The strain gauge is a passive transducer used to pressure, which converts the change in pressure into a change in resistance as the metal strain gauge deforms because of the pressure applied.
The principle of the strain gauge is the Piezoresistive effect, which means “pressure-sensitive resistance,” or a resistance that changes value with applied pressure. The strain gauge is a classic example of a piezoresistive element.
Electrical resistance of any conductor is proportional to the ratio of length over cross-sectional area (R ∝ l/A), which means that tensile deformation (stretching) will increase electrical resistance by simultaneously increasing length and decreasing cross-sectional area while compressive deformation (squishing) will decrease electrical resistance by simultaneously decreasing length and increasing cross-sectional area.
The majority of strain gauges are foil type, available in a wide choice and shape and sizes to suit a variety of application.
Strain gauges in their infancy were metal wires supported by a frame. Advances in the technology of bonding materials mean that the wire can adhere directly to the strained surface. Since the measurement of strain involves the deformation of metal, the strain material need not be limited to being a wire. As such, further developments also involve metal foil gauges. Bonded strain gauges are the more commonly used type.
There is the Wheatstone bridge arrangement where the change in pressure is detected as a change in the measured voltage:
The change in the resistance of the strain gauge breaks the balance of the Wheatstone’s bridge and change the voltage V. The voltage V is proportional to the pressure change in the strain gauge.
Attaching a strain gauge to a diaphragm results in a device that changes resistance with applied pressure. Pressure forces the diaphragm to deform, which in turn causes the strain gauge to change resistance. By measuring this change in resistance, we can infer the amount of pressure applied to the diaphragm.
As strain gauges are temperature sensitive, temperature compensation is required. One of the most common forms of temperature compensation is to use a Wheatstone bridge. Apart from the sensing gauge, a dummy gauge is used which is not subjected to the forces but is also affected by temperature variations. In the bridge arrangement the dummy gauge cancels with the sensing gauge and eliminates temperature variations in the measurement:
- Residual stress
- Vibration measurement
- Torque measurement
- Strain measurement
- Compression and tension measurement
- No moving part
- Wide range, 7.5kPa to 1400 Mpa
- Inaccuracy of 0.1%
- Small in size
- Stable devices with fast response
- Good over-range capability
- Unstable due to bonding material
- Temperature sensitive
- Thermoelastic strain causes hysteresis