We all know Coriolis flow meters are powerful field devices. If you read our earlier article, you learned the basics of how they work in mass flow measurement. Today, let’s go further and learn more about how they can measure density. We’ll talk a little about some cool technology to measure viscosity as well.
First, let’s review how Coriolis meters measure mass flow. The meter has a drive coil in the middle of the measurement tube, to move the tube at its natural frequency. At each end of the tube, a pickoff measures the movement. With no flow, the tube moves at its natural frequency, and the signals measured by the pickoffs remain in phase. When fluid flows through the tube, the Coriolis effect causes a twist, and the signals shift out of phase. So the differences between the signal waves are proportional to the mass flow! If you want the deep explanation, then check out the first part of this series on Coriolis meters.
We also discussed the different tube shapes you can find on the market. Every vendor has a sales pitch to explain why this shape outperforms that one. You can listen to their spiels if you want, but keep in mind that you still need to scale out the meter with your process specs: How much space do you have to install the sensor? How much pressure loss can you take? How much money can you spend? Stuff like that.
Now, let’s go a step forward to see how this flow meter measures density with excellent accuracy in most cases!
So how does it measure density?
Let me give you an analogy. Say you have two identical buckets. One bucket has water in it, the other mercury. Each bucket hangs from the ceiling on a steel spring. You can tell – without looking in either bucket, just by watching the movement of the springs – that each bucket contains something different.
Now, the drive coil in the Coriolis meter makes the measurement tube move in its own frequency, right? So when you change the density of the fluid, it’ll change the resonance of the tube. A higher density will reduce it, and a lower density will increase it.
During the calibration process, developers use different fluids to bring their meters to high accuracy. For example, the Micro Motion Elite from Emerson and the PROMASS F 300/500 from Endress+Hauser both have an accuracy of +-0.0005 grams per cubic centimeter.
And because the temperature can affect these measurements, Coriolis meters can measure and compensate for temperature. They can also calculate specific density values such as Brix, Plato, Baumé, and API. We’ll tackle those in another article, so let’s move on!
It measures viscosity too?
First off, not all brands can measure viscosity. Endress+Hauser has patented technology to do this in the PROMASS I. You can find other technologies measuring viscosity, but here you have one device to measure that along with flow, temperature, and density. Everybody likes a multi-purpose device. But will you get good value for those functions in your process? Only you can determine that.
Anyway, E+H calls 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 torsion 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 and calculates it into numbers we can read.
Where can you use it?
The Coriolis flow meter measures liquid density and not gas. A technology called MEMS (micro-electro-mechanical system) can measure gas density, if you need that. And if you just want density measurement, then you don’t need to install a Coriolis meter directly in the production line. You can save money by installing a small Coriolis in a bypass.
Like density, viscosity measurement can indicate process quality and help improve process performance. By installing your flow meter directly in the line, and you can get flow, density, temperature, and viscosity. But if you only want to measure viscosity, then you can do the bypass trick here too. You just need to decide which technology will benefit you most.
This video has more information about Coriolis flow meters: