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Strain Gauge

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Several different technologies exist for the conversion of fluid pressure into an electrical signal response. These technologies form the basis of today’s electronic pressure transmitters. One of such technology is the strain gauge discussed here.
A strain gauge is what may be described as a ‘’piezoresistive element’’. This means its resistance changes with changes in applied pressure. Basically, a strain gauge uses the change of electrical resistance of a material (wire, foil or film), under strain to measure pressure.

The 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 will decrease electrical resistance by simultaneously decreasing length and increasing cross-sectional area.
The complete strain gauge pressure-measuring device includes:
  • A sensing element (Bourdon’s tube, bellows or diaphragm) 
  • A strain gauge attached to the element 
  • A stable power source and a readout device 
In practical devices using the strain gauge technology, the strain gauge is attached to a diaphragm (technology differ for different manufacturers). This 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.

In earlier manufactured brands, metals were used as the strain gauge element. Within their elastic limits, many metals exhibit good spring characteristics. Metals, however, are subject to fatigue over repeated cycles of strain (tension and compression), and they will begin to "flow (plastic deformation)" if strained beyond their elastic limit. This is a common source of error in metallic based strain gauges.

Modern manufacturing techniques have it made possible for the construction of strain gauges made of silicon instead of metal. Silicon exhibits very linear spring characteristics over its narrow range of motion, and a high resistance to fatigue. This characteristic is desirable in actual applications as a failed sensor will necessitate the need for its replacement (whereas a metallic strain sensor may give the false impression of continued function after an over-stress operation).

A simplified diagram of a diaphragm / strain gauge pressure sensor arrangement is shown below:
Foxboro manufactures a line of pressure transmitters using the strain gauge as the pressure sensing element. You might check it out here Foxboro pressure transmitter

One major disadvantage of the strain gauge is that all their applications require regulated power supplies for the excitation voltage, although this is commonly internal with the sensing circuits.
You can also learn more about the evolution of the strain gauge from  Omega