Absolute precision in particle size analysis can only be achieved when the two principle parameters of length and concentration (e.g. weight) are directly traceable to International Standards. Furthermore, the particles to be analysed must be spherical so their size can be described by a single dimension. Techniques that fulfil these criteria are called “primary” methods and include Microscopy, Sieve Analysis and Sedimentation Analysis. This paper evaluates the accuracy of a 100- stage spinning riffler using electroformed sieves and then examines Sonic Sieving as a primary particle sizing method in the certification of a 20-100 micron spherical reference standard.