Sir,

    I am facing the problem with accuracy of measurement of water levels. It shows very high reading sometimes. Sensor is mounted correctly...not to tight or other problem....water is also steady without spikes. Weir construction is also perfect. Sensor works perfectly, when we test it other places. 

         I think...some ultrasonic interference could be there....but not sure.....

Please give the possible reasons for such problem....and possible solutions....

Thanks

----------------------------------------------------------------------------------------------------
Split from The Probe, Ultrasonic Level Transmitter Interference Question.

The January 06, 2015 issue of European Automation ran the article The Top Five Things that Cause Inverter Failure.  The 5th reason cited is ultrasonics that are generated inside the drive:

"The final problem on the list is one that contributes to the mechanical stress placed on an inverter. Ultrasonic vibrations originating in the cores of inductive components cause friction, adding to the unwanted heat generated by the device and further damaging components in the inverter."

http://www.euautomation.com/uk/automated/article/the-top-five-things-that-cause-inverter-failure

The article is not scholarly dissertation on the problem, just a heads-up recognition that VFDs can generate ultrasonics of significant strength.   

Acoustic noise generated by VFDs used to be in the audible spectrum/range.  Higher switching frequencies have shifted the noise to the ultrasonic frequency range.

The LUT 400 transmitter has some noise troubleshooting points (includes both acoustic and electrical noise) [pages 240-242 paper are attached as a pdf].  They describe a test for determining the source of the noise using a couple pieces of cardboard placed over the face of the transducer.

The listed solutions for reducing noise are insulating/filter/deflecting the noise or mounting the transducer in a rubber bushing if the acoustic noise is coming  the physical mounting connection.