Level tutorial: Magnetostrictive level sensors

05/17/2007


Sometimes an old technology can receive an update that leap-frogs it over more modern techniques. Such is the case with magnetostrictive level sensors. While the idea of using a moving float seems old fashioned, the level of precision this technology offers and its versatility make it a compelling approach for continuous level monitoring in the right applications. (Like all these tutorials, this information is general and you should talk to specific vendors for details.)

Magnetostrictive level sensors use a doughnut shaped float that moves up and down on a shaft that is actually a wave guide. Inside the shaft is a wire that carries an electrical pulse. The float has a permanent magnet which sets up a field around the waveguide. When the pulse travels down the waveguide, part of it is reflected back by the magnetic field. The rest continues down to the end of the waveguide and is reflected back from there. The sensor measures the time difference between the pulse going down and the two reflections coming back to calculate the float position. Once a sensor system is set up, it requires no additional calibration. Units are mostly stainless steel; however plastic versions are also available.

Magnetostrictive technology grew out of position sensing for machine tools, so it can be exceptionally precise. When combined with a float tuned to the specific gravity of the process liquid, they are capable of an error band well belowmay be in the bottom of a tank or there might be mixed products that settle into layers. Creating a float that sinks in some liquids but floats in others is tricky, but manufacturers can build floats to differentiate between liquids with as little as a 0.1 difference in specific gravity.

The obvious downside is that the float has to be able to move freely, so any contents that could glue it in place will be problematic. Floats can be quite large, up to the size of a soccer ball if necessary, so they can have a lot of buoyancy when needed. Moreover, the waveguide and float are in the product, so that eliminates some applications. Nonetheless, sanitary versions are available, so the design can be configured for critical food and pharmaceutical uses.

This technology has been particularly popular in oil and gas applications for large storage tank monitoring, but is spreading to other areas. It is generally less expensive and as accurate as an equivalent radar sensor if the other drawbacks are not an issue.

Magnetostrictive level sensors are available from:


—Edited by Peter Welander, process industries editor, PWelander@cfemedia.com , Control Engineering Weekly News (Register here and scroll down to select your choice of eNewsletters free.)





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