They consist of a triaxial mounting frame into which three vibrating wire displacement gauges are mounted and connected to individual anchors via a wireline, which allows independent movement in all directions, irrespective of each other.
Each displacement gauge comprises a sensor outer body tube and an inner free-sliding rod, which is connected at the internal end to a vibrating wire sensor by a spring. At the external end of the rod, anchors are attached which can be fixed either side of the crack to be monitored.
The gauge operates on the principle that a tensioned wire, when plucked, vibrates at its resonant frequency. The square of this frequency is proportional to the strain in the wire. Around the wire is a magnetic coil which when pulsed by a vibrating readout or data logger interface plucks the wire and measures the resultant resonant frequency of vibration.
A change in distance causes the inner free-sliding rod to move within the outer body, which changes the tension on the spring and the vibrating wire thus altering the resonant frequency of the wire.