through direct measurement is surprisingly difficult to achieve, requiring steady state data on the feed, product, and tailings solids rates. Variability in the movement of the solids through at least one stream of the separator, e.g. due to an automated valve opening and closing, can lead to significant error. In general, samples need to be taken for a sufficient period, and in general the feed solids rate needs to be measured last to avoid process disturbance. Similar issues arise with directly measuring the recovery Ri of a target species i, which is another parameter of great interest.