Two pH sensors are not reading the same value when tested in the same solution after calibration

Offset between pH sensors after calibration

Even following the calibration of a pH sensors, it is possible that there will not be exact agreement between two pH sensors.

The pH sensor works by reading the voltage that is generated by the pH probe that is connected to it. The amplifier box that is the heart of the sensor, reads this voltage down to a 1 mV precision. When the sensor is calibrated, the voltage reading is assigned a pH value that is input within the calibration process. A new probe will tend to have a larger difference in the voltage generated for different pH solutions that the electrode is placed in. For the probes that are able to generate a larger voltage difference for the same difference in pH, these probes are able to discern smaller steps in pH.

Because of these different minimum step sizes in the reported pH, two sensors with two different probes may not agree exactly (down within the two to three decimal place reading). The video provides further explanation of the responsiveness of the pH electrodes.

If you are concerned about the readings from a particular pH amplifier box, you can test two amplifier boxes with the same pH electrode within the same solution. A given pH probe within the same solution should provide the same mV reading regardless of the pH amplifier box that is attached to that electrode.