CORIOLIS MASS FLOW METERS

A Coriolis flow meter sensor is typically constructed of two internal measurement tubes. Liquid or gas flowing in a process is evenly split at the entrance to the Coriolis sensor, flows through the two tubes, and is reunited again at the exit of the flow meter. These two parallel tubes form a curve inside the meter body. When the flow meter is powered up, these tubes are osculated back and forth against each other using a drive coil or coils. When process liquid flows through the Coriolis sensor, the force of the liquid flowing in the tube opposes the oscillation of the tubes, causing the tubes to twist or distort.

This is where the physics kicks in. Newton's law 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.

A process fluid can be just about anything—turpentine, sugar solution, petroleum, milk, even heavy water—and it’s often necessary to measure just how much of it is flowing at any one time. That is where Coriolis meters come in. A Coriolis flow meter primarily measures mass flow. Mass 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 a process is evenly split at the entrance to the Coriolis sensor, flows through the two tubes, and is reunited again at the exit of the flow meter. These two parallel tubes form 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 or coils. When process liquid flows through the Coriolis sensor, the force of the liquid flowing in the tube opposes 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.

Firstly, unlike other designs, Coriolis meters aren't impacted by any change in viscosity. This means that any changes in the process material won’t affect the measurement accuracy. This is in contrast to situations when an orifice plate is used, for example. Additionally, Coriolis flow meters have a greatly decreased pressure drop when compared to other options such as PD meters. In fact, the only time that pressure is lost is when the flow is divided between the two tubes.

Secondly, Coriolis flow meters are more accurate and precise than PD meters, averaging between 0.1 and 0.2 percent accuracy in output readings. This level of accuracy leads to significant cost savings. Furthermore, Coriolis meters have an excellent turndown ratio of about 100:1, while the industry standard lags behind at only 10:1. This considerably increases accuracy.

Finally, Coriolis flow meters are more durable than PD meters. This is because the liquids and gases don't go through tubes with excessive moving parts, meaning normal wear and tear is low and corrosion is avoided.

In summary, Coriolis flow meters are efficient, effective, accurate, precise, and durable.

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.

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 their tubes, however, the relationship between the two sine waves changes and their zero crossover points move away from each other, with the sine waves exhibiting a phase shift or becoming asynchronous with each other.

Phase shift means that the two sine waves are not occurring at the same time, and this difference in time is caused by the distortion of the meter tubes that occurs when fluid is flowing through them. Of course, this time difference is very small-imperceptible to the naked eye and impossible to measure with typical time-keeping devices.

The magnitude of this phase shift is, however, the fundamental measurement made in a Coriolis mass flow meter. Special high-speed electronic circuitry in the meter is able to measure this phase shift time difference, with better quality meters having resolution down to nanoseconds (billionths of a second). 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.

It should be noted that because of how Coriolis mass flow meters measure flow rate, they are bidirectional. Flow can be reversed without any impact on measurement accuracy or need for reconfiguration.

Picture 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 weight down and then let go as they bounce back up, do they move at the same speed? No.

The heavier weight will move more slowly. The lighter weight accelerates more quickly because it has less mass, and less inertia to overcome. This means it will reach the top of its ascent first, at which point gravity will cause it to drop back down again until the spring arrests its fall and pulls it back up again. It is bouncing or oscillating up and down at a higher frequency than the heavier weight.

Exactly the same thing happens in a Coriolis meter. When the tubes begin to oscillate, they accelerate until they reach maximum velocity, then decelerate and stop, and the cycle repeats. 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, or at a lower frequency.

In other words, the frequency of the sine waves is an indicator of density. Coriolis meters measure 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.

Gallons are a volumetric unit, and one that we use in our daily lives for items such as milk, gasoline, or water. However, a gallon of milk has a different weight than a gallon of gallon of gasoline or a gallon of water. Multiplying the mass by the density of a fluid gives the fluid’s volume.

Because Coriolis meters measure both of these parameters, it's a simple exercise to configure a flow meter's electronics to produce a volumetric flow measurement.