Listen in tutorial: Electromechanical level sensor / switches

26 August 2008

We often consider high-tech types of instruments but readers appreciate basic technologies too. For example: electromechanical level devices that use vibrating probe or paddle wheel sensors still have many useful applications.

Listen in: Don Koeneman, application engineer for Ametek’s Drexelbrook division, comments on electromechanical level devices.

These two technologies are similar in that they insert a moving device into the tank contents and determine the presence of liquid or dry material by the way in which it interferes with the motion. A vibrating sensor extends a small tuning fork or rod into the material that is attached to a piezoelectric crystal. If there is nothing around the probe, it can vibrate at its normal frequency. If immersed in liquid or solid material, it can’t vibrate as easily, and the system will detect the difference.

A paddlewheel sensor extends a multi-bladed wheel or a flag into the tank and turns it slowly. Immersion in liquid or solid material will interfere with its rotation, which can be detected.

These approaches have some limitations:
 They provide only a point level reading, so use them in appropriate applications.
 Both use moving parts so there is some mechanical delicacy. Don’t position them where product can fall on the probe or protect it with a baffle.
 Both are relatively high power consumers. If you’re concerned about having the lowest power devices possible, there are better choices.
 Equipment vibration can be problematic.
 Product build-up on the probe can interfere with accuracy.
 Things that move need maintenance.

But there are some characteristics that keep customers coming back:
 They are generally very reliable;
 Typically low cost;
 Often no calibration is required;
 Liquid/solid characteristics (conductivity, dielectric, etc.) are largely inconsequential;
 Relatively immune to foam, (although this varies);
 Can be made in sanitary configurations;
 Easy to position and place; and
 Many configurations are available.

These are generalities, so discuss specifics for your process with your instrumentation provider. Vibrating and paddle wheel sensors are available from a variety of suppliers

You can search online at Control Engineering Europe's Sitefind

—Peter Welander, process industries editor


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