Ultrasonic flow meters have many applications, from process flow to custody flow. It is also a common practice to use a clamp-on ultrasonic flow meter to verify other flow meters or for temporary measurement. The article explains what you need to know about the basics of ultrasonic flow measurement and its applications.
There are two types of ultrasonic flow meters: Doppler and transit time ultrasonic flow meters. But first, let’s find out more about the working principle. Then we can go through in-depth variations and typical uses. Later, we will highlight the pros and cons and give examples of brands on the market.
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What is an ultrasonic flow meter?
The ultrasonic flow meter is a volumetric flow measurement device with a wide range of applications for liquids and gases.
Ultrasonic flow meters can be good alternatives to both electromagnetic and vortex flow meters. If we can’t deploy an electromagnetic flow meter, 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.
Ultrasonic flow meter types
As mentioned earlier, an ultrasonic flow meter measures using one of two methods, Doppler or transit time. Vendors might offer you both types in their portfolios or only one. Of course, both options have good and bad points, so the ideal choice will depend on an application.
Usually, when people explain the Doppler effect, they use an ambulance as their example. So, we’ll use a different example: the police car! As the car moves closer to us, its siren sounds louder (audio frequency), even though it doesn’t actually increase the volume. When it moves away, the siren wanes. We call this frequency shift the Doppler effect.
Now, the Doppler method needs particles or bubbles in the fluid. The transmitter’s signal reflects back off the bubbles or particles moving through the pipe, with a slightly changed frequency. The transmitter reads those changes and calculates flow.
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. We also need to remember that a signal reflected by a particle may hit another particle or bubble before returning.
Transit time method
Contrarily to the Doppler method, the fluid in a process using the transit time method should not have particles or bubbles. Usually, there is a maximum percentage of bubbles and particles that still allow for accurate measurement.
This method works based on the propagation velocity of sound waves. Imagine that you’re in the middle of a northbound crowd and you want to head south. Exerting the same amount of effort to make your way, you will still end up moving slower because of the contrary movement of the crowd.
With the transit time method, the ultrasonic flow meter has two sensors that transmit and receive signals simultaneously. At zero flow, you’ll have no transit time delay between both sensors. They transmit and receive signals equally.
When there is 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 the sensors, you can calculate the flow velocity based on the difference in 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 ultrasonic flow meters to measure the flow on the pipe’s wall or models to measure gas flow.
Clamp-on ultrasonic flow meter
If you want non-intrusive technology, a clamp-on ultrasonic flow meter can be an ideal choice. Of course, a clamp-on may not provide the accuracy you need, so make sure you know your process requirements before buying.
Keep in mind, an ultrasonic flow meter can do complex as well as simple measurements. You can find sophisticated versions on the market, dedicated to custody applications and other high-stake 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.
On the downside, you have to prepare the pipe to receive the clamp-on flow meter’s sensor. You will also have to 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 is an excellent choice to measure temporary flow and verify other meters. However, I’ve seen some service companies offering calibration services using clamp-on devices. However, you can’t calibrate a flow meter with an accuracy of +-0.05 percent very well using another one with an accuracy of +-2 percent.
On the upside, it’s a great tool to verify other meters if you have questions about their accuracy or to measure the flow for a short period of time.
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Advantages and disadvantages of ultrasonic flow meters
- 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
If you need help picking the right flow meter for your application, take a look at our new flow meter smart assistant.
To know more about ultrasonic flow meters, you can get in touch with our engineers and we will be happy to help.