A flash-derived conductivity k = αρcₚ inherits uncertainty from three inputs. Here the first term is deliberately narrowed to the non-geometry part of the diffusivity determination (timing, model and fitting), fixed at ±1.5% for illustration, because a reported α uncertainty usually already contains the thickness term through α ∝ L² — adding a full α uncertainty and a separate thickness term would double-count it. Thickness is therefore propagated once, explicitly, with its sensitivity coefficient of 2.
Teaching implementation of the law of propagation of uncertainty for k = αρcₚ with α ∝ L². The first term is the non-geometry part of the diffusivity determination, held at ±1.5% for illustration; thickness is propagated separately with a sensitivity coefficient of 2. If a report quotes a combined α uncertainty that already includes geometry, the thickness term must not be added again. Real budgets add model, calibration and correlation terms per the GUM.