[intermediate_noob]

This has been on my "Hey, this would be cool to do..." list for a very long time so I thought I would share some links:
http://www.ph-meter.info/pH-meter-construction
http://damien.douxchamps.net/elec/ph_meter/

The two above came from this site...do not know how up to date it is though:

http://www.home.zonnet.nl/rsetteur/aquarium/karel/ph/index_ph.htm

And personally, the one I would build would be this one:

http://www.66pacific.com/ph/ph_1.htm

I understand that you want this to be for a computer, and that last one would basically output to voltage that you would read as pH. If you could have that, then just basically write a program to start logging it.

Hope this helps!

intermediate_noob

 

[intermediate_noob]

pH probes are very temperature dependent so you will need to also have a temperature input that is accurate to .1 F. Then your software will have to calibrate it's readings. This is all pretty doable the only problem I forsee is the very short life expectancy of common pH probes. There are some industrial processes that require constant pH monitering and use more robust probes but they ar very expensive. On cheap probes I wear them out in a year of using them for 10 minutes a day and storing them in storage solution and needs calibration daily. My good meter has had the same probe for 6 years and is still in calibration from 2002.

It has been a long time since I was taught how a pH probe works. But if I have any old grey matter left, I think the probe is a current source and not a voltage source.

I have never heard of pH probes being temperature dependent. If that were the case, wouldn't the commercial probes (I own two Milwaukee's and Pinpoint) would also have temperature settings. If you head over to the last link in my previous post there is an entire section on how the pH probe works and the premise behind the monitor/controller.

 

[intermediate_noob]

Here is that pH Electrode article that I was talking about:

http://www.omega.com/techref/ph-3.html