Equipment Calibration Lapse on a Coating Line
A fabricator operates an in-house blast and coating line for a project requiring an inorganic zinc primer at a specified dry film thickness of 3-5 mils. The shop has two coating thickness gauges. The calibration log shows gauge A was calibrated 14 months ago; the manufacturer recommends annual calibration. Gauge B was calibrated 2 months ago but its calibration certificate lists a different probe than the one currently attached. The coating crew has been using gauge A for the last 6 weeks on this project, recording readings of 3.5-4.2 mils. The QC manager discovers the lapse during a routine audit and asks whether the 6 weeks of readings are valid. The project is 60% through coating. The owner's inspector is scheduled for a site visit in 3 days.
1. Evaluate whether the 6 weeks of coating thickness readings are valid and what the QC manager should do before the owner's inspector arrives.
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The readings are not reliable as recorded. Gauge A is 2 months past its recommended annual calibration, so its accuracy is unverified for the entire 6-week period. Gauge B's certificate does not match the attached probe, so it cannot be used as a valid cross-check without resolving the probe mismatch.
The QC manager should:
- Stop relying on gauge A for acceptance decisions until recalibrated.
- Send gauge A for calibration immediately and resolve gauge B's probe/certificate mismatch.
- Re-measure a sample of the coated steel with a verified gauge to determine whether the 3.5-4.2 mil readings hold. If they do, the coating is likely acceptable and the records can be corrected with a note explaining the calibration issue. If they do not, the affected steel needs evaluation and possible rework.
- Document the finding and corrective action before the owner's inspector arrives.
Hiding the lapse or presenting the 6 weeks of readings as valid would be a misrepresentation. Disclosing it with a corrective action plan is the defensible path.
2. The production manager says re-measuring 60% of the project is too expensive and proposes to 'just recalibrate and move on.' How should the QC manager weigh cost against the equipment-control requirement?
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Recalibrating and moving on fixes future readings but leaves the past 6 weeks unverified. The equipment-control requirement exists precisely so that measurements used for acceptance are traceable to a known-accurate instrument. If the coating is later found thin in service, the fabricator has no defensible record.
The cost of re-measurement is bounded: a sampling plan (statistically valid sample of the coated members, not 100%) can verify the population at far less cost than full re-measurement. If the sample confirms the readings, the fabricator corrects the records and continues. If the sample shows thin coating, the fabricator reworks only the affected areas. Either way, the cost is far below the cost of a failed coating discovered after the owner's inspector or in service. The QC manager should propose the sampling plan as the middle path and document the decision.