The electrode used for the pH measures is the SBE 18 built by Sea-Bird Electronics. It is a glass electrode Ag AgCl.
The pH measurement is delicate and obtaining correct and comparable values is a tedious work. The worst uncertainty comes from the stability of the measures because the sensor needs to be calibrated quite often. As a matter of fact, we note a drift that can be corrected using a regular calibration. To recalibrate the sensor, see note n° 18-1 from Sea-Bird .