TEC-NOT-001 Strain gages and ideal bridges

This paper introduces the basics of strain gage theory and the terminology used. It examines how these gages can be used in a variety of ways with Wheatstone bridges, focusing on how strain can cause a change in resistance, which in turn causes a voltage change across the bridge that can be measured. Also, the relationship between the output voltage and the change in resistance is described for the more popular bridge configurations. The following topics are discussed:

  • “1.1 Stress and strain” on page 1
  • “1.2 Strain and fractional change in resistance” on page 2
  • “1.3 Sources of error in the gage” on page 3
  • “1.4 The Wheatstone bridge and Poisson’s ratio” on page 4
  • “1.5 Resistance as a function of fractional change” on page 5
  • “1.6 Equations for two active gages, one of which is Poisson” on page 6
  • “1.7 Straight line approximations” on page 9
  • “1.8 Conclusion” on page 9
  • “1.9 References” on page 9