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ph meter

Discussion in 'Electronic Basics' started by delounge, Jul 21, 2003.

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  1. delounge

    delounge Guest

    i need to build a simple ph meter i wonder if someone can help me?!?!?!

    i have found this

    http://www.cyberreefguru.com/electronics/phmeter1.gif

    it is not completely clear to me.

    on the left there is the probe rite? why does it look like a resistor? is
    actually a resitor or what do i have to use?
    second question. how do i mesure the ph? multimeter is that correct? the so
    which are my terminal for measuring?

    thank you for your kind help!
     
  2. Baphomet

    Baphomet Guest

    Measurement would be from the output of the opamp to gnd. I guess a multimeter could be used. You would need to use two samples of known ph like an acid and a base, and hope to hell the circuit is linear.
     
  3. This schematic is very misleading. A pH probe is a two terminal
    device, much as a volt meter has to have two leads. One lead is
    essentially a contact to the liquid, to act as a reference point,
    while the second lead is a half cell that generates voltage when it
    comes in contact with the liquid. The voltage it produces gets more
    positive as the hydrogen ion concentration of the liquid increases.
    The pH meter is a high impedance volt meter that amplifies and
    displays the magnitude of this voltage. See:

    http://www.omega.com/techref/ph.html

    So the circle with a resistance through it, labeled pH probe is
    actually a pair of probes, one that is the reference probe and one
    that is the hydrogen ion half cell. It is shown as a resistance,
    probably because the cell formed by these two probes and the liquid
    between them has a very high internal resistance, on the order of 100
    meg ohms.

    I think with a bit of searching, you can find a better design of a ph
    meter than this.
     
  4. Gary Lecomte

    Gary Lecomte Guest


    Just Buy one, Cheaper in the long run. Additionally, be prepared to
    buy a new one every few years. These PH cells don't last more than a
    few years.

    Take care.....Gary
     
  5. Bob Masta

    Bob Masta Guest

    The high-impedance meter may be the easy part.
    The pH electrode is made from special permeable
    glass, and is something you would be best buying
    anyway. They tend to be very fragile and need special
    care in handling and storage. Last time I checked
    I think you could get one for around US$50, but that
    was a "few" years ago.




    Bob Masta
    tech(AT)daqarta(DOT)com

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Shareware from Interstellar Research
    www.daqarta.com
     
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