationship between the two sine waves changes and their zero crossover points move away from each other and the sine waves exhibit a phase shift or become asynchronous with each other.  Where they were in phase with no fluid flow, there is a phase shift when fluid is flowing.

Time Difference and Mass Flow Rate

Coriolis Flow Meter

Phase shift means that the two sine waves are not occurring at the same time and this difference in time is caused by the twisting/distortion of the meter tubes when fluid is flowing through them. Of course, this time difference is very small, imperceptible to the naked eye and definitely can’t be measured on a wristwatch. It is, however, the fundamental measurement made in a Coriolis mass flow meter. Special high-speed electronic circuitry in the Coriolis mass flow meter is able to measure this phase shift time difference, with better quality meters measuring with resolution down to nanoseconds. This measured time difference is called the ΔT (“Delta T”).

As it turns out, Delta T is directly proportional to the mass flow rate within the meter tubes. The greater the time difference between the sine waves, the greater the mass flow rate.

The Weight on a Spring Analogy

So what about density measurement in a Coriolis flow meter?  Here, even more, physics come into play. Think about what happens when you have a 10-pound weight on a spring alongside a 2-pound weight on a spring.  The springs are identical. If you pull the weights down a little and then let go so they bounce back up, are they going to move at the same speed? That’s right: The heavier weight will move more slowly – more Newton’s Laws of Motion. The lighter weight accelerates faster because it has less mass. This means it will reach the top of its ascent first and of course, gravity will cause it to drop back down again until the spring arrests its fall and brings it back up again – and so on. It is bouncing or oscillating up and down at a higher frequency than the heavier weight. Exactly the same thing happens in the Coriolis meter.  When the tubes oscillate, they [obviously] move back and forth, that is they accelerate, reach maximum velocity, decelerate, stop and then do exactly the same thing in reverse.  When the fluid within them has less density, they will move faster, or at a higher frequency. When the fluid is heavier, they will move more slowly.

In other words, the frequency of the sine waves is an indicator of density.  Coriolis meters measure the meter tube frequency and provide flowing density as a valuable secondary measurement. Fluids can have similar makeups but different densities. Chocolate milk, for example, will have a slightly different density than regular milk.

But what if I want to measure in Gallons?

Gallons are a volumetric unit and one that we use in our daily lives for things like milk, gasoline, and water.  Note though, that a gallon of water has a different weight to a gallon of gasoline and both have a different weight to a gallon of water.  Multiplying the mass and the density of the fluid together will always give us an answer in volume. Coriolis meters measure both of these parameters, so it’s a relatively simple exercise to come up with a volumetric flow measurement in the flow meter’s electronics.

Delivering Highly Accurate Measurements

Using the principles of physics and with highly precise time measurement, Coriolis meters make exceptionally accurate measurements of mass flow and fluid density simultaneously. Reporting volumetric flow is just a case of multiplying these two highly precise measurements together. And that’s how a Coriolis meter gets its super-precise measurements.

There are quite a few manufacturers of Coriolis mass flow meters, with Rheonik being a leading, reputable brand. One of a few select brands that’s known throughout the industry for high quality, South Fork Instruments is proud to sell this outstanding product. Rheonik started producing flow meters in 1984 using unique Omega-shaped tubes within their Coriolis sensor.  With a series of “first’s” behind them – first large meter, first CNG meter, first high temperature meter, first 20,000psi rated meter, first Tantalum meter, first color screened transmitter – Rheonik continues to innovate and deliver highly accurate Coriolis flow meters useful in a broad range of applications and markets with measurements you can count on. Let us help you learn more about choosing the right one for your measurement need.

South Fork Instruments is a proud supplier of highly accurate instrumentation and control products, and our highly-trained staff enjoys breaking down the basics on our blog and helping your company implement these useful products to help improve your measurement needs and product quality.

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