A pH probe interacts with the liquid while doing the pH measurement, it means that over time the probe will wear out and has to be replaced. Indication is that the sensor is getting old is when it takes more then 5 seconds to have a stabilized reading in one of the calibration liquids. Lifetime of the probe is generally around a year, all depends on how many measurements are taken a day and how the sensor is maintained and stored. After use the sensor should always be stored in KCL liquid.