How is calibration uncertainty calculated and what is the test uncertainty ratio?

Calibration uncertainty considers: standard uncertainty (reference device accuracy), stability (drift since last calibration), resolution (display or ADC quantization), environmental effects (temperature, humidity on device under test and standard), and repeatability of measurement process. Test Uncertainty Ratio (TUR) = tolerance/measurement uncertainty; TUR >= 4:1 traditionally required (4:1 rule). Lower TUR requires guard-banding: decision rules that adjust acceptance limits to account for measurement uncertainty. ILAC-G8 provides guidance on guard-banding. Calibration certificate must state uncertainty and methods used.