Why to calibrate strain gauges

Discussion in 'General Electronics Chat' started by tooma, Mar 5, 2017.

  1. tooma

    Thread Starter New Member

    Mar 5, 2017
    6
    0
    Hi,

    I need to measure the strain in concrete beams. For this purpose I have purchased data acquisition system with a complementary 1/4 whetstone bridge circuit. The complementary circuit consists of 3 resistors of 120 ohm each. My strain gauge is 120 ohm and the gauge factor (k) is 2.11. The data acquisition system is modern and digital. Now, after connecting the strain gauge to the complementary circuit and exerts excitation (Ue) of 2.0 V (output) to the whetstone circuit and then loading the beam so that strain happens and the data acquisition system reads the voltage from the whetstone bridge (Ua)(input) that varies due to the strain in the strain gauge, my question is: Is it possible to use the formula:
    strain=4 Ua/(k . Ue)
    Why should I do calibration? and why the strain is not programmed to be calculated automatically to the above formula?
    The data acquisition system requests the k, Ue and the whetstone bridge type (1/4, 1/2, or full) so that it has all the necessary information for calculating the strain but will not work without calibration data!
     
  2. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    6,644
    2,928
    You are going to ask a bunch of anonymous people on an electronics forum how to calibrate an unknown design of a concrete beam? And if your calibration is wrong could end up killing people? Really?
     
  3. tooma

    Thread Starter New Member

    Mar 5, 2017
    6
    0
    My question is not about calibration of concrete beam (to be more accurate, calibration of concrete beam design does not exist). I am asking about the calibration of strain gauges in general, and to follow the forum rules, I was specific in telling the details and was descriptive.
     
  4. kubeek

    Expert

    Sep 20, 2005
    5,266
    986
    The thing is that the strain gauge is not exactly 120.00000 ohms, and the other three resistors in the bridge are not as well. That is why you need to measure two points to get a correct number, preferably one at zero load and another at a known load, then you can be sure how what the aquisition system measures corresponds reality.
     
    tooma likes this.
  5. tooma

    Thread Starter New Member

    Mar 5, 2017
    6
    0
    Theoretically, the strain calculations does not need the values of whetstone bridge circuit resistors:
    strain=4 Ua/(k . Ue)
     
  6. kubeek

    Expert

    Sep 20, 2005
    5,266
    986
    Yes, but that equation (if I understand correctly what it means) only works if the four resistors are exactly the same. Since they will allways be off to some degree, you should calibrate that error out.
     
    tooma likes this.
  7. tooma

    Thread Starter New Member

    Mar 5, 2017
    6
    0
    If the four resistors are not exactly the same, the initial bridge voltage (Ua initial) will not be zero. It will be easier to subtract this initial value from all readings to overcome this discrepancy than making a calibration process.
     
  8. kubeek

    Expert

    Sep 20, 2005
    5,266
    986
    Yes, that is the calibration of the zero point. But the gain of the bridge will also be different and that will make every measurement multiplied by some coefficient close to but not equal to one. And that will make the strain gauge inaccurate, so say the voltage corresponds to 50kN but the actual measured value was 55kN. That is why you calibrate out the zero AND one known point.
     
    tooma likes this.
  9. tooma

    Thread Starter New Member

    Mar 5, 2017
    6
    0
    In Wheatstone bridge, if the bridge is not balanced, this is caused by the difference in the voltages from the electrical resistances on R1, R2 and R3, R4. This calculated as:
    Va=Ve (R1/(R1+R2)-R4/(R3+R4))
    i.e, the factor can easily be verified if both Va (excitation) and Ve (measured by the DAQ)
     
  10. EM Fields

    Active Member

    Jun 8, 2016
    583
    154
    A strain gauge is merely a device with a resistance that changes when it's stretched, and its zero strain resistance will change depending on how it's glued to the concrete beam, the ambient temperature (especially after it's been glued down and the beam expands and shrinks because of temperature), and the phase of the moon.

    Consequently, you should perform a zero load measurement with the beam unloaded and, ideally, another measurement with the beam under a known stress, preferably under full load - or beyond - in order to keep from having to extrapolate into the unknown.

    And why should it?

    If you started out on a motor trip and you knew the fuel economy of your vehicle, how could you possibly determine your fuel budget if you didn't know (under perfect conditions) the distance to your destination?
     
    tooma likes this.
  11. #12

    Expert

    Nov 30, 2010
    18,078
    9,617
    Strain gauges (and most other analog sensors) have variations between them on the day they are born, and they tend to change a bit across time. It is standard practice to provide adjustments for zero and amplification (span) over the range of interest. You can pretend that having a formula makes all strain gauges exactly the same for both zero and load under all conditions, but you do it at your own peril.
     
    tooma and kubeek like this.
  12. EM Fields

    Active Member

    Jun 8, 2016
    583
    154
    Or that of other trusting souls.
     
    tooma and #12 like this.
  13. tooma

    Thread Starter New Member

    Mar 5, 2017
    6
    0
    Now it is clear. Thank you all for answering my question.
     
Loading...