what is calibration
The Metrology, an educational department within , has developed a unit of the Metrology Fundamentals series to address the confusion in calibration practice. The unit aims to clarify the different concepts of calibration, including official definitions and those in common use. The primary technical basis for this document is ASME B89.7.1-2016, Guidelines for Addressing Measurement Uncertainty. The official definition of calibration is the International Vocabulary of Metrology (VIM), which defines calibration as an operation that establishes a relation between quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties.
There are two important steps in calibration: relating measured values from measuring equipment to those from calibrated measurement standards, which is the critical connection between calibration and traceability. Calibration enables units of measurement on equipment to be traced back to an official reference, like NIST in the U.S. The second step in the definition clarifies that the information gained in the first step is only as good as how it is used. Therefore, whatever information is used as the calibration is indeed the calibration, and therefore we need to explore the ways that calibration information is used.
Calibrations can be confusing due to different expectations between the requesting and providing organizations. ASME B89.7.1 explains that calibration measurements have three purposes: determining reference values, determining test values for verification to tolerance, and determining necessary adjustments to measuring equipment. Reference values, such as the nominal value on a gage block, are determined and reported on a calibration certificate.
Verification to tolerance limits is the most important aspect of calibration, and the necessary calibration method may differ from the method used to determine and assign a reference value. The issued calibration certificate should include measured values, tolerances, and a statement of conformance. A new calibration certificate and calibration label can be issued even if the equipment is out of tolerance.
The biggest confusion in calibration is with adjustments to measuring equipment accuracy. Many U.S. industry members believe calibration includes necessary adjustments to bring the equipment back to manufacturer specifications. To avoid confusion, clear expectations and careful usage of the word "calibration" when requesting calibration services.