How do I know when I should get a replacement pH electrode? As pH (and ISE) electrodes age, the voltage responsiveness of the probe will decrease. An older sensor will have a smaller change in the output voltage for the same change in pH of the solution that the probe is placed in. It is still possible to calibrate to the full range of pH's that one could calibrate to when the probe was new, but the number of levels that can be reported between the calibration points gets smaller as the probe ages. If the probe becomes seriously depleted, then the small voltage changes that can occur from electrostatic charges in the area around the electrode can cause changes to the displayed pH reading.
Replacement pH electrodes can be purchased at the following link:
You can also try the procedures at the following TechNote to try to restore the electrode.