Dan's Dozen - my list of LOE (Loss of Echo) causes:

1) Is the correct transducer model's code specified in P004?

2a) Location of transducer - see the transducer application manual for suggested locations, especially with respect to location next to a vertical wall and the effect of transducer location 'seeing' a dished (parabolic) bottom, whose echo will not reflect back.

3) Is surface that is reflecting an echo within the transducer's maximum range?  (A 6m transducer cannot 'see' 10m distance)

4) Aiming of transducer - it's aimed where it cannot return an echo. Re-aim.

5)  Transducer is overtightened in its threaded fitting.

6)  Excessive dust is attenuating the acoustic pulse and its echo.

7)  Something is in the blanking zone.

8a) Water or ice on the face of the transducer
     excessive water from shop air used for purge collects on xducer face

8b) Fog or condensed water when the dew point temperature drops below the tank's blanket atmosphere temperature.

9)  Carbon Dioxide (CO2) atmosphere.  Ultrasonic energy is absorbed like welfare checks by CO2.

10)  Faulty transducer with intermittent performance or a transducer affected by temperature change.

11)  LOE can be the result of attempting to read a dished bottom or when an open channel flow empties to zero (for the latter P801 is a range extension parameter).

12)  Are there 2 echoes arriving that can cannot be distinguised one from another? (echo profile needed)