Proceq Profometer 4 Manual [best] -

A: The half-cell probe typically lasts 5-7 years. The manual includes a "Check" function. If the potential drifts more than 20 mV when measured against a known standard, replace it.

Accuracy depends on knowing the rebar diameter. If you don't know it, a common default is 16mm or #5. proceq profometer 4 manual