The Calibration That Wasn't
You are the only technologist on shift at a water treatment pilot plant. A turbidimeter used to verify filter effluent against a 0.3 NTU permit limit has a calibration sticker dated eight months ago; the manufacturer specifies quarterly calibration with a primary standard. The plant's spare instrument is out for repair until next week. Current readings are 0.18 to 0.22 NTU, comfortably below the limit, and the operator tells you the instrument 'has been reading fine for months' and asks you to keep logging results so the pilot data set stays continuous for the regulator. You have primary standards in the lab refrigerator, expired two months ago.
1. Can you continue logging the readings as permit-quality data? What must you do first?
Read the worked answer
No. The instrument is eight months past a quarterly calibration requirement, and the readings are therefore indicative, not verified. 'Reading fine' is not evidence of accuracy - a drifting turbidimeter can read plausibly while biased in either direction, which is exactly why the calibration interval exists. Before any reading is treated as compliance data, you must calibrate with a valid primary standard. The standards on hand are expired, and expired standards cannot establish traceability, so the first action is to obtain in-date primary standards. Until then, you may record readings clearly labeled as uncalibrated/indicative, but they must not enter the compliance data set. Logging them as permit-quality data would misrepresent their status to the regulator.
2. How do you manage the data gap and the regulator's need for a continuous data set?
Read the worked answer
Distinguish continuity of the record from validity of the numbers. Maintain the record continuously, but annotate the affected period as collected with an out-of-calibration instrument and therefore not usable for compliance demonstration. Pursue in-date standards urgently; once obtained, calibrate, verify with a check standard, and resume valid logging. If the pilot's scientific value depends on the gap, note that the gap is a known limitation, not a hidden one. Notify the project lead and, if the permit requires reporting of monitoring downtime, follow that notification requirement rather than waiting to be asked. A documented, honestly labeled gap is defensible; a continuous series of unverified numbers presented as compliance data is not, and it is the kind of record that unravels under audit.