Measure the right side of treatment
An upstream reading helps characterize what enters the treatment stage. It cannot establish what leaves that stage. Identify an accessible downstream point that represents the RO feed and allows paired reference sampling. Account for the physical distance and delay between that point and the membrane system.
Specify low-level performance
Ask for measurement uncertainty, detection capability and response behavior near the intended action threshold. A broad measuring range or a small displayed increment is not sufficient evidence. Confirm the method in the actual feed water and include the residual species that matters to the application.
Agree on the breakthrough response
Define who investigates a rising reading, what reference confirmation is needed and how the plant operates when a measurement is invalid. If automatic diversion or shutdown is proposed, commission the complete controls path. A monitor by itself does not guarantee protection of an RO membrane.
