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NCSL International - Governance
Instrument Adjustment Policies
Paul Reese PSE Operations, Baxter Healthcare Corporation
Instrument adjustment policies play a key role in the reliability of calibrated instruments to maintain their accuracy over a specified time interval. Periodic review and adjustment of assigned calibration intervals is required by national standard ANSI/NCSL Z540.3 and is employed to manage the End of Period Reliability (EOPR) to acceptable levels. Instrument adjustment policies may also be implemented with various guardband strategies to manage false accept risk. However, policies and guidance addressing the routine adjustment of in-tolerance instruments are not so well established. National and international calibration standards ANSI/NCSL Z540.3 and ISO/IEC-17025 do not mandate any particular adjustment policy with regard to intolerance equipment. Evidence has been previously presented where routine adjustment of in-tolerance items may even degrade performance. Yet, this important part of the overall calibration process is often left to the discretion of the calibrating technician based on heuristic assessment. Astute adjustment decisions require knowledge of the random vs. systematic nature of instrument error. Instruments dominated by systematic effects, such as drift, benefit from adjustment, while those displaying more random behavior may not. Monte Carlo methods are used here to investigate the effect of various adjustment thresholds on in-tolerance instruments.
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