The definitive guide to Coriolis flow meter
Welcome to our third article of the definitive guide series. If you haven’t seen the other two guides, then you’re definitely missing out. We have definitive guides for differential pressure transmitters and time-of-flight devices.
Now let’s talk flow measurement. And when we talk about flow, one of the stars in the market is the Coriolis flow meter because of its versatility. It can measure a liquid’s mass flow, density, and temperature all in the same device.
Okay, let’s not give any spoilers on this article! Shall we begin?
PART 1: The Coriolis effect
As always we’ll start the guide with the physics and math behind the principle in our case the Coriolis effect. If you don’t really dig this topic, then feel free to jump to the next part, where we talk about the device. For all the rest of you ready to take a trip back to school, let’s dig into it.
You might have heard about the Coriolis effect in high school, but at that time you probably never imagined it would have anything to do with process automation or flow measurement. Teachers use the Coriolis effect to explain wind current deflection in geography classes. If you don’t recall it, don’t worry. I wasn’t the best in geography either.
As you might have guessed, the Coriolis effect is named after a scientist, the Frenchman Gaspard-Gustave de Coriolis. Other fellow scientists had found out about this concept before Gaspard, but he wrote the first mathematical expression on the Coriolis effect in a paper on water wheels published in 1835.
Even though you might have forgotten about Coriolis effect, you probably remember Newton’s laws of motion, right? For those of us too embarrassed to admit it, I’ll do a little recap of the relevant bits. The first law of motion, also known as the law of inertia, states that an object will stay in the same state, either resting or moving uniformly in a straight line, if no external forces affect it. The important bit to remember here – in a straight line.
Got the law fresh in your mind again? Good, but this law just applies to an inertial frame of reference. What if we have a rotating frame? Now Coriolis effect comes in. Let me illustrate with an example.
Let’s say you’re playing catch with a friend in a merry-go-round, with your friend in the middle and you on the outer edge. For the purpose of this example, we’ll say you have no wind at the moment, which means no external forces interfere with the ball’s path.
The merry-go-round is still and your friend throws the ball to you. With no wind interference, the ball goes from the center of the merry-go-round out to you in a straight line.
Now that other friend who loves to disturb people’s games comes around. We all had that friend in our childhoods, didn’t we? Anyway, this annoying friend spins the merry-go-round. This time, when your friend throws the ball at you, it’ll still go in a straight line in an inertial reference frame. However, in your perspective, now a rotating perspective, the ball makes a curve. And guess what creates this curve? Yeah, you got it. The Coriolis effect.
Analyzing the example, we can see that the Coriolis “force” describes a matter of perception. The ball still goes straight, but you see it making a curve because you’re moving. Since there’s no actual force affecting the ball, many people now call it Coriolis effect instead of force. But for mathematical purposes, we still say Coriolis force for the inertial or fictitious force.
Although no force affects the ball trajectory in the merry-go-round, you and your friend in the rotating frame see it making a curve. We can calculate the acceleration you see because the ball’s inertia is proportional to (a) the velocity of the ball in the straight line and (b) the velocity of the merry-go-round’s rotation.
We call this the Coriolis acceleration and use the following formula to reveal it:
ac = 2*ω*v
- ac = Coriolis acceleration
- ω = rotational speed (merry-go-round)
- v = velocity perpendicular to the axis of rotation (ball in a straight line)
If we go back to Newton’s laws of motion, to the second one specifically, we can find a relation between force and acceleration:
F = m*a
- F = force
- m = mass
- a = acceleration
So when we multiply both sides by the mass of the object, in our example the ball, and replace the acceleration with the Coriolis formula, we can find the Coriolis force with the resulting formula:
Fc = m*2*ω*v
So to wrap up, the Coriolis force is proportional to the angular velocity, rotational velocity, and mass. And that’s why you can use a Coriolis flow meter as a mass flow meter. Look, we finally got to talk about flow meters!
Okay, you know you can use the Coriolis effect to measure flow. That’s why we have this article, right? We’ll explain how it works in the next part. But first, let’s have a look at other applications for it.
We used the merry-go-round example to explain the effect. However, we have another – very big – object that also creates a rotating reference frame by spinning around an axis. Can you guess which object I mean? You know. The very planet you stand on! Or sit or whatever.
Meteorologists also use the Coriolis effect. Winds blow across the Earth from the poles (high-pressure systems) to the equator (low-pressure systems). However, at the equator, they move faster than at the poles, because a point in the equator has to travel farther in 24 hours than a point near one of the poles.
So, let’s recall the merry-go-round example, but with the Earth as the rotating frame. If you’re at the North Pole and throw a ball to your friend at the equator, then the trajectory will look like it curves right, because he’s traveling faster than you. And if your friend throws the ball back, then it will seem to him that the ball also curves right, because you’re slower than him. So the winds in the southern hemisphere deflect to the left and in the northern hemisphere to the right because of the Coriolis effect.
