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We take a cylindrical concrete sample and apply compression load on it, perpendicular to its head. To measure its strain we use 2 Lvdt(strain measurement devices)on both sides of the concrete sample . Surprisingly they give us different numbers (close but not the same). Why is that? Firstly I think that it is because concrete sample might not be fully homogeneous (in preparation the mixture process might not have been perfectly mixed. The second reason is that it might have some bulk defects inside. Let me know what you think

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    $\begingroup$ How close is "close" and have you calibrated them against each other to know if they just give different readings? $\endgroup$
    – hazzey
    Dec 2 '16 at 20:20
  • $\begingroup$ The instruments are electronic, so they dont need calibration. $\endgroup$
    – Enea
    Dec 4 '16 at 12:41
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    $\begingroup$ wherever did you get the idea eletronic sensors dont need calibration?? $\endgroup$
    – agentp
    Dec 5 '16 at 2:21
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With any measurement system, there are errors. Based on what details have been given there are plenty of potential error sources. A few that I can think of off the top of my head are:

  • Physical differences in the sensors
  • Errors in their mounting locations
  • Calibration errors
  • Differences between the sig. cond. and/or A/D hardware for each sensor
  • Differences in the dimensions of the test specimens
  • Irregularities in the test specimens
  • All manner of variations in setup
  • Operator dependent error
  • Etc.

If you're really interested in understanding what you're measuring, my recommendation is to take the time to do a real Measurement System Analysis (MSA) to understand the limitations of the measurements you're taking and what the sources of your error really are.

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The reason is because of errors. The first one is that the specimen might not be completely homogeneous(there might more cement on one side than the other for example). The second reason is that, the loading might not be in the center of the specimen, this means that one side will have to carry more loading than the other side.

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