| Hausner ratio | The ratio of tapped density to loose poured bulk density. |
| hydrodynamic cluster | The phenomenon of multiple, closely-grouped, individual particles falling through a fluid with a higher terminal velocity than that of the individual particles. Note that a focused flow stream achieves a higher fall velocity than the terminal velocity of an individual particle. |
| major principal stress | The largest principal stress acting on a bulk solid. |
| minor principal stress | The principal stress acting on a bulk solid at 90 degrees to the major principal stress. |
| oscillating hopper sample divider | A method of securing a representative sample by oscillating feed from a hopper over two contiguous chutes leading into separate collectors. |
| parent population | The overall bulk system from which a powder sample is taken. |
| permeability | The ease with which a porous mass e.g. a powder bed or compact, permits the passage of a fluid such as air. This feature has a major influence on the flow behaviour of fine powders. Changes in volume of the bulk essentially are reflected in changes in the voidage. This must initially respond to the volume variation by pressure change of the ambient fluid. A positive void pressure acts to partially support the particle mass. This support reduces particle-to-particle contact pressures and surface interferences that resist shear. In extreme circumstances, the shear strength of the bulk is negated and a highly dilated mass behaves as a fluid. A void pressure less than ambient, as generated by the shear of a settled bed, opposes the expansion of the bulk and hence increases its resistance to expansion and flow. |
| powder density, absolute | (See absolute powder density). |
| powder density, apparent | (See apparent powder density). |
| powder density, bulk | (See bulk density). |
| powder density, tapped | (See tapped density). |
| powder volume, apparent | (See apparent powder volume). |
| pressed density | (See density, pressed). |
| principal stress | A stress acting in a plane that is not subjected to a shear stress. |
| principal stress, major | (See major principal stress) |
| principal stress, minor | (See minor principal stress) |
| pyrophoric | The property of a powder to self combust when exposed to oxygen. |
| ratio, compression | (See compression ratio). |
| recovery factor | The change in dimension of a non-metallic compact on ejection from its die or mould. |
| representative sample | A quantity taken from a larger amount, that fairly reflects the qualities relative to the conditions of interest of the whole. |
Glossary of Terms in Powder & Bulk Technology
3. Powder Properties
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