# DIY Electronic pH Indicator



## hoppycalif (Apr 7, 2005)

Has anyone here tried making a DIY Electronic pH meter? Like this, for example: http://www.66pacific.com/ph/simplest_ph.aspx. How did it work? What are the disadvantages of using this?


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## BigBee (Jul 13, 2006)

Hi hoppycalif, one thing I don't understand is why can't we measure the probe directly with the digital multimeter? The probe gives 59mV/pH and his montage 54.55 mV/pH, that means a little lower resolution. An other thing is, that we still need to have a pH probe for this.


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## hoppycalif (Apr 7, 2005)

The pH probe doesn't produce any voltage itself, as I understand it. Instead it modifies the voltage applied to it. It is that change in voltage that the meter reads. (I could easily be wrong, but that's what it looks like.) Also, I just realized that you need a temperature reference for this to work accurately. There is probably an easy way to do that, but I don't know what it is.


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## BigBee (Jul 13, 2006)

I think you're wrong. The pH probe does produce a voltage, the article mentions +/- 59 mV/pH referenced to pH=7. The circuit is an amplifier with an amplification factor of almost 1 (unity gain), thus merely repeating the voltage. It's true that it has a high input impedance which you definitely must have when measuring voltage, but that's already implemented in the digital multimeter (measuring voltage).


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## hoppycalif (Apr 7, 2005)

You obviously understand this stuff far better than I do. I still wonder if any of us has actually tried this and how well it worked.


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## g8wayg8r (Feb 11, 2008)

He is correct. The probe does produce a voltage as the referenced article indicates. It's just that the current is so small that you need a good op amp. The basis of the circuit is to wire in a gain of about 17 so one unit of ph gives you a one volt change. Then, its a matter of comparing that to a standard (your set point) to turn the relay on and off. 

I built the circuit he referenced in his article and it works. He has done a nice job modifying it so you only need a standard AC adapter; no positive and negative voltages like from a modified computer power supply or center tap transformer with a rectifier bridge. It gets buy using seven volts as a reference ground, not an earth ground. I'm not sure about the safety considerations there so I'll just leave it at that. 

I'm currently (no pun intended) trying to wire his circuit but I can't get it to work. I suspect I'm having trouble with the reference ground but it may be the chip that I have. It's generic and I think I've identified the pens properly since I'm running seven volts from one of the four amps. So, it could be that I can't get it wired properly on the breadboard. Who knows. I'll let you know how it turns out eventually.


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## BigBee (Jul 13, 2006)

Actually you need the 17 times amplification (plus an offset of 7V) only if you want to read the pH directly (so the multimeter shows the same value as the pH), otherwise you have to make that graph and look up the pH value after you measure.
If you want to build a controller you will also have to amplify it to the range of the voltages you have in the rest of the controlling circuit.


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