I’ll take a guess and say that you’re experiencing a level transmitter FMR51 error because someone configured the radar improperly. The FMR51 from Endress+Hauser uses time-of-flight method (ToF) to measure the distance from the reference point (process connection) to the product surface. Then the antenna emits impulses to reflect off the product surface back to the radar. If you have a level switch, mixer, or other things inside the tank, the radar can identify them as levels, because they’ll create peaks in the level signal.

It’s simple to solve this problem. An envelope curve will help your radar understand what you have in your tank. Once you analyze the diagnostic, you can show the radar the proper level. If you followed all the installation steps correctly and still have a problem, you should do an envelope curve analysis.

Envelope curve

An envelope curve provides a graphic showing the relationship between the distance and the decibels (dB), for free space radar, or millivolts (mV), for guided radar. The graphic will show all peaks in the tank, and the peaks represent the product level and all interferences.

The red line shows the product level and the blue line dynamic mapping. Most of the time, you can use dynamic mapping to measure correctly. Lastly, the black line shows the static mapping, where we’ll work.

Level transmitter FMR51 error
Envelope Curve (Copyright Endress+Hauser)


Once you analyze the envelope curve, you need to do the static mapping to eliminate all noise and interference, keeping only the product level. The best way to do it is with the tank empty, but it’s usually not feasible to empty a tank just for mapping. Instead, you can do a partial mapping.

This procedure allows you to tell the radar to ignore the interferences and find only the level. Normally the peak of the level should be higher than the propellers, but this will depend on the dielectric constant.

Check out the ToF video below for more information:

If you’d like to see how to make and save envelope curves with an Endress+Hauser transmitter, have a look at this video:

And to learn how to do the mapping using the display, watch this:

If you have more questions, give us a call and our engineers will help.

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