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In earlier editorials, I've examined modern industrial requirements for powder characterization and assessed how well different test methods answer to the resulting brief. In this editorial, I want to focus on tapped density methods and their advantages and limitations when it comes to investigating powder flow properties.

Tapped density is based on measurement of the increase in bulk density induced by tapping a powder sample. Bulk density of the sample is first measured in a baseline state, and then again after a defined tapping process. Carr's index and the Hausner ratio are other ways of representing the ratio of tapped to untapped density, and they enable classification of the powder according to a predefined scale: a Carr's index of less than 15, for example, would indicate good flowability.

Such techniques have a number of practical advantages. They are quick, relatively easy to carry out, and often inexpensive. They assess an important aspect of powder behavior density change as a result of vibration or unidirectional tapping, which always happens during transport and processing. However, they only coarsely differentiate cohesive from free-flowing samples. Moreover, attempting to apply such data to predict the flowability of different samples within the processing environment quickly highlights some important limitations of the technique.