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Strain gage question

Discussion in 'Electronic Basics' started by Maxwell, Jan 22, 2007.

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  1. Maxwell

    Maxwell Guest

    Can you get accurate, relative or repeatable readings, from a strain gage
    with a inexpensive digital volt meter?

    I would like to build an electronic scale using a strain gage, but I could
    care less about the units on the display. I just want to be able to
    accurately measure say, 400 pounds of product, again and again for a
    batching operation.

    Could I load the scale with a 400 pound weight, observe the reading on the
    volt meter, remove the 400 pound weight, and load the scale with product
    until the same meter reading is observed?

    Would battery life cause the need for frequent recalibration? If so, would a
    low buck 9v power supply eliminate this?
     
  2. Self contradicting sentence as you don't include the permissible errors
    of readings.
    Every part of the system has effect on readings so all of them have to
    be accurate and stable during the "life time" cycle.
    Or include a calibrating known deflection of reading (on strain gauge
    bridge a stable resistor in parralel with one leg of the bridge) to give
    you the starting point.

    Been there, done it!

    HTH

    Stanislaw
     
  3. Maxwell

    Maxwell Guest

    I should have mentioned that, my current method is probably no more than 5%
    accurate. So anything less than that would be a plus.
    Well I have a good bit, but the fine points are beyond me. I understand
    strain gages change very little in resistance throughout there usable range,
    and usually require bridges and very high quality power supplies for
    ultimate use, but then they yeild results as accurate as .03% in many cases
    on a 5000 lb load cell. That's why I'm posing the question here, and framing
    it as I have. One percent accuracy should be more than adaquate for my
    needs. So will most ohm meters costing less than $100 have the ability to
    measure say 600 steps in the useful range of a strain gage?
     
  4. Maxwell

    Maxwell Guest

    Permissiable error of one percent would be very nice. As for calabration, we
    can just load the scale with a know quantity most any time. That's what we
    do now. But now we are using a 1.125 dia, oil filled cylinder with a 1000
    lb. pressure gage. But I don't think we are getting closer than 5%, and it
    seems especially suseptable to temperature change.
     
  5. John Larkin

    John Larkin Guest

    A typical full-bridge load cell will give 5 millivolts of output per
    volt of excitation at full load. So 9 volts of excitation gives 45
    millivolts of signal. Some DVMs have enough sensitivity to be useful
    with such signals.

    The output will droop as the battery dies, so a regulated power supply
    would be a better idea.

    Buy a load cell that's rated at or just a bit over your expected load.

    You can buy a complete load cell excitation/display box fairly cheap;
    they're a commodity.

    You will generally have to adjust for zero offset ("tare") and
    full-scale (using a known test weight) calibration, as load cells
    aren't always very accurate. They usually are extremely linear and
    stable.

    John
     
  6. Now you _are_ talking sense!
    5% is NOT in my vocabulary as measuring range. In my lifetime in testing
    anything greater than 1% was a failure static and/or dynamic.
    So now the temperature creeped in, next will come something else, so do
    yourself a favor, do it once but do it as perfect as possible to
    eliminate insecurities, it will pay in the long run.
    Are you measuring now by reading pressure on circular gauge or having
    pressure transducer and getting the readings from that?
    And just a stupid querry, how time consuming is this activity, if it is
    daily routine for hours it costs in people's time plus errors in
    results. As you carry the calibrating quantity initially why not to use
    it on a lever as counterweight and watch for microswitch opening as
    indication of equality?

    Just talking

    Stanislaw
     
  7. Maxwell

    Maxwell Guest

    Isn't a load cell and excitation/display box about $700?
    If not, how cheap and where.
     
  8. Maxwell

    Maxwell Guest

    Measuring now by mechanical gage, but suspect 5% errro is primarily caused
    by friction in the cylinder.

    Measuring operation is done at least 30 times an 8 hour day, and increasing.

    Space constraints eliminate the possibility of a counter balance.
     
  9. John Fields

    John Fields Guest

    ---
    The viability of the whole thing depends on the granularity of the
    measurement and what kind of repeatability/stability you're looking
    for.

    I suggest you Google "Strain gage tutorial" to find out what you're
    up against.
     
  10. John Larkin

    John Larkin Guest

    Omega has indicator boxes for around $250, and I'd expect there is
    cheaper stuff around.

    If you want really cheap, try ebay.

    Have you heard of google?

    John
     
  11. Don't you _?know?_. So what improvement you want to apply?
    As mentioned before, do it once and do it properly. If the hydraulic
    system has some faults replace it with load cell, most reasonable
    quality load cells are self temperature compensated (one of your
    complains), many include precise calibrating resistor inside to give
    indication of part scale reading of the full range.
    So a three point mechanical suport with one point beying a load cell
    gives you a weghing possibility. One point attachment can be used by
    fully hanging the load on a load cell. Not electronics, mechanics;^)

    Have fun

    Stanislaw.
     
  12. When given the possible 5% error you can either gain or cheat one and a
    half units per day ( maximum taken as sample), so make some calculations
    how much it will cost you over the continuation of this activity,
    against some investment in reducing the errors.

    Have fun

    Stanislaw.
     
  13. Maxwell

    Maxwell Guest

    Thanks for you help.
     
  14. jasen

    jasen Guest

    If you'll be satisfied with 1% accuracy use a mechanical balance.

    Bye.
    Jasen
     
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