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pH Meter — Parallax Forums

pH Meter

mechstampmechstamp Posts: 5
edited 2009-10-28 18:06 in Learn with BlocklyProp
Good Afternoon,
I am fairly new to the world of circuits and micro controllers and am looking for a little help if possible. I am trying to design a pH meter using the basic stamp. Right now I have my BOE as sort of a digital voltmeter using a POT and AD converter. I would like my input to be the pH of a solution and be able to continuously monitor it. Does anyone have any advice for me? Thanks so much

Comments

  • FranklinFranklin Posts: 4,747
    edited 2009-10-21 21:50
    You need to get a pH sensor and depending on which one you get you need to design a circuit to read it. You need to start with the sensor.

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    - Stephen
  • mechstampmechstamp Posts: 5
    edited 2009-10-21 21:52
    do you have any suggestions on which pH sensor i should use?
  • FranklinFranklin Posts: 4,747
    edited 2009-10-22 02:14
    Always wanted one but the probes are too expensive and require frequent recalibration so I never got very far into it.

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    - Stephen
  • John R.John R. Posts: 1,376
    edited 2009-10-24 20:24
    There are really only two types to choose from. The "cheap" way is to use any of the pH probes with the BNC connectors on. This are "unconditioned" probes, and making a long story short, put out a VERY low current voltage based on the pH being measured. They range in price from about $30 US and go up to several hundred, depending on construction, accuracy, etc. Just about all of these can use the same circuit to condition and read the signal. See a very nice write up by Dr. Tracey Allen here: www.emesystems.com/OL2ph.htm An internet search will turn up other options. There are now single chip amplifiers that can work. I can't find the spec sheet right now, I'll try and add it latter, but Analog Devices makes one. [noparse][[/noparse]edit - found it, see link below]

    Here's a couple other links that may be helpful:

    Depending on how accurate you want/need to get, the required circuit is not all that terrible, but it does take some finesse to minimize stray signals and interference. The signal coming from the probe is very weak, and it doesn't take much to make things fluctuate.

    Again, depending on how well you care for the probe, and how accurate you need to be, recalibration is not all that big a deal. The "secret" is to make the electronics so that you can get things stable and "close" (as close as you can to optimal). Then use software for any "tweaking".

    The other factor is temperature. There are a couple ways to do this, depending on what you're measuring, and again, how accurate you need to be. You can either make the compensation in software, or there are special resistors made specifically for this, and if you can get the resistor the same temperature as the liquid being measured, you're "good to go".

    The other choice is to find a pH probe/system with a "conditioner" or "amplifier" built in. This takes away the worry of the above circuit, but can add significantly to the cost. Don't expect to find one for less than $100.

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    John R.
    Click here to see my Nomad Build Log

    Post Edited (John R.) : 10/24/2009 8:38:00 PM GMT
  • Greg LaPollaGreg LaPolla Posts: 320
    edited 2009-10-25 04:53
    I just recently built a ph sensor circuit for the propeller. here is a good place to start. That circuit is vary simple. You can get everything from digikey. Replace the CA3160 with a LF356N. the CA3160 is no longer in production. The bulk of the code is there there already and is pretty easy to follow once you grasp it.

    You will also need and analog to digital converter. Again look here for info on the TLC2543.

    The above PH circuit should work with most standard off the shelf PH probes.

    Greg
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2009-10-27 15:50
    I'd been meaning to do this for some time, so I went and edited that page, here, to take out the obsolete CA3160. In its place, I suggested the LMC6082, which is a mosFET input op amp with an input bias current of around 10 femtoAmps. Alternatively, the CA3140 is still available (Ib = 2 pA), or there is the readily available LF356 (Ib = 30 pA) that Greg suggested, and most any FET or mosFET input op amp will work.

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    Tracy Allen
    www.emesystems.com
  • mechstampmechstamp Posts: 5
    edited 2009-10-28 18:06
    Thank you all very much for your help. These resources are excellent.
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