Different values from pH probes

[mistral]

I have 2 pH probes, one connected to the expansion box (pH2), and the other one connected to an extension slot in the expansion box (pH3).
I have calibrated both probes in the same solutions (7 and 9), but when I put them in my sump there is a difference of about 0.35, one measuring 7.98 (pH3) and the other 8.34 (pH2). I have repeated the calibration several times, without any change.

How can I solve this issue?

Thanks in advance!

[Antipodes]

What do they read in the calibration fluid?

[mistral]

They read the correct values in the calibration fluid. When the calibration ends, I check always the value measured in the last calibration fluid (9) and the value is correct!

[Matthias]

With the time pH-probes lose their linearity of the output signal.
If this deviation is not acceptable then probes need to be replaced.

[mistral]

Thanks Matthias.
But, after what time the probes are no longer reliable, and the measured values are not accurate?

[Antipodes]

Could be anywhere from six months to 12 months.
My last probe only lasted 6 months!
Was dry when I unpacked it from box and was never really any good. (GHL PH probe)

[mistral]

And how about the other probes?

[Antipodes]

My temp probe, redox probe and conductivity are all 4 years old.
In theory the redox probe should have been replaced, as they use a similar membrane as the PH probe.
But if the cable doesn't get damaged on the temp and conductivity, they should last a decade or 2.

[Matthias]

https://www.aquariumcomputer.com/en-...ts-shop/probes

all mentioned in section "waranty"