Field grade reference electrodes contain a saturated salt solution in a gypsum-bentonite gel. CuSO4 is the salt in copper/copper sulphate electrodes; KCl is the salt in silver/silver chloride electrodes. The accuracy of a reference electrode depends upon this salt solution remaining saturated. During use, salt will diffuse out from the reference electrode which can affect the concentration in the gel. The design life of a reference electrode is an estimate of the time based on testing it would take for enough salt to diffuse out from the inner core to lower the salt concentration to below saturation. At EDI, we use several techniques to extend this time as much as possible. One of these techniques is to increase the amount of salt reserve contained in the gel. This is one reason why longer life electrodes have physically bigger housings. Download our paper Factors Affecting the Accuracy of Reference Electrodes from the Technical section of our website to learn more.