Extended pH/ORP Sensor Life
Desalination is the process of removing dissolved mineral salts from seawater and brackish water for human consumption. Most major markets now employ reverse osmosis (RO) membrane separation in place of energy intensive evaporation/distillation processes as up to 99% of dissolved solids in raw seawater can be separated out by RO membranes. Most RO membranes in pH sensors today are made of polyamides and can tolerate any pH level between 2 and 10. However, pH sensors can be damaged by free chlorine injected upstream in the seawater stream as a biocide. For this reason, sodium sulfite is introduced right before the RO membranes to remove any free chlorine. Residual free chlorine (RFC) is an oxidizer and will register an oxidation reduction potential (ORP) of approximately 500-700 mV. RFC elimination can be controlled by a sodium sulfite pump until the ORP value reaches 50-350 mV. Furthermore, pH can be monitored as well. The Rosemount 389 pH/ORP Sensor features a triple reference junction and is ideal for delaying reference electrode poisoning from chloride ions in seawater, resulting in longer pH sensor life.
Desalination & pH/ORP Measurement with Rosemount pH/ORP Sensors & Transmitters
Which pH sensor is applicable?
The reference electrode in a pH sensor provides a stable voltage potential independent of the process. The reference electrode degrades over time, requiring eventual replacement. The Rosemount 389 pH/ORP Sensor contains three reference junctions for delay of reference poisoning by chloride ions in seawater, while the Rosemount RBI pH Sensor contains five reference junctions to further slow degradation.
More than one type of pH sensor can be selected for a particular process. While many pH sensors are discarded after use, some pH sensors can be rebuilt and reused. The Rosemount 3500P pH/ORP Sensor contains just two reference junctions, but the reference junction and reference electrolyte are rebuildable.
This video outlines the maintenance process for deciding between a replaceable or rebuildable sensor.
