Calibration
Calibration is formally defined by the International Vocabulary of Metrology (VIM) as an operation that establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties. In simpler terms, it is the rigorous comparison of a device under test (DUT) against a reference standard of known, higher accuracy.
Such a standard could be another, more precise measurement device, a device that generates a highly stable physical quantity (like a precision voltage reference or a calibrated acoustic calibrator), or a physical artifact (like a gauge block or standard mass).
The Purpose of Calibration
All measuring instruments degrade over time due to wear and tear, environmental changes (temperature, humidity), mechanical shock, or electrical drift. Calibration does not necessarily mean adjusting an instrument so it reads "correctly." Rather, its primary goals are to:
- Quantify Error: Determine how far the instrument's reading deviates from the true value of the standard.
- Establish Uncertainty: Calculate the statistical confidence level of the measurement (e.g., "The reading is 10.0 V ± 0.1 V with a 95% confidence level").
- Ensure Traceability: Document an unbroken chain of comparisons back to national or international SI standards.
Calibration vs. Adjustment vs. Verification
These terms are often confused in industry, but they have distinct metrological meanings:
- Calibration: The act of comparing the DUT to the standard and recording the results (As-Found data).
- Adjustment: The physical or software manipulation of the DUT to bring its indications within a specified tolerance. If an adjustment is made, a second calibration (As-Left) must be performed.
- Verification: The provision of objective evidence that a given item fulfills specified requirements (e.g., checking if the instrument meets the manufacturer's published specifications).