For any laboratory with instruments that measure properties of materials, for example, maintaining proper calibration is important to ensure valid results. Knowledge that the instrument is stable over time requires calibration records which many keep, yet few analyze the log. This project applies statistical process control methods to determine stability of an instrument.
Calibration data (collected daily) for an instrument that measures surface tension (using the Du Noüy ring method) of ethanol were analyzed over time to determine stability of the measuring device. Five daily measurements were taken each week over the course of 20 weeks, for a total of 100 individual observations. The accepted value for surface tension of ethanol is 22.39 dyne/cm. The overall average of the data reported is 22.38 dyne/cm and thus the accuracy of the measuring device is confirmed being within 0.1 dyne/cm. The x-bar and R charts demonstrate the variation of the measurements over time, and the x-bar chart reveals two data points that fall outside of the statistical control limits, suggesting that there is assignable cause in the measurement in addition to natural random variation. Further investigative work will be necessary to determine the root cause(s) of the excessive variation. The measurement process as is can reliably yield surface tension measurements in the range of ethanol to within the existing +/- 0.5 dyne/cm specification limits with the instrument under test.