Types of Strain Gauge

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2.2 Types of Strain Gauge
Types of strain gauges are classified into foil strain gauges, metal strain gauges and semiconductor strain gauges, etc.
2.2.1 Foil Strain Gauge
The foil strain gauge has metal foil on the electric insulator of the thin resin and gauge leads attached, as shown in Fig. 2.1 below.

Fig: 2.1 Foil strain gauge
The strain gauge is bonded to the measuring object with a dedicated adhesive. Strain occurring on the measuring site is transferred to the strain sensing element via adhesive and the resin base. For accurate measurement, the strain gauge and adhesive should be compatible with the measuring material and operating conditions such as temperature, etc.
2.2.2 Metal strain gauge
The change of resistance in an electrical conductor due to the impact of mechanical stress, as discovered by Wheatstone, became the subject of study in the 1930s.
A number of advancements were experimented and finally Ruge took a very thin resistance wire, stuck it in a meander shape on to some thin tissue paper and terminated the ends with thicker connections. In order to study the characteristics of this prototype device, he glued it to a bending beam and compared the measurements with a traditional strain measuring device. He discovered a good correlation with a linear relationship between strain and the displayed values over the complete measurement range, both with positive and with negative, i.e. compressive, strain, including good zero-point stability. Hence the “electrical resistance strain gauge with bonded grid” was invented (fig: 2.2).

Fig: 2.2 Metal strain gauge

2.2.3 Semiconductor Strain Gauge

In the year 1970, the first semiconductor strain gauges were developed for the use in automotive industry. Semiconductor strain gauges’ exhibit following key features:

  • Unlike other strain gauges, semiconductor strain gages are based upon the piezo resistive effects of silicon or germanium and measure the change in resistance with stress as opposed to strain.
  • The semiconductor bonded strain gage is a wafer with the resistance element diffused into a substrate of silicon.
  • No backing is provided for the wafer element and bonding it to the strained surface needs extra care since only a thin layer of epoxy is used to attach it.
  • Size of a semiconductor strain gauge is much smaller and the cost much lower than for a metallic foil sensor.
  • Advantages: include higher unit resistance and sensitivity.
  • Disadvantages: Greater sensitivity to temperature variations and tendency to drift as compared to metallic foil sensors. Also, the resistance-to-strain relationship is nonlinear, varying 10-20% from a straight-line equation. However, by means of computer-controlled instrumentation, these limitations can be overcome via software compensation.

Fig: 2.3 Semiconductor strain gauge.

2.2.4 Vapor-Deposited (Thin-Film) Strain Gauge

This is provided by vapor deposition techniques. In this case, the measuring element is directly deposited onto the measurement point under a vacuum by the vaporization of the alloy constituents. The applications are restricted to the production of transducers.


Fig: 2.4 Thin-film strain gauge

2.2.5 Capacitive Strain Gauge

The capacitive strain gauge (fig: 2.5) is considered as an alternative to conventional strain gauges for use at high temperatures beyond the limit of metal strain gages. There are three known versions presently:
  • A British development by the Central Electricity Research Laboratories (C.E.R.L.) in cooperation with the company Planer. Here a plate capacitor is used where the plate separation changes depending on the strain to be measured.
  • An American development from Boeing Aircraft which is constructed as a differential capacitor.
  • A German development from Interatom. This is also constructed as a plate capacitor. Capacitive transducers are fixed to the test object with spot welding techniques. It is possible to obtain good results with capacitive strain gages in the temperature range up to about 500°C. The results are in the range up to 800°C.

Fig: 2.5 Capacitive strain gauge

2.2.6 Piezoelectric Strain Gauge

Piezoelectric strain gauges are active devices. The strain sensing material is barium titanate. When compared to piezoelectric transducers that use quartz as the sensing material, the strain gauge provides an electrical charge on its surfaces which is proportional to strain and that can be measured with charge amplifiers.


Fig: 2.6 Piezoelectric strain gauge

2.2.7 Photo elastic Strain Gage

A strip fabricated from optically stressed active material exhibits an isochromatic field as a result of a “frozen”, continually increasing stress. The isochromatic get displaced as a result of the strain. The degree of displacement which is read off a scale is a measure of the strain. This kind of strain gages are made in the USA. They do not have any specific benefits and are no longer commercially available.

Fig: 2.7 Photo elastic Strain Gage

2.2.8 Mechanical Strain Gauge

These devices are seen infrequently but have a long tradition. They can usually only be applied to larger objects due to their construction. The measurement effect is shown by a trace scratched on a metal plate or on a glass cylinder, which can only be studied at the end of the test under a microscope. This disadvantage is offset somewhat by the large temperature range.

Fig: 2.8 Mechanical Strain gauge

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