Gage Drift Time to Out-of-Tolerance Calculator

How to use it

Enter the permissible error limit for the instrument, the as-left error from the last calibration and the drift rate per month. Drift is best estimated from at least three calibrations: take the change in as-found error divided by the months between them. Choose a safety fraction so the interval ends before the predicted crossing. The result shows months to the limit, projected error at your proposed interval and a suggested interval.

• Use as-found data, not as-left, for the drift rate.
• Use the absolute error so direction does not matter.
• Revisit after each calibration as the data improves.

Worked example

A micrometer has a limit of 0.010 mm, was left at 0.002 mm and drifts 0.0006 mm per month. Margin is 0.008 mm, so it reaches the limit in about 13.3 months. A 12 month interval projects an error of 0.0092 mm, which is 92 percent of the limit, close for comfort. An 80 percent safety fraction suggests 10 months. If the drift were due to wear on a contact face, repair might be a better answer than a shorter interval.