Coriolis mass flow meter
You’ve heard a lot from us on flow measurement so far. We started with differential pressure (DP) transmitters in flow measurement and their advantages and disadvantages. However, most companies sell their products based on weight instead of volume, because factors such as temperature, pressure, and density can affect volume. So to avoid “losing” product to the time of day or some other variable, companies usually measure by mass.
You can do mass measurement with combined products, like the slurry in mining industries. Companies often use magmeters for volumetric flow measurement. You also have densimeters, such as gamma density meters or vibration forks combined with other items. The meter receives the density data and calculates the mass flow. You can also include temperature and pressure here. However, in this case you measure mass indirectly rather than directly.
This brings us to the Coriolis mass flow meter. Raphael Freitas explained the Coriolis effect in an earlier article. I’ll throw in a quick recap, using Raphael’s example.
You and a friend are on a rotating platform on different sides. With the platform at rest and no wind blowing, when you throw a ball for your friend, the ball goes straight to him, right? Now if the platform is rotating and you throw the same ball for your friend, the ball will go straight, but in your perspective, the ball makes a curve. We call this the Coriolis effect.
This video demonstrates the effect well:
How does the Coriolis effect measure mass flow?
Okay, let’s go to the flow meters! We’ll use the standard twin-tube Coriolis meter for now; later we can talk about the other variations.
First, inside the flow tube, you have a drive coil that vibrates the measurement tube at its natural frequency. Then, at each end of the measurement tube, you have the inlet pickoff and outlet pickoff to measure the tube’s movement. Now let’s create two situations to understand how the meter works.
Imagine that you stopped your process and have no flow through the measurement tube. The drive coil continues to vibrate the tube, so the pickoffs will generate volts during the tube movement, creating signal waves. These signal waves create the movement of one tube relative to the other, as you can see in the image below:
Here, both pickoffs generate in-phase signals. We can also say they’re in synchronized motion, indicating zero flow in the meter.
In the second scenario, you restart the process to have flow through the tube. The flow creates the Coriolis effect, where we have a difference of time between the inlet pickoff and outlet pickoff, causing a twist.
Analyzing the signal wave created by both pickoffs, we see that the signal waves have phase-shifted from each other and become unsynchronized. Now we can measure the time delay between the signal wave in microseconds, and the difference of time is directly proportional to the mass flow rate! You’ll also need a temperature measurement to calculate a compensation factor.
Additionally, the pipe frequency can directly influence the measurement. I dug up an Endress+Hauser handbook that says we’ll find the range of vibration in a pipe around 50 to 150 hertz. Coriolis flow meters that work in the same range need some sort of vibration inhibitor installed to avoid problems. However, some measurement tubes work in high frequencies like 600 to 1000 hertz.
Why do we need different tubes?
As mentioned before, different vendors offer variations on the twin-tube Coriolis meter, with straight, looped, or slightly bowed tubes. Each option comes with a pitch from the vendors on how this style can improve your application. Depending on the shape, you may need extra space to install the meter, but sometimes a compact design will bring a higher pressure loss than a bigger format. Therefore, you’ll need to – say it with me now – scale out and find the right option for your process.
You also can find single-tube measurement systems on the market. This style has lower pressure loss, is easier to clean, and won’t split the fluid inside. However, it usually has less accuracy and repeatability than the twin tubes.
So what are the pros and cons to a Coriolis meter?
- Is easy to implement in gas and fluid flow measurement
- Doesn’t require inlet and outlet runs
- Can measure mass flow, volume flow, density, and temperature
- Supports measurement independent of fluid viscosity and density
- Measures mass flow directly
- Costs more than most other options
- Has a limited temperature range
- Has limited applications in multi-phase fluids
The Coriolis flow meter is an outstanding solution in the automation world for general flow measurement. We talked here about how it measures mass flow, but it can also measure density. Endress+Hauser even has a version that can measure viscosity. Stay tuned to read more about those!