User`s guide

113
sensor is directly proportional to the change in resistance of the gauge used, as shown in Eq 7.When
unstressed, usual strain gauge resistances range from 30 Ohms to 3 kOhms.
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

An ideal strain gage is small in size and mass, low in cost, easily attached, and highly sensitive to strain
but insensitive to ambient or process temperature variations. The ideal strain gauge would undergo
change in resistance only because of the deformations of the surface to which the sensor is coupled.
However, in real applications, there are many factors which influence detected resistance such as
temperature, material properties, the adhesive that bonds the gage to the surface, and the stability of the
metal.
The strain sensitivity, which is also known as the gage factor (GF) of the sensor, is given by:




where R is the resistance of the gauge without deformation, dR is the change in resistance caused by
strain, and 

is the strain to be measured. Therefore, the strain can be expressed as:







2.4.2 Materials and Selection of Strain Gauges
Typical materials for strain gages include: constantan (copper-nickel alloy), nichrome v (nickel-chrome
alloy), platinum alloys (usually tungsten), isoelastic (nickel-iron alloy), karma-type alloy wires (nickel-
chrome alloy), foils, and semiconductor materials. The most popular alloys for strain gages are copper-
nickel alloys and nickel-chromium alloys.
Temperature change can affect the internal structure of strain-sensing material, and also can amend
properties of the material of the surface the strain gage is attached to. When there is a temperature change
while a measurement is being made, the effects can cause large errors in data unless proper precautions
are taken.
Each material has unique reaction to temperature change, as illustrated in figure below. Variation in
expansion coefficients between the gage and base materials may cause dimensional changes in the sensor
element. Therefore, it is a good practice to select strain gauge made of same type of material as the base
structure.
Eq.64
Eq.65
Eq.66