ULI 200 series transmitters are 3 wire type instruments.
Anultrasonic wave is pulsed 5 times a second. The reflection of this pulse is received by the sensor. The time taken for the wave to travel to the object and back to the sensor is proportional to the distance of the object from the sensor.
The compact size, narrow beam angle and small dead band ensures that these sensors will work reliably for small tanks.