Insights
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 explained
Traceability means a result can be related to the SI through an unbroken, documented chain of calibrations, each with a stated uncertainty. Here is what that requires in practice.
Type A vs Type B uncertainty evaluation
Type A uses statistics on repeated observations; Type B uses any other information. Both produce standard uncertainties that combine the same way.
What the coverage factor k means
The coverage factor multiplies the combined standard uncertainty to give an expanded uncertainty with a stated level of confidence. k = 2 is roughly 95 % for a normal distribution.
Rectangular, triangular and normal distributions in uncertainty budgets
The assumed distribution sets the divisor that turns a limit into a standard uncertainty: √3 for rectangular, √6 for triangular, k for normal.
Sensitivity coefficients: how input uncertainties reach the result
A sensitivity coefficient converts an input's uncertainty into its effect on the output. For a direct temperature comparison it is 1; for derived quantities it comes from the model.
Correlated inputs in uncertainty evaluation
Root-sum-of-squares assumes independence. When two inputs move together — the same reference used twice, for example — the combination must include 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 — defined 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 except time; reproducibility changes operators, instruments, locations or days. The gap between them reveals hidden influences.
Drift: how instruments change between calibrations
Drift is a slow change in indication over time. It is estimated from calibration history and enters the budget as an uncertainty for the interval — or is corrected if it is predictable.