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pH sensor

jabezsdavidjabezsdavid Posts: 11
edited 2010-06-17 07:16 in Accessories
I am facing problem in pH sensor which was designed according to the circuit which is shown http://www.emesys.com/OL2ph.htm and the reading which i am getting is more than 14 about 15.45. And could you please help me what would have being wrong in the circuit. In the circuit I used .1uF for the capacitor in place of .1 and this only change i made....

Comments

  • FranklinFranklin Posts: 4,747
    edited 2010-06-15 17:56
    Could you post YOUR circuit and your code? It will help us find your problem. Without that all I can say is that you did something wrong.

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    - Stephen
  • John R.John R. Posts: 1,376
    edited 2010-06-15 19:11
    pH probes, and the associated circuits are VERY sensitive, and if not constructed carefully, prone to inaccuracies and/or fluctuation. This is especially true of the circuit from the probe, through and including the first Op Amp (the 6081).

    Is this circuit a "real" circuit, or something you put together on a breadboard?

    Have you been able to trim the circuit to 1.0 on the output with the input shorted?

    If you're actually trying to take a measurement, what is the pH of the solution you're measuring? (obviously less than 14)

    What are you using to measure the output?

    Have you verified that you have good + and - 5 Volts from the voltage regulators?

    Have you verified the circuit you actually constructed against the schematic?

    This circuit does work, I've personally buit it, along with some other variations.

    As Franklin mentioned, posting your circuit and code (if applicable) would be helpful. You should be able to test this circuit without code before you try to connect it to a microprocessor.

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    John R.
    Click here to see my Nomad Build Log
  • dossicdossic Posts: 38
    edited 2010-06-17 07:14
    Hi!

    I agree about the problem of measuring pH. pH sensors (glass electrodes) have a very high impedance, sometimes higher than 500 MOhm, and need a first Opamp stage with very high input resistance and very low bias current. Clearly, this circuit is highly prone to collect noise and special precautions need to be undertaken.
    Secondly, I do not know why you are measuring pH around 14. a 1M NaOH solution is extremely aggressive and it may readily deatroy the glass bulb. You should start using a buffer solution at 7 and then t pH11 or 4, to measure slope og mV/pH line.

    Let me know for further help

    Carlo
  • dossicdossic Posts: 38
    edited 2010-06-17 07:16
    oops.... sorry for my bad "italianized" English!

    Carlo
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