Hanna Instruments portable meter for pH and the raw millivolt signal behind it, one model.
HI8424
This Hanna Instruments portable meter measures pH and the raw millivolt signal behind it.
Reading millivolts directly is how you find out whether an electrode is still good: its slope and offset are visible in the raw signal long before the pH readings look obviously wrong. It is also the mode used for ORP and for ISE work.
An electrochemical meter is three things that fail separately: the electrode, which ages and eventually dies; the meter, which rarely goes wrong; and the calibration, which is only as recent as the last time somebody did it. Almost every bad reading traces to the first or the third.
Budget for electrodes as consumables, store them wet in storage solution rather than water, and calibrate against fresh buffers at the temperature you will measure at.
A healthy pH electrode reads close to 0 mV in pH 7 buffer; a large offset there means replacement, not more calibration.
Supplied as one model.