The SeaFET V2 and Shallow SeapHOx V2’s external reference electrode is directly exposed to the surrounding environment. This Ag/AgCl electrode has a high sensitivity to chloride ions, which vary in seawater at different salinities. To get an accurate measurement from the external reference electrode, the concentration of chloride ions must be known—this can be accomplished when accurate salinity data is available.
Housed within the SeaFET V2’s DuraFET, the internal reference electrode is immersed in a bath of saturated KCl gel that is physically separated from the environment. The KCl gel exposes the Ag/AgCl internal electrode to a relatively constant chloride concentration, allowing the sensor to measure pH regardless of environmental salinity.
Note: For a better understanding of how the reference electrodes contribute to pH measurements, watch our video “Explaining the ISFET” for a high-level description of the ISFET circuit.
To simplify, if accurate salinity data is available (typically if you are using a SeapHOx V2), the external pH value is generally more accurate. If salinity is unknown, (typically if you are using a standalone SeaFET V2), the internal pH value is a better choice. Ultimately both pH values should be within 0.05 pH of each other after post-processing.