Metallic strain gauges are electrical resistors on a carrier material. The resistance value is defined by the length and cross section of the conductive path. The carrier material primarily consists of a base body with a weakened surface, which is subjected to deformation due to the pressure: the membrane.
The strain gauges are firmly connected to the membrane and express the mechanical deformation as a change in their resistance value. During expansion, the conductive path protracts and the cross section tapers – the electrical resistance increases. During compression, the cross section of the conductive path increases and the length diminishes – the resistance decreases.
There are usually 4 strain gauges on the membrane, which are connected to form a Wheatstone bridge (Fig. 1). They work against each other, i.e. during deformation of the membrane, the outer strain gauges are compressed and the inner ones are expanded. Thanks to the bridge circuit, the output signal is four times stronger than it would be with a single strain gauge.