Measurement Process Characterization
2.4. Gauge R & R studies
2.4.5. Analysis of bias
|Definition||Drift can be defined (VIM) as a slow change in the response of a gauge.|
|Instruments used as comparators for calibration||
Short-term drift can be a problem for comparator measurements. The
cause is frequently heat build-up in the instrument during the time of
measurement. It would be difficult, and probably unproductive, to try
to pinpoint the extent of such drift with a gauge study. The simplest
solution is to use drift-free
designs for collecting calibration data. These designs mitigate
the effect of linear drift on the results.
Long-term drift should not be a problem for comparator measurements because such drift would be constant during a calibration design and would cancel in the difference measurements.
|Instruments corrected by linear calibration||For instruments whose readings are corrected by a linear calibration line, drift can be detected using a control chart technique and measurements on three or more check standards.|
|Drift in direct reading instruments and uncertainty analysis||
For other instruments, measurements can be made on a daily basis on
two or more check standards over a preset time period, say, one month.
These measurements are plotted on a time scale to determine the extent
and nature of any drift. Drift rarely continues unabated at the same
rate and in the same direction for a long time period.
Thus, the expectation from such an experiment is to document the maximum change that is likely to occur during a set time period and plan adjustments to the instrument accordingly. A further impact of the findings is that uncorrected drift is treated as a type A component in the uncertainty analysis.