What should I know about electromagnetic flow meters (magmeter)?
So many devices – how do you choose! Don’t worry, we can help. You know all options have pros and cons, not only from brand to brand but also from process to process. You also know that high-end models usually offer more than low-budget devices – but not always!
And you need to remember that installation and pressure loss may factor into your shopping. Yes, some customers forget this fact, so don’t make the same mistake. Most of all, you need the right principle for your application to get reliable flow measurement.
We have a definitive guide to Coriolis flow meters here at Visaya and a bunch of content talking about different flow principles. So today, let’s dig into the world of the electromagnetic flow meter – magmeter from here on out- and learn its strengths and weaknesses so you can decide if this principle will work for you.
How does a magmeter work?
Yes, we’ll get to installation, application, advantages, and disadvantages in a bit. First off, you need to understand how a magmeter works so that you can immediately assess whether it will work for you.
A magmeter uses Faraday’s law, also known as the law of electromagnetism. Okay, let’s break that down. Imagine you have a magnetic field – easy to get, right? – and you want to see how it interacts with an electric circuit. In this interaction, you create an electromotive force (EMF), also known as electromagnetic induction. Faraday also discovered more stuff, like the concept that the electric voltage is directly proportional to the velocity of the movement and strength of the magnetic field. Super cool!
Now, the magmeter borrows this law to measure volumetric flow, which means you can only use it in conductive fluids. It has two field coils, installed on opposite sides of the meter, and two electrodes, one on each wall, to measure voltage. The coils create a magnetic field in the measuring tube. When the fluid passes through the field, it creates a voltage, measured using the electrodes on each wall. This voltage is directly proportional to flow velocity. Ta da!
Where can a magmeter work?
You can use magmeters in tons of products in all kinds of segments. You just need a minimum conductivity and a need to know your volumetric flow. I mean, you can measure mass too, but we’ll hit that point later.
As for price, I like to say that magmeters give some of the best bang for your buck if you can fit one in your process. Vortex flow meters come second, and maybe between them, you can find ultrasonics and so on. Of course, some company portfolios may have devices which defy my statements, so make sure you do your research!
One point about magmeters: the liner that insulates the measuring tube. All magmeters have them. So with your product in contact all the time with that liner, you need a material that will withstand your product. If you use the wrong material, you can run into corrosion and abrasion, shortening the life of the measuring tube. Also, your process temperature may factor in, so pay attention.
The electrodes have similar needs, as they too remain in constant contact with the fluid. Usually you have two electrodes to measure the voltage, but you can have a ground electrode on the bottom of the measuring tube and empty pipe detection on the opposite side.
These facts can eliminate magmeters as an option for your process, so during the sizing process, you need to check them out.
Volume or mass flow?
Even though magmeters start out as volumetric flow devices, they can measure mass flow, too. But how? The answer is simpler than you can imagine – just plug in the density of the product, and the magmeter will calculate the mass flow! Okay, maybe you need a bit more than that.
First off, you have to set the density value. Some magmeters can do it themselves, but usually as a high-end feature. You may need a density meter for an assist.
Second off, when you set the density value, you need to make sure this value remains stable, or your accuracy will suck. Read the density from a density meter and calculate the mass flow based on the right information.
Mining companies often use magmeters to measure slurry flow, with certain liners to reduce abrasion and a densimeter to send data for the magmeter to translate into an online mass flow measurement. I’ll dive into “special” applications in another article.
What installation does a magmeter need?
You need to meet some installation requirements to keep good accuracy throughout the device’s life. Unlike Coriolis flow meters, you have a bunch of stuff to deal with for a magmeter.
The best way to install it is vertically, with the product going up, so you have a filled or empty pipe. Half-filled pipes can cause magmeter problems, for sure. Of course, you can install it horizontally, as long as you avoid half-filled pipes. You may even need to modify your pipe. Some vendors claim that their meters will work on half-filled, so if you foresee that as an issue, then look into those claims.
Moreover, high temps in your process mean that you should install the device upside down. It doesn’t mind cold, so business as usual there. On top of all that, you have outlet and inlet runs! Again, some devices claim that they can skip this bit, but typically you need at least five times the meter size before the meter and two times after. This point can vary, so follow the recommendations in the manual from the vendor.
And finally, some applications need ground rings, but for others the ground electrode will suffice. You definitely need good grounding, but we need to cover this topic carefully, so let’s save it for another article.
Magmeter advantages and disadvantages
As I always say, you can find good and bad things in all principles, models, and brands. The perfect flow meter does not exist – so far, anyway. At least, I’ve never seen one. Have you?
On to the lists!
Advantages of magmeters
- Working principle independent of temperature, pressure, and viscosity
- Measures from water to slurry
- No pressure loss
- No moving parts
- Good size range
Disadvantages of magmeters
- Minimum conductivity required
- Incrustation can cause errors
- Low conductivity has low accuracy
And to finish us off, these videos from different vendors will tell you more about magmeters. If you have questions, then reach out to me on social media!