Emerson Rosemount 5408 Reference Manual page 125

Change the upper null zone
The Upper Null Zone defines a zone close to the transmitter where echoes are ignored.
This zone can be extended to block out disturbing echoes at the top of the tank.
Note
Make sure the Upper Range Value (100%/20 mA) value is below the Upper Null Zone. Measurements
are not performed within the Upper Null Zone.
Rosemount 5408 and 5408:SIS Level Transmitters
Figure 7-9: First Endpoint
A. First endpoint
Service and Troubleshooting
115