Honda HL2000 ultrasonic signal has characteristic that it reflects where the density changes.
Submerged sensor transmit ultrasonic signal, then the signal reflects at the interface inside the tank. Some part of signal penetrate into the sludge and it also reflects at the rake or bottom of the tank.
Ultrasonic signal transfers in the unclear water. There is no need to contact to the measuring object (sludge), the measurement can be done without sinking the sensor deep into the tank.