Just like the people who use them, gauges should have periodic physical examinations. Sometimes, gauge calibration is needed to identify the seriousness of a known problem, and sometimes it uncovers problems you didn’t know existed.

But as with a human exam, the main reason for the annual checkup is to prevent problems from occurring in the first place.

The accuracy of a gauge can only be known by reference to a higher standard. Thus, gauges are set to masters that are more accurate than the gauge. These masters are certified against gauge blocks produced to a higher standard of accuracy, ultimately traceable to nationally or internationally recognized “absolute” standards that define the size of the dimensional unit. This is the line of traceability, which must be followed for calibration to be valid.

Calibration is used to determine how closely a gauge adheres to the standard. When applied to a master ring, disc, or a gauge block, it reveals the difference between the nominal and the actual sizes. When applied to a measuring instrument such as a comparator, calibration reveals the relationship between gauge input and output. In other words, the difference between the actual size of the part and what the gauge says it is.

Gauges go out of calibration through normal use: parts wear and mechanisms become contaminated. A gauge might have a design flaw so that joints loosen frequently, and the gauge becomes incapable of holding calibration. Accidents and rough handling also put gauges out of calibration.

No gauge, therefore, can be relied upon if it hasn’t been calibrated or if its calibration history is unknown. Annual calibration is considered the minimum, but for gauges that are used in demanding environments, gauges that are used by several operators or for many different parts, and gauges used in high-volume applications, shorter intervals are needed. Frequent calibration is also required for gauging parts that will be used in critical applications, and where the cost of being wrong is high.

Large companies that own hundreds or thousands of gauges sometimes have their own calibration departments. But this is rarely an economical option for machine shops. In addition to specialized equipment, in-house calibration programs require a willingness to devote substantial employee resources to the task.

Calibration service providers are usually a more economical approach. Smaller gauges can be shipped to the provider; large instruments must be checked while in place. Calibration houses also help shops by maintaining a comprehensive calibration program to ensure that every gauge in the facility is checked according to schedule, and that proper records are kept.

General guidelines for instrument calibration procedures appear in the ISO and ANSI standards. While every gauge has its own specific procedures as outlined in the owner’s manual, calibration procedures also must be application-specific. In other words, identical gauges that are used in different ways may require different procedures.

For example, if a gauge is used only to confirm that parts fall within a tolerance band, it may be sufficient to calibrate it only at the upper and lower tolerance limits. On the other hand, if the same gauge is used to collect data for SPC, and the accuracy of all measurements is important, then simple calibration might be insufficient. A test of linearity over the entire range might be needed.

The conditions under which calibration occurs should duplicate the conditions under which the gauge is used. Calibration in a high-tech gauging lab may be misleading if the gauge normally lives next to a blast furnace. Similarly, a snap gauge that’s normally used to measure round parts should be calibrated against a master disc or ring, and not with a gauge block. The gauge block could produce misleading results by bridging across worn areas on gauge contacts, while a round master would duplicate the actual gauging conditions and produce reliable results.

Therefore, before calibration begins the technician should be provided with a part print and a description of the gauging procedure. Next, they should check the calibration record to confirm that the instrument serial number and specifications agree with the instrument at hand. The gauge will then be cleaned and visually inspected for burrs, nicks, and scratches. Defects must be stoned out, and mechanisms checked for freedom of movement. If the instrument has been moved from another area, it must be given time to stabilize.

All of these measures help ensure that calibration will be accurate. But this must not lead to a false sense of security: Gauge calibration won’t eliminate all measuring errors. As we have seen before, gauging isn’t simply hardware; it’s a process. Calibration lends control over the instrument and the standard or master, but gauge users must continue to seek control over the environment, the workpiece, and the gauge operator.