Ultrasonic vs electromagnetic flowmeters

Our flow measurement library:

When your process consists of aggressive or abrasive fluids you would definitely prefer a flowmeter that doesn’t obstruct the path of flow. Or if your flow architecture demands flow measurement, but there is no room for pressure drop, the full bore flowmeters come directly into mind. The solutions that are popular in those cases are ultrasonic and electromagnetic flowmeters (also known as magmeters or magnetic flowmeters). A comparison.

Accuracy of electromagnetic and ultrasonic flowmeters

Accuracy performance of both magmeter and ultrasonic flowmeters are excellent. When measuring gases, temperature and pressure compensation is needed on ultrasonic flowmeters. Accuracy depends heavily on the way the meter is constructed. So its important to find a balance between the needed accuracy and costs. Ultrasonic flowmeters, for instance, can have up to 32 channels, which results in uncertainties of around +- 0,021%, but that comes at a price. Ultrasonic flowmeters can be compliant to MID regulations and are capable of custody transfer measurements.

Limitations of both flowmeters

The biggest limitation for electromagnetic flowmeters is the fact that the measured medium must be electrically conductive. And signal to noise ratio decreases dramatically when conductivity gets poor.
Ultrasonic flowmeters may have problems with measuring fluids with suspended solids, particles or bubbles which interrupt the path of the sound signal. As the sensors are installed in the pipe wall, they create sensor pockets which can accumulate dirt. Or in case of measuring gas, the sensor pockets may flood with condensate. These problems can often be overcome, but need attention when designing a system.
Electromagnetic flowmeters need an insulating lining in the pipe. When measuring aggressive or abrasive substances this needs attention. Nowadays extremely hard linings made of composite ceramics are available which makes them suitable for virtualy any liquid medium.

Installation aspects of full bore flowmeters

Flow disturbers like bends and valves cause inaccuracy in flowmeters. The magnetic flowmeter is much less sensitive for this phenomena. When there isn’t much space available magmeters might be the better choice, since they only need one diameter length upstream and two downstream. In most cases the inaccuracy caused by flowprofile is linear so the resulting error can be compensated for in the flowcomputer.
Ultrasonic meters typically need five diameters pipe upstream and 10 downstream.
An important consideration for magmeter installation is to ensure an electrically quiet atmosphere, with good grounding and shielded cables. For ultrasonic flowmeters a sonically quiet environment is necessary and placement of valves requires special attention.
Both types of flowmeters have problems with lightning strikes and power surges.
Both types of flowmeters are well suited for hazardous areas and can get ATEX certificated.

How about flowmeter OPEX (maintanance)?

Both flowmeters have no moving parts and are not prone to abrasive erosion. And magnet flowmeters are known for their incredible long operating life.

And ultrasonic APEX (investment) versus electromagnetic?

Both flowmeter technologies can bevery cost effective flow measurement. Cost comparison depends strongly on project parameters.