A focus tube is recommended for tank applications that experience a drastic level change when the vessel is underway.
The TLM100 transmits an ultrasonic pulse that allows the device to calculate the distance from the sensor to the fluid surface (similar to depth sounders). When the vessel begins to plane, the tank level changes. The liquid shifts to one side of the tank leaving, a sloped surface which will cause the sound pulses from the TLM100 will bounce around the tank and not back to the TLM100 sensor, as shown in the following illustration:
A focus tube helps control this change within the tank. The focus tube helps guarantee that a signal will return back to the sensor, eliminating this issue of signal loss, as shown below: