Map the stage boundaries
Name the upstream and downstream take-offs, monitor IDs and treatment equipment between them. Record recirculation, bypasses and streams joining between the points. A change across a boundary with an added stream cannot be attributed to removal alone.
Pair the water, not just the clock
Document the intended pairing window, sample-line delays and estimated travel through the stage. During a change, two readings with the same timestamp may describe different water. State any uncertainty in the pairing rather than hiding it in an average.
Retain the measurement basis
For each sample, retain the method, target-oil calibration, optical channel where relevant, preparation, reporting units and sample ID. Keep the water-matrix and oil description alongside the results. Mark a change of method or calibration as a new comparison basis.
Record the operating window
Link the pair to throughput, stage availability and relevant operating changes recorded by the site. Retain the original readings and the comparison calculation. Describe a difference as an observation until the process and measurement evidence supports the interpretation.