It is extremely important for us to maintain calipers and micrometers carefully. They are delicate instruments which are constructed to exacting tolerances and designed for long service. Keep them free of foreign particles whenever possible and when not in use, store them in their appropriate container. If you allow them to be left unprotected on a bench, they will probably be damaged.A light coating of high-grade instrument oil will also go a long way to extend the life of the tool. Be aware, resist the urge to spray the tools with some of the protectarant type oils. Instead, these products are designed to place a coating on the material so as to resist rusting. Were this coating to attach to the threads or teeth of the micrometer or caliper you will notice a change in the calibration. If the tools are to be stored for a period of time, the placing of a silica gel pack into the toolbox draw will also help to retard rusting. How often do you check the devices? With the advent of ISO certification this may be already predetermined by individual companies but a rule to follow on micrometers and calipers is to periodically throughout the day, close the measuring surfaces to assure that you are on zero. With mechanical tools you are not so apt to be far away but the addition of the floating zero to the electronic tools makes this more important as the button could have inadvertently been pressed. For calibration, your company may have established a frequency for checking calibration. If a tool is under the control of an individual and is used in a normal measuring capacity (not abrasive), it normally needs to be checked by a technician once a year. If the tool is loaned from a central tool crib and is in use by many individuals. If this is the case, you should calibrate them twice a year.