In the past, navigators used this effect to forecast the so-called “trade winds” that influenced travel between Europe and South America. Airplanes and rockets also experience it, so pilots must take the Earth’s rotation into account when flying long distances. If they try to travel in a straight line between two places, they’ll probably miss the target!
This video shows the Coriolis effect:
PART 2: Mass flow measurement
Now that we know all about the Coriolis effect or force, let’s see how we use it to measure flow – finally! For the sake of this explanation, we’ll use the standard twin-tube Coriolis flow meter; later, we can get into the other variations.
In the flow tube of a Coriolis flow meter, you have a drive coil which vibrates the tube at its natural frequency. The tube has inlet and outlet pickoffs at either end. These pickoffs measure the tube’s movement, the frequency at which the tube vibrates. To understand how to measure the flow from these pickoff signals, we’ll consider two situations, no flow and with flow.
Imagine that you had to stop your process and had no flow through the measurement tube. Even without flow, the drive coil of the meter continues to vibrate the tube. So the pickoffs will still create the signal waves from the tube’s vibration. The signal waves create the movement of one tube relative to the other, as shown in this picture:
In this case, the pickoffs generate in-phase signals, which means they’re in synchronized motion. This indicates no flow in the meter.
Now you restart the process and once again have flow through the tube. The flow, in this case, creates the Coriolis effect, causing a little twist of a time difference between the inlet and outlet pickoffs.
Now, if we analyze the signals we can see that the waves have phase-shifted from each other and are not synchronized anymore. With this tiny delay between the two waves, we can calculate the mass flow rate. The rate is directly proportional to the time delay. You’ll also need a temperature measurement to calculate the compensation factor.
As a side note, the pipe frequency can directly influence the measurement. We dug up an Endress+Hauser handbook and found that we’ll find the range of vibration in a pipe around 50 to 150 hertz. If your Coriolis flow meter works in the same range, you’ll need some sort of vibration inhibitor to avoid problems. However, some measurement tubes work at high frequencies like 600 to 1000 hertz.
Why do we need different tubes?
As mentioned before, you’ll find variations on twin-tube Coriolis flow meter in the market – straight, looped, or bowed tubes. For each option, you’ll get pitches from the vendors on how this style can improve your application. Depending on the shape of your choice, you might need extra space to install the meter. But sometimes compact designs will bring higher pressure losses compared to bigger formats. That means that you’ll need to – say it with me now – scale out to find the right option for your process.
You’ll also find single-tube measurement systems on the market. This style is easier to clean, has lower pressure loss, and won’t split the fluid inside. However, it usually has a lower accuracy and repeatability than twin tubes.
Pros and cons of a Coriolis flow meter
- 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
This video has more information about Coriolis flow meter:
PART 3: Density measurement
A Coriolis flow meter can measure not only flow but also density. Some of them even measure viscosity! Just as we use time delay to measure mass flow, we can use another aspect of a signal to calculate the density of a fluid flowing through a pipe. Can you guess which? No? I’ll give you a bit more information and see if you can get it.
As usual, we’ll use an analogy to make it easier to visualize. Imagine you have two identical buckets, one filled with water and the other with an equal amount of mercury. Now hang each from a steel spring. You can tell – just by watching the movement of the springs – which bucket has water and which has mercury. Have a look at the image below and try to guess again which signal aspect relates to density.
Got it now? If you guessed frequency, kudos to you. The drive coil in the Coriolis flow meter makes the measurement tube move at its own frequency, right? So when you change the density of the fluid, the resonance frequency of the tube also changes. A higher density will reduce it, and a lower density increases it.
During the calibration process of a Coriolis flow meter, developers use different fluids to increase the accuracy of their devices. For example, both the PROMASS F 300/500 from Endress+Hauser and the Micro Motion Elite from Emerson have an accuracy of +-0.0005 grams per cubic meter.
Remember that I mentioned that a Coriolis flow meter can measure temperature? Yeah, that also has to do with density. If you change a liquid’s temperature, then its density changes too. So a Coriolis flow meter can measure the temperature of the fluid and compensate for it. They can also calculate specific density values such as Brix, Plato, Baumé, and API.
It measures viscosity too?
Jein! This German word perfectly answers this question. It means both yes (ja) and no (nein).
Some brands can measure viscosity. Endress+Hauser, for example, has patented technology to do so in the PROMASS I. They call this tech the Torsion Mode Balanced (TMB) system. The magic happens using torsional action. The middle of the tube has a counter-oscillating mass. This motion exerts a shear force on the fluid flowing through the tube. The fluid’s viscosity changes the oscillation, and the meter reads this change, calculates it, and converts it into numbers we can read.
