Evaporation deposited thin metal strain guages: evaporation deposited guages are used mostly of the fabrication of transducers. they are of sputter deposited variety. both processes begin with a suitable elastic metal element. the elastic metal converts the physical quantity into a strain. for example in a pressure transducer, a thin, circular metal diaphragm is formed. the strain guage elements are directly on the strain surface and they are not separately attached as in case of bonded strain gauges.

in the evaporation process, the diaphragm is placed in a vacuum chamber with some insulating material. heat is applied until the insulating material vaporizes and then condenses, forming a thin dielectric film on the diaphragm. suitably shaped templates are placed over the diaphragm, and the evaporation and condensation processes are repeated with the metallic gauge material, forming the desired strain gauge pattern on top of the insulating substrate.

in the sputtering process, a thin dielectric layer is deposited in vacuum over the entire diaphragm surface. the detailed mechanism of deposition is, however, entirely different from the evaporation method. the complete layer of metallic guage is sputtered on top of the dielectric material without using any substrate.the diaphragms are now removed from the vacuum chamber, and micro imaging techniques using Photostatting materials are used to form the gauge pattern.

It is usual to describe the sensitivity of strain gauge in terms of its character termed as gauge factor.
Gauge Factor is defined as the ratio of unit change in resistance to unit change of length and is denoted by the letter K. so K is basically resistance change in the material to the strain applied on it.

Source: from various journals and websites...

Strain Guage

some basic instrumentation stuff which we have studied and used in college labs and also in our workplace.. Well, this is about strain gauges.

Basic Theory
if a metal conductor is stretched or compressed, its resistance changes on account of the fact that both length and diameter of conductor change. also there is a change in the value of resistivity of the conductor under strain and this property is called peizo- resistive effect. the strain gauges (peizoresistive gauges) are used for the measurement of strain and associated stress in experimental analysis. moreover, many other detectors like load cells, torque meters, diaphragm type pressure gauges, temperature sensors, accelerometers, etc. use this principle.

 the change in value of resistance by straining the gauge may be partly explained by the normal dimensional behavior of elastic material. if a strip of elastic material is subjected to tension or positive strain, then its longitudinal dimension will increase while the area of cross section will decrease. since the resistance is proportional to its length and inversely proportional to the area of cross section, the resistance increases in case of positive strain.

Types of strain gauges:
1. unbonded metal strain gauge
2. bonded metal strain gauge
3. bonded metal foil strain gauge
4. vacuum deposited thin metal film strain gauge
5. sputter deposited thin metal strain gauge
6. bonded semiconductor strain gauge
7. diffused metal strain gauge

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