Strain Gauge Extensiometer

An extensiometer is shown mounted to a sample
with elastic bands. Knife edges on the beams in
contact with the sample are a defined distance
apart, the Gauge length. The change in this length
due to the deformation of the sample is measured with
a strain gauge bridge fixed to a deformable component
in the head of the device. Deformation causes a
change in the resistance of the gauges, unbalancing
the Wheatstone bridge circuit, and giving rise to an output voltage proportional to the deformation. The ratio of the change in length to the gauge length is the average strain of the sample between the knife edges.

The extensiometer must be oriented so that the fixed (black) beam is at the top and the electrical cable comes from the top of he device. Once the extensiometer is fixed in position, the cable should be supported so that its full weight is removed from the extensiometer and the extensiometer is free to move without being pulled by the cable.