Re-Analysis of the Uncertainty of the 0.895 µm Diameter (NIST SRM(®) 1690) and the 0.269 µm Diameter (NIST SRM(®) 1691) Sphere Standards

The uncertainties of the mean diameters of the nominal 1.0 µm SRM(®) 1690 polystyrene spheres and of the nominal 0.3 µm SRM(®) 1691 polystyrene spheres are recomputed using the current NIST Guidelines for computing uncertainty. The revised expanded uncertainty (approximately 95 % confidence level) for SRM(®) 1690 polystyrene spheres is equal to 0.005 µm compared to previous value of 0.008 µm. The revised expanded uncertainty for SRM(®) 1691 is equal to 0.004 µm compared to the previous value of 0.007 µm. The major cause of the reduction in the uncertainty for the 1.0 µm spheres is from a decrease in the recomputed uncertainty of the refractive index of the polystyrene spheres. The 1.0 µm spheres were used in calibrating the electron microscope used to size the 0.3 µm spheres, and the reduction in the uncertainty of 1.0 µm SRM(®) uncertainty was the biggest factor in the decrease in the uncertainty of the 0.3 µm spheres.