How to determine bridge voltage for strain measurements?

Discussion in 'General Electronics Chat' started by manuu84, Jun 19, 2009.

  1. manuu84

    Thread Starter New Member

    Jun 16, 2009
    5
    0
    Hi, I am wondering how to determine the input voltage for a wheatstone bridge? Is there standard rules for the voltage that is applied? I am assuming I'd like to keep the current low while trying to optimize my output resolution. However, I was hoping to learn about some sort of "gold standard"?

    These are the specs from the transducer:

    Operating Temperature Range 150 to 400C

    Storage Temperature Range 250 to +700C

    Sensitivity 5.0~VNImm Hg i 1%

    Nonlinearity and Hysteresis + 1.5% of reading or i 1 mm Hg, whichever is greater

    Excitation Impedance 350 ohms + 10% with typical Baxter Monitor Cable attached

    Signal Impedance 300 +5%

    Zero Offset <125 mm Hg

    Zero Thermal Drift ~<10.3 mm H~0C

    Output Drift 11 mm Hg per 8 hours after 20 second warm-up

    Sensitivity Thermal Drift < 1 0.1 %/~C

    Natural Frequency 40 Hz nominal for a standard kit (48712N); >200 Hz for transducer alone
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I am more than surprised that the spec's do not explicitly mention the max applied voltage. Most 350 ohm strain gauge bridges take 10 volts excitation, but this one could be an exception.

    Generally, the excitation voltage is applied all the time. For 350 ohm gauges, the sensitivity is about 3 mv/V. The difference signal is applied to an instrumentation amp to be amplified to a level consistent with the measuring gear.

    Sensitivity can be increased by applying more voltage. The bridge will burn up and die if the duty cycle is not limited, though.

    You want to learn more about strain gauge bridges, go to www.omega.com and hunt around. They know everything there is to know.
     
  3. awright

    Well-Known Member

    Jul 5, 2006
    84
    7
    beenthere said it all (mostly).

    As he said, a nominal voltage of ten volts is a good starting point for a 350 ohm strain gauge bridge. See if this gives you sensitivity, noise and drift that is satisfactory. There are complex tradeoffs between voltage applied to the bridge, sensitivity, thermal stability, and life expectancy. As you increase voltage you increase the equilibrium temperature of the SG elements, increase the time to reach thermal equilibrium after applying power to the transducer, and decrease the thermal stability of the transducer. If you go far enough, you shorten the life expectancy of the SG elements, possibly to the point of burnout.

    A SG bridge mounted on a thin pressure transducer diaphragm can safely dissipate much less power than a SG bridge on a solid aluminum load cell bar or steel aircraft weighing cell, so I'd be cautious about increasing voltage on your pressure transducer. If you are not getting the sensitivity you require, consider using a more sinsitive transducer, rather than increasing voltage across the bridge.

    I'd recommend just sticking with 10 volts as an upper limit.

    awright
     
  4. manuu84

    Thread Starter New Member

    Jun 16, 2009
    5
    0
    I should probably modify my initial thread in that it is actually a pressure transducer with semiconductor resistors. Does that affect the bridge voltage?

    Thanks
     
  5. gotumal

    Active Member

    Mar 24, 2008
    99
    0
    No, I don't think so. But they should have mentioned the max voltage (or power dissipation) to be applied and not mere a sensitivity. Could you give the link for the datasheet?
     
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