The Model 4675OC Precision Settlement Monitoring System was designed to measure differential settlements with a very high degree of accuracy and resolution, making it suitable for applications of a highly critical nature where the expected settlements are small. The system uses a 3 inch diameter horizontal open channel pipe, which is half filled with water or antifreeze solution and fixed to the structure or tunnel under observation. Sensors are situated at intervals along the pipe and connected hydraulically to the water in the pipe through short lengths of tubing.