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Hi all

I got an apex controller and have two lab grade ph probes attached to it

I have calibrated them both multiple times.

After calibration the both read within +/- 0.02 of 7.01 in the calibration fluid.

Once I put them in the tank they are 0.13 apart.

The one on the main port reads 7.13 and the second one is reading 7.26

Over the past few days they have moved in lock step, but always right around 0.13 apart.

Any ideas as to what is causing the different readings and anything I can do to get them to be closer?

Thanks all
 

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If you swap ports, do the readings change? That would at least tell you if it is the probe or the controller that is drifting.

Not sure about Apex, but I have a Milwaukee that has both a high and low point calibration (7.0 and 4.0 solutions).

Another random thought: how far apart are the probes? Could be possible that one or the other is picking up some electrical interference somewhere? Just a wild guess...
 

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You remembered to rinse the probe with RODI water then blow residue off and wipe dry (q-tip glass electrode) before calibrating to your first and second solution, right? Are your probes made by the same company? Are you using fresh, unused buffer solution every time (and not "recycling it")? If so then you will be using four total quantities of solution. Are you putting both probe in the same solution at once or calibrating each one completely separately? They could be interfering with each other if placed in the same solution at the same time. Is your tank grounded?

Have you tried putting them right next to each other to see if they both read the same? Did you equalize the temp of your calibration solutions to the temp of your tank water? Were they properly stored before you used them?

You could average them and go with 7.195 in case nothing else solves your problem. You could always return both for two new ones on the grounds that since you have no idea which is correct, that you can't trust either one.


What is the second probe even for? Why would you use two if you expect them to read the same pH at all times anyway?
 

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Any ideas as to what is causing the different readings and anything I can do to get them to be closer?
I believe that pH and other probes have a limited lifespan. If your probes have not been used for the same length of time, and have been in different solutions, they may have aged differently, leading to the different readings you are now getting.
 
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