On average we state that a pH electrode will last 1-2 years. It is very possible to have a probe work longer. As long a pH electrode has a good offset (mV value in pH 7.01) and slope (mV difference from pH 4- pH 7) then the probe can be used.
A good offset value is +/- 30 mV while slope should be greater than 160 mV (90%).
Since the meter is calibrated manually and it does not have a GLP feature it can be difficult to determine the pH electrode characteristic. I would recommend the following:
1) Short the BNC connector on the meter with a paperclip. That is put one end in the center hole and touch the outside of the connector. This is 0 mV.
2) Dial pH 7.01 calibration trimmer until pH 7.01 is displayed
3) Reconnect the pH electrode to the meter and place in pH 7.01 fresh buffer
4) Record the pH value in the buffer.
It is now possible to determine the offset of the pH electrode. Ideally the meter would display pH 7.01 since the theoretical mV at pH 7.01 is 0 mV at 25 oC. An acceptable offset of +/-30 mV would be displayed as a pH from 6.5 to 7.5. Meters will typically allow a calibration of +/- 60 mV which would show as a pH reading around 6 (+60 mV) and 8 (-60 mV).
If you see reading outside pH 6.5 to 7.5 then the electrode needs to be cleaned. Any coating on the glass will shift the voltage read by the probe.
Instability in readings that occurs with aging are due to various factors including:
1) Dirty electrode – as there is a build up on the glass the measurement circuit is impeded.
2) Clogged junction – on the side of the probe should be a white plug. That is the junction. When it gets clogged the measurement circuit gets impeded.
3) Storage - It is very important to store the pH electrode in storage solution. The storage solution maintains a hydrated layer on the glass bulb, hydration of the junction, and hydration of the gel used inside the probe.
Below is a link to some basic maintenance information:
http://hannainst.com/ph-electrode-maintenance-calibration-guideProducts for Beer Analysis.
http://hannainst.com/beer-phRecommended cleaning solutions
http://hannainst.com/industries/beer/ph/maintenance-solutions/cleaning.html If you have any questions then feel free to message me.