The Magnetic Extensometer is installed at locations where monitoring of settlement or heave is required. The Magnetic Extensometer System comprises a probe, a graduated tape on a reel and an access pipe along which magnetic targets are positioned at pre-determined intervals. The reed switches within the probe detect the magnets as it moves along the length access pipe. When a magnet is detected, the switch circuit closes causing a light to display and a buzzer to sound at the reel.
There are different models of magnetic targets available; three-legged and six-legged spider magnets, retained by leaf springs for use in boreholes, and plate magnets for placing in soil or fill when adding
further sections of access tube during construction.