Measurement System Stability

 

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Measurement System Stability

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Measurement System Stability

Stability helps evaluate how a measurement device performs over time.

  • Stability looks at whether the bias changes over time. If the bias does change, the device is less repeatable over time.

  • Stability studies also help determine the proper increment between calibration and repair intervals.

Stability can be analyzed using an X-Bar & R or IX & MR (individuals and moving range) control chart.

A reliable, stable standard is needed to begin a stability study.

  • The same standard will be used during the entire study.

  • The standard must not degrade over time, wear as it is measured, or have a short shelf life.

Control chart method of evaluating stability:

  • Measure the sample 3 to 5 times. Calculate X-Bar & R and plot on a control chart.

  • Repeat daily for 20 days.

  • After 20 sets of measurements, calculate the control limits and add them to the chart.

  • Are there any out-of-control conditions indicating instability? If so, the device is not stable.

Use the control chart to check the interval for calibration.

  • Use the control chart at regular intervals (e.g. weekly) to make sure the device remains stable.

  • If a (previously) stable device becomes unstable, you will have exceeded the safe interval of calibration or repair.

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