Ultrasonic flow meters and their applications
Here we are again! You ready to learn more about another principle of flow measurement? Ultrasonic flow meters show up in many applications, from process flow to custody flow. And companies often use portable ultrasonics to verify other flow meters or for temporary measurement.
You’ll find two concepts within this principle, Doppler and transit time. But first, let’s find out more about the principle itself. Then we can go through in-depth variations and typical uses. Later, we can highlight the pros and cons and give examples of brands on the market.
Stick with me, and let’s learn more about this flow meter!
What is an ultrasonic flow meter?
The ultrasonic flow meter is a volumetric flow device with a wide range of applications in liquids and gases. I like to position the ultrasonic between the magmeter and the vortex flow meter. Of course, a vendor or a user may not agree with my point of view. I’m cool with that.
But consider: If you can’t deploy a magmeter, then an ultrasonic makes an excellent second option. And if the product won’t support the ultrasonic, then you’ll probably look to the vortex.
Best of all, if you want non-intrusive technology, then get a clamp-on meter. Just click and go! Of course, a clamp-on may not provide the accuracy you need, so check before you buy.
Keep in mind, ultrasonics can do complex as well as simple. You can find on the market sophisticated versions dedicated to custody application and other such measurements. You may need other devices to support it with process data such as pressure, temperature, and gas composition to keep it accurate, but it’ll do the job.
How does an ultrasonic flow meter work?
As mentioned earlier, an ultrasonic meter measures using one of two methods, Doppler or transit time. Vendors might offer you both solutions in their portfolios or only one. Of course, both options have good and bad points, so your best choice will depend on your application.
Usually, when people explain the Doppler effect, they use an ambulance as their example. So, I’ll use a different example – the police car! As the car moves closer to you, its siren sounds louder (audio frequency), even though it doesn’t actually increase the volume. When it moves away from you, the siren wanes. We call these frequency shifts the Doppler effect.
Now the Doppler method needs particulates or bubbles in the fluid. When the transmitter sends its ultrasonic signal, the signal will reflect off the bubbles or particles in motion through the pipe, changing its frequency. The transmitter then reads those changes and calculates the flow from that data.
This reflected frequency shift is proportional to the velocity of the particles/bubbles, and we assume that the flow velocity matches the particle velocity. In a real application, many particles or bubbles may have differing velocities. You also need to remember that a signal reflected by a particle may hit another particle or bubble before returning.
Transit time method
Unlike the Doppler method, the fluid in a process using the transit time method must not have particles or bubbles. Usually you have a minimum percentage of bubbles and particles that you can squeak by with, but more than that will affect the measurement.
This method works based on the propagation velocity of sound waves. Imagine you’re in the middle of a northbound crowd and you want to head south. So, you either put more energy into walking against the flow rather than with it or you wind up walking slower.
With this method, the ultrasonic meter has two sensors that transmit and receive signals simultaneously. If you start with zero flow, then you’ll have no transit time delay between both sensors. They transmit and receive signals equally.
However, with flow in the pipe, the sensor emitting the signal in the flow direction will receive its signal sooner than the sensor emitting against the flow. If you know the distance between both sensors – and you should! – you can calculate the flow velocity based on the difference of transit time.
Depending on the model and vendor, you can have more than two sensors in the device. You can also have inline or clamp-on meters to measure the flow on the pipe’s wall or models to measure gas flow.
Clamp-on ultrasonic flow meter
Some industries use clamp-on ultrasonics more than inline. You save a lot of time on installation but lose some accuracy. However, if your application doesn’t need high accuracy, then you might like a clamp-on too.
So how does it work? Well, the sound waves can pass through the pipes and not damage or even change anything. As I said, in some applications a no-contact flow meter sounds like the best choice.
On the downside, you have to prepare the pipe to receive the sensor, know the thickness and material of the pipe, and make sure you don’t have incrustation. Still, you can install a clamp-on with a local flow transmitter or a portable ultrasonic flow meter.
Portable ultrasonic flow meters
The portable ultrasonic flow meter makes an excellent option to measure temporary flow and verify other meters. However, I’ve seen some service companies offering calibration services using clamp-on devices. I gotta know – how can you calibrate a flow meter with an accuracy of +-0.05 percent using another flow meter with an accuracy of +-2 percent? You can’t, can you?
On the upside, it’s a great tool to verify other meters if you have a question about their accuracy or to measure the flow for a short period of time.
Advantages and disadvantages of ultrasonic flow meters
Like it says on the box. Read them and decide for yourself whether an ultrasonic would work for you.
- No pressure loss
- No contact with the fluid necessary
- Small to large nominal diameters
- Some temperature limitations
- Deposits in the sensor or pipe can affect the measurement
- Results will depend on the flow profile