Need help designing pH meter

Discussion in 'The Projects Forum' started by l1nuxg33k, Sep 27, 2006.

  1. l1nuxg33k

    Thread Starter New Member

    Sep 27, 2006
    7
    0
    Hi, I have been designing a pH meter.
    I have the circuit for the meter part (a 10-step bargraph) LM3914
    http://www.national.com/pf/LM/LM3914.html
    But the electrodes output 0-300mV based on the pH.
    The IC above has a 1.2V minimum full scale voltage.

    Can someone please recommend an OP-Amp that runs on a 3V or 12V battery, not requiring any negative voltages, that can
    input=0 - 300mV
    output=0 - >1.2V (I'm not sure about the max. voltage from
    http://www.national.com/pf/LM/LM3914.html)

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    You're in for an uphill battle. A calomel electrode outputs something like 59.6 mV/pH - but it's negative and positive on either side of 7. A pH of 14 is over 400 mV - 414 comes to mind. Acidic pH's are negative, with 0 being -414 mV.

    As long as you only want to measure alkalinity and not too far past pH 10, you're ok. Any op amp can run with the negative power pin grounded. You will need a large input resistance to keep from loading the electrode. If you think 10^13 is enough, try an OPA134PA. They're about $2.75 from Digi-Key.
     
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