Amongst the pile of tools in my garage is a nice six-inch caliper, which I use for the occasional minor project around the house. And even though I work in one of the finest dimensional calibration laboratories in the country, where we calibrate thousands of calipers every year, my own personal caliper has not been calibrated in over 20 years. Some people are shocked when they hear this—they ask me how can an expert in calibration not even take care of his own caliper? Hypocrite? Heretic? My response is that I am knowledgeable enough about calibration, and understand the risks with calibration intervals, that I know it is not worth the trouble to calibrate this particular caliper.
In general, calibration is all about managing risk in measuring systems. The extent and importance of calibration depends on the role of the measuring system and the risks that can occur if the measuring system is no longer performing as expected. Every organization has a different threshold of acceptable risk and this has a significant impact on the setup of the calibration system. Regarding the caliper in my garage, I mostly use it to measure small drill bits and Allen wrenches for minor household projects (as my eyes have gotten old and the small print is hard to see on those little things). In my case, the impact of making a bad measurement with my caliper is minor—the worst situation is maybe putting too big of a screw hole in the wall and then having something else that needs to be fixed. The probability of my caliper being so far off that it will impact my low accuracy needs is negligible. In contrast, some manufacturing companies calibrate their calipers with intervals as low as every three months. The more frequent use of the caliper, combined with a more serious impact of being wrong in measurement, demands a shorter calibration interval.