DIY pH controller?

I'm not sure, but I would suspect that a common probe reacts to the resistance change relative to ph in water. This would assume a constant voltage and the control circuit sensitive to changes in amperage. I'll buy the beer and supply the bench meter if you're willing to sacrifice a couple of probes.

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.

Edit: This link shows it as a voltage source:

http://www.ph-meter.info/pH-meter-construction

colinsk said:

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.

Click to expand...

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.

I my probes there is a temperature probe (metal) and a pH probe (glass).

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

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

It sounds like a perfect job for a pic or a stamp. If fact would love to have multiple inputs to measure temp(perhaps even control) , light( measure and control), and other parameters for more than one aquarium and report back to a computer via wireless, rs-485 or other methods. I thought about this as I have been starting back up. I used to use X11 technology to control my lights.

colinsk said:

pH probes are very temperature dependent so you will need to also have a temperature input that is accurate to .1 F.

Click to expand...

It would depend on the accuracy you'd like to achieve. There is a table on my Milwaukee 7.01 calibration solution which shows dependency in question; within the range from freezing to boiling water the numbers are 6.98 to 7.14, which translates to +-2ppm CO2 error using popular KH/PH formula for KH4. Again, this error is for 0-95degC range; if we look at the same table in normal aquarium 20-40degC range, the difference in pH would be 6.98-7.04 which brings CO2 error to sub-ppm making it insignificant.

I found this link a very interesting read on the basics.
http://www.thekrib.com/Plants/CO2/hurley-co2.html

Give him a buzz and see if you can get his advise on the controller part.

Good Idea