Just remember that other technologies on the market can measure viscosity too. But here you have a device to measure that along with flow, density, and temperature. Everybody likes a multi-purpose device, right? Again, whether you’ll get good value for all those functions in your process, only you can determine.
Where can I use it?
A Coriolis flow meter can measure the density of liquids, but not gases. If you need gas density, then you can use a technology called MEMS (micro-electro-mechanical system). And if you use your Coriolis flow meter to measure just density, then you don’t need to install it directly on the production line. You can save some money by installing a small Coriolis meter in a bypass.
Like density, viscosity measurement can indicate process quality and help improve process performance. If you install your Coriolis flow meter directly in the line, then you can measure flow, density, temperature, and maybe viscosity all at once. If you want to measure just viscosity, then you can use the bypass trick. You just need to decide which technology will benefit you the most.
PART 4: Fancy Coriolis flow meter
Trying to find the right instrument for your application can sometimes be tricky, as you have many options and brands to choose from. So we decided to give you a list with some fancy meters to check out. And by “fancy” we mean that the device has advanced functions like digital protocols or meter verification. Some of these features can help your daily field activities.
So without further ado, let’s get to the devices, listed in no particular order. If you think we missed a good meter, then contact us through one of our social media profiles.
Proline PROMASS 300/500
This new series launched by Endress+Hauser has the 300 (integrated version) and 500 (remote version) to bring you a bunch of cool stuff. We did a product review on the Promass 300 combined with the I sensor – the one which can also measure viscosity. If you want more details about this device, then you can check it out here (Link to the PR).
By the way, you can combine the the Promass 300/500 with an alphabet soup of sensors – E or Q or whatever else – for different applications. Have a look at some of the features in this line:
- Wireless communication: You can set up the transmitter using your phone, tablet, or laptop. It also has optional WLAN, which allows you to set up without software.
- Flexible input/output (I/O): No fixed I/O means you have the freedom to set up as you like with options such as pulse, frequency, and status.
- Display color: If you have a problem with the device or the measurement, the display color switches to red to warn you.
- Heartbeat Technology (HBT): You can run device verification directly in your process. Three options give you data on the condition of your device. We have an article on HBT, too.
If you want to learn more about the Promass 300 and 500, click here.
ROTAMASS Total Insight
Next, on our list, we have the new ROTAMASS Total Insight. It brings significant improvement to Yokogawa’s line of Coriolis flow meter. We did a product review on it too. You can read that here. You have two options with this line, Essential and Ultimate. As the names suggest, the Essential comes with the basics. If you want all the whiz-bang, then pick the Ultimate.
Here you have the most relevant features:
- Micro-SD card: You can easily save and upload information through your laptop, then read and analyze the data.
- Tube Health Check: Following the boom of self-diagnostics in Coriolis flow meter, Yokogawa beefed up with its own version to help you avoid problems and unscheduled downtime. It’ll give you a complete report, too.
The FC430 isn’t the top option in the Siemens portfolio, but it is a new device and thus brings new technology to save time in the field.
Some of its highlights:
- USB port: You can easily get data from the device using a simple USB stick! This feature is fairly fresh in flow meters today.
- Micro-SD card: You can store all your data, from factory settings to the latest process variables during operations. Great for creating an audit trail, which people always forget about until hit with an audit. Trust me, you want this now, not later. More information
Yet another device which we also reviewed here at Visaya. To learn more about all its features, you can find the review here. A great Coriolis flow meter from KROHNE, it has all kinds of bells and whistles.
Take a look at its list of fun frills:
- Automatic test: This Coriolis flow meter has some sort of system that verifies the transmitter and sensor, guaranteeing that both work properly. Too bad I couldn’t find any information on it online.
- Redundant data storage: You can save the same configuration in two different memory caches. This redundancy gives your data more security.
- Entrained Gas Management (EGM): This is a big challenge for a lot of mass flow meter vendors. So it’s impressive that the crew from KROHNE developed this technology to ensure continuous measuring even if you have entrained gas in the liquid.
Micro Motion 5700
Last but not least, we have the new Micro Motion 5700 from Emerson. This device brings great features and a wide variety of communication options for seamless integration. Even better, you can get the transmitter as integrated or remote. It also has a very cool display, and Emerson focused on the user experience in its development.
Check out the outstanding functions:
- USB connection: Again, you can use a simple USB stick to download or upload configurations and check diagnostics.
- Smart Meter Verification (SMV): This feature monitors the condition of the device so you know when to do maintenance or calibration. It also creates a history of all verifications.
- Flexible I/O channels: You can configure your I/O to current, frequency, pulse, and more.
- Ethernet protocol: You can integrate the Micro Motion into your system using EtherNet/IP, PROFINET, or Modbus TCP/IP.
Oh, almost forgot! We also did a product review on it. You can check it out here.