Carr index

The Carr index is an indication of the compressibility of a powder. It is calculated by the formula $C=100frac\{V\_B-V\_T\}\{V\_B\}$, where $V\_B$ is the freely settled volume of a given mass of powder, and $V\_T$ is the tapped volume of the same mass of powder. It can also be expressed as $C=100\; imes(1-frac\{\; ho\_B\}\{\; ho\_T\})$, where $ho\_B$ is the freely settled bulk density of the powder, and $ho\_T$ is the tapped bulk density of the powder.

The Carr index is frequently used in pharmaceutics as an indication of the flowability of a powder. A Carr index greater than 25% is considered to be an indication of poor flowability, and below 15%, of good flowability. [cite book |author=Kanig, Joseph L.; Lachman, Leon; Lieberman, Herbert A. |title=The Theory and Practice of Industrial Pharmacy |publisher=Lea & Febiger |location=Philadelphia |year=1986 |edition=3 |isbn=0-8121-0977-5]

The Carr index is related to the Hausner ratio, another indication of flowability, by the formula $C=100\; imes(1-1/H)$.

Both the Hausner ratio and the Carr index are sometimes criticized because their relationships to flowability were established empirically and do not have a strong theoretical basis. Use of these measures persists, however, because the equipment required to perform the analysis is relatively cheap and the technique is easy to learn.

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