Metrology
Traceability, uncertainty and the language of measurement.
Measurement uncertainty explained
What an uncertainty statement means, how it is built up, and why a coverage factor matters.
Metrological traceability under ISO/IEC 17025
An unbroken, documented chain of calibrations back to the SI, each with its own stated uncertainty. What ISO/IEC 17025 actually requires you to show.
Type A vs Type B uncertainty: the difference that matters
Type A comes from repeated measurements, Type B from everything else. It changes nothing about how they combine, and everything about degrees of freedom.
Coverage factor k explained: why k = 2 means about 95 %
The coverage factor multiplies combined standard uncertainty to give expanded uncertainty. k = 2 is about 95 %, but only when the distribution is normal.
Rectangular, triangular and normal distributions in uncertainty budgets
The distribution sets the divisor that turns a limit into a standard uncertainty: √3 rectangular, √6 triangular, √2 U-shaped. Say which you chose.
Sensitivity coefficients: how input uncertainties reach the result
A sensitivity coefficient converts an input's uncertainty into its effect on the result. For a direct temperature comparison it is 1; elsewhere, derive it.
Correlated inputs in uncertainty evaluation
Root-sum-of-squares assumes independence. When two inputs move together, such as the same reference used twice, the combination needs a correlation term.
Reporting uncertainty: rounding and significant figures
Report the expanded uncertainty to at most two significant figures, round the result to the same decimal place, and state the coverage factor.
Ten VIM terms every calibration user should know
Measurand, indication, error, correction, accuracy, precision, trueness, resolution, drift and traceability, as the international vocabulary defines them.
Accuracy vs precision in measurement
Precision is about scatter; accuracy is about closeness to the true value. An instrument can be precise and wrong, or accurate on average and noisy.
Repeatability vs reproducibility
Repeatability holds everything constant but time; reproducibility changes operators, instruments, locations or days. The gap reveals hidden influences.
Drift: how instruments change between calibrations
Drift is a slow change in indication, estimated from calibration history. It enters the budget as an uncertainty, or is corrected when it is predictable.