Compressibility Index and Hausner ratio

Carr’s index and Hausner ratio 

Both the Compressibility Index and Hausner ratio are intended to be indicators of the flowability of bulk solids.

  • They used as an indirect measure of bulk density, size and shape, surface area, moisture content, and cohesiveness
  • The Carr Index other name is Carr’s index or Carr’s Compressibility Index.
  • Since the flowability of solids is complicated and dependent on numerous factors, it is not always easy for scientists to characterize the solid flowability using the limited data they often have at their disposal during design or problem-solving.
  • The compressibility index and the Hausner ratio are achieved by calculating both the bulk volume and the tapped volume of a powder.

What is the Carr index equation and Hausner Ratio ? How to calculate the Carr’s Index (Compressibility Index) and Hausner Ratio ?

Calculation of as per USP

Method 1:

The basic process is to measure the initial apparent volume (VO), and the final tapped volume (Vf ) , of the powder after tapping the material until no further volume changes take place. There are, however, several variants in the method of estimating the both,

The Hausner ratio and compressibility index are calculated as follows:

Compressibility Index and Hausner Ratio as per USP

Method 2:

Calculation by bulk density (ρbulk) and tapped density (ρtapped) as follows:

Compressibility Index and Hausner Ratio as per USP

Hausner Ratio and Carr Index Interpretation and Relation to Flowability

A variation of these techniques measures the rate of consolidation in addition to or instead of the change in volume that occurs on tapping. The standard flowability scale is as per the below table,

Flow CharacterHausner RatioCompressibility Index (%)
Excellent / Very Free Flow1.00 – 1.11<10
Good / Free Flow1.12 – 1.1811-15
Fair1.19 – 1.2516-20
Passable1.26 – 1.3421-25
Poor Flow / Cohesive1.35 – 1.4526-31
Very Poor Flow / Very Cohesive1.46 – 1.5932-37
Approximatively no flow> 1.60> 38

Investigational Considerations for the Compressibility Index and Hausner Ratio

Both are dependent on the methods utilized, not the fundamental qualities of the powder.  The following crucial factors that affect the calculation of the final tapped volume, Vf, the bulk density, bulk, and the tapped density, tapped, are discussed in the literature now in print:

  • For desired density: Number of times the powder is tapped
  • The diameter of the cylinder utilised
  • The mass of the material used in the test
  • The rotation of the sample during tapping

Suggested Procedure for Compressibility Index and Hausner Ratio as per USP

Use a 250-mL volumetric cylinder with a test material with a weight of 100 g. Smaller weights and quantities can be employed, but modifications to the approach should be explained along with the outcomes. An average of three (3) determinations is recommended.


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