= Measurement calibration
<Measurement calibration> establishes a relation between an instrument's indications and reference values, with the relevant <measurement uncertainty>. It is distinct from changing the instrument until it reads correctly: an instrument can be calibrated by determining a response curve and applying a correction afterward.
A practical <voltage> experiment connects a meter to several reference values across its working range and repeats the sequence in both directions. Fitting $V_{\rm indicated}=aV_{\rm reference}+b$ estimates a scale factor and <measurement zero offset>; departures from that relation test the response model. Repetition at later times checks <instrumental drift>. Uncertainty in the reference, the fitted parameters, and unresolved departures belongs in the <measurement uncertainty budget>, even if the displayed readings appear very stable.<Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results by Barry N. Taylor and Chris E. Kuyatt>[Appendix D.4: calibration and the measurement model; section 4: reference information in uncertainty evaluation]{ref}
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