How should a plant use as found calibration results to detect instrument deterioration?
As found readings show the instrument condition before adjustment. Trending them reveals drift that a simple pass or fail certificate can hide.
Preserve original measurements
Capture tag, range, units, test points, reference standard, ambient conditions and uncertainty before adjustment. Record as found and as left separately. A successful adjustment does not erase an earlier out of tolerance result.
Evaluate the process consequence
Compare errors against approved function specific tolerances, particularly at normal operating points. Investigate possible effects on process control, alarms, quality or protective functions during the interval since the last acceptable test.
Find repeatable patterns
Trend error magnitude and direction over several cycles. Consider sensor aging, plugged impulse lines, vibration, temperature, wiring and calibration equipment. A single reading is insufficient to infer a failure trend.
Industrial example
A pressure transmitter reads progressively high at the normal operating point over three cycles. Instrument engineering checks its impulse line and temperature exposure before deciding whether to change the interval or replace it.
Close the corrective action
Record impact assessment, root cause hypothesis, repair or replacement, final verification and next review date. Any interval extension needs supporting history and approval.
Implementation checklist
- Keep as found readings before adjustment
- Check function specific tolerances and uncertainty
- Trend drift over successive calibrations
- Document operating impact and corrective action
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