During normal operation, the blue LED should be off and the 4-20 mA signal output should be relatively stable within the span.
If the sensor does NOT receive a valid acoustic signal return, the blue LED will turn on, and the signal output will go to 22 mA until the sensor acquires the level. This is generally caused by the presence of substantial foam, vapor and/or turbulence.
If the sensor DOES receive a valid acoustic signal return, but the level falls below the calibrated 4 mA empty set point, the blue LED will rapidly flash on and off, and the signal output will go to 22 mA until the level rises back into the span. The solution is to perform a factory reset on the sensor, re-calibrate the span and place the 4 mA empty set point lower in the tank.
Factory reset washes out any previous user calibration and returns the sensor to the original set points. Connect the white wire to the black wire (see manual), point the sensor into open space with no reflective targets, power it up for 2 seconds, and then power it down. The blue LED will not flash during reset.