YSI ProDSS [626964] pH/ORP Sensor Replacement Module. The user-replaceable ProDSS pH/ORP Module is used with the 626904 ProDSS pH/ORP Sensor. ProDSS pH and pH/ORP feature replaceable sensor modules due to the electrolyte depleting characteristics necessary to make such measurements. YSI recommends that users replace these modules as necessary, typically one to two years if stored properly in the provided storage solution when not in use. Working life will depend on the conditions of the deployment environment.