The Impact of Background Echoes on Ultrasonic Level Measurement


Photography is a pretty good way to illustrate the importance of background.

Look at the two photographs here. In one, the background is minimal, and focuses your eye on the subject matter. In the other, the background seriously detracts from the subject. Where should you be focusing? What’s most important?

Non-intrusive background makes focus point clear

Hard to discerne the difference between the action and all the stuff happening in the background

 

But unlike photography, where a good background helps you focus on the subject, in the world of non-contact ultrasonic level measurement, even a “good” background has a negative influence.  Background never contributes to a level reading, it only detracts. But Siemens has a built-in function to “cure” for the influence of backgrounds in their level devices.

Ultrasonic level instruments are always mounted on the top of the vessel, looking down at the material or liquid.  The instrument transmits a “ping” of energy and looks for an echo to return off the material in the tank.  The problem is that the return echo profile includes not only the material echo, but echoes from any and all other objects in the tank.

Tanks can have a lot of stuff on the inside – ladder rungs, agitator blades, and baffles.  The echoes from all the other stuff are real, but they’re unwanted, so we refer to them as False echoes. Except for agitator blades, the non-material stuff is stationary; it’s always there, in the same location. And even though agitator blades move, they’re stationary in the sense that they’re always at the same distance from the sensor at the top of the tank. 

The classic way to deal with these unwanted false echoes is to tell the instrument to take a scan of the tank with all its background warts and pimples. The background scan profiles the stationary echoes and can then be used to adjust the baseline curve, known as the time-varying threshold or TVT curve, to ignore them. Only signals above the TVT baseline are used for calculating distance or true level.

The background echoes are ‘added’ to the adjusted TVT which allows the instrument’s algorithm to ignore the permanent stationary echoes and concentrate on the remaining True echo from the material’s level.

TVT curve adjusted to suppress false echoes.

 

Running a false echo scan

Because false echo suppression is so important to accurate level detection, Siemens lists it in the quick-start, right after the six primary setup parameters.

Remember, this routine needs to be run when there’s a relatively low level for an obvious reason: there won’t be any echoes from stationary objects that are already covered by the material in the tank. 

Running a background scan is an easy two-step process:  Tell the instrument the distance over which it should run the profile (P838), and then do it (P837).  The distance you need to know is the length in engineering units from the transducer face to the material surface minus 0.5m (18″).

It’s becoming apparent to me that people are reluctant to initiate the sequence, and I think know why.  Siemens calls the background scan “Auto False Echo Suppression”, which can be a scary term. It sounds like something that could really screw things up. 

But, auto false echo suppression is there to maximize the instrument’s performance and is a major contributor to reliable level echo processing.  Using it does NOT require any user decisions or input other than the distance value; it runs a profile and does the TVT adjustment all on its own. 

Customers who use Siemens SIMATIC PDM software for commissioning their ultrasonic level instruments have told us that the manual covers the handheld programmer procedure for running the Auto False Echo Suppression routine, but that it isn’t clear how to run the routine in PDM software.  (The radar manuals have a procedure and show a screen shot).  

To address this, I’ve written a short technical note that shows the simple step-by-step procedure for running Auto False Echo Suppression in PDM.

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Lesman Instrument Company is the authorized Siemens representative in Illinois, Indiana, Missouri, Paducah KY, Wisconsin, and Michigan’s Upper Peninsula. If you are located outside that area, you can find your local sales office or get technical assistance by calling 800-365-8766.

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