NORAC UC4.5-BC-EV5 Part 1 - Manual (Page 10)

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5.2 Ultrasonic Sensor Serial Number Arrangement
When installing the sensors, start with the smallest serial number on the left-hand side, and
proceed to the largest serial number on the right hand side. Each sensor has a serial number
stamped on the sensor housing.
Apply a light coating of the supplied Permatex Anti-seize grease to all threaded
parts upon installation.
Figure 6: Sensor Serial Number Arrangement