A Step in the Right Direction: The Hausner Ratio
The Hausner Ratio is a particular method of measuring the fluidity of a material. It is most often used to measure powders and granular solids. This ratio is found by measuring the bulk density (LB) of a material and then dividing that by its tapped density (LT). It allows industrial chemists to easily judge the “flowability” or “powder behavior” of a material, which can be affected by a variety of things. The ratio is named after the researcher who developed the method, Ferdinand Hausner.
The tapped density of a material is measured by tapping a sample of it in a cylindrical container, releasing it, and repeating the process until the volume doesn’t change any more—known as the “fully tapped density”. The bulk density is found by measuring the uncompacted volume of the material and then dividing it by the sample weight. The bulk density is a measure of the density of the material when it is free-flowing, for instance after suspended in air for a certain amount of time, or gently vibrated.
The Hausner Ratio is directly related to the angle of repose for the material in question. This is the critical angle at which the material tends to slide down a slope, when it is allowed to form a pile. It is calculated by creating a pile of the material and then measuring the angle that is formed. Greater angles of repose indicate that the material possesses greater flowability, while lesser angles indicate poorer flowability.
The aim of using the Hausner Ratio is to determine whether or not a material is sufficiently fluid. The ratio can be used to compare different materials, or the same material over different times, to understand how its flowability is changing. It can also be used to determine if a sample is affected by humidity, if too bulky, or if too small.
The idea behind the Hausner Ratio is that a high ratio indicates that the sample has poor flowability—that is, it does not move well when tapped. On the other hand, a low ratio indicates that it is relatively fluid—that is, it moves well when tapped. In either case, there may be additional causes for the lack of fluidity, such as the size or shape of the particles of the material, or the presence of impurities.
The Hausner Ratio can provide important information about the relative fluidity of materials. It is a simple and effective measure, and is often an important step in the development process for new products. Additionally, it can give insight into how existing products can be improved, simplifying the production and improving the consistency of quality.
