The fill extensometer Model ERI consists of a telescopic outer PVC pipe fitted with two end flanges and an inner stainless steel rod. One end of the rod is attached to a flange, while the other end of the rod is attached to a displacement sensor, which is attached to the other flange. A signal cable attached to the sensor links the sensor to a readout at the surface. As displacement occurs, the telescoping outer pipe moves with the soil, causing the rod to compress or extend the displacement sensor.
These fill extensometers are generally installed horizontally in trenches, but sometimes vertically in boreholes. The standard lengths of the fill extensometer Model ERI are between 3 meters and 6 meters. Extensometers can be assembled in series using threaded rods inserted in the holes on each.