Bulk Density Tester for Powders – Scott Volumeter

The bulk density of powders can be extremely difficult to measure since the slightest disturbance may result in a change in the result. This is the result of the relationship between the particles that constitute the powder bulk. This same relationship affects the ability of the powder to flow. The bulk density of a powder may be described as the density of the powder ‘as poured’ into a measuring vessel and is expressed in terms of grams per mL by dividing the weight of a given “untapped” powder sample by its volume.

Tapped density, on the other hand, is the density attained after ‘tamping down’: this is normally measured using an instrument that lifts and then drops a measuring cylinder containing the powder through a fixed distance. Please view the JVi Series tester for more details. A comparison of the bulk and tapped densities of powders can give an indication of the type of interaction present between the various particles making up the powder mass and hence provide an index of powder flowability.

Construction 

The apparatus consists of:

• A stainless steel top funnel having an integral ss 18-mesh screen (10-mesh screen available as an option)
• A baffle box containing four glass baffle plates over which the powder slides and bounces as it passes
• A stainless steel bottom funnel to direct the powder into the receiving cup
• A cylindrical receiving cup having a capacity of 25 ml /- 0.05 ml

Mode of Operation

• Weigh the empty receiving cup and place it in position
• Slowly pour the powder through the upper funnel until it overflows the receiving cup. (Note: Use a minimum of 35 cubic cm)
• Level the top of the receiving cup with a spatula such that it is completely full being careful not to compress or shake the powder
• Re-weigh the receiving cup and its contents
• Calculate the bulk density in terms of grams per ml by dividing the weight of the powder by the volume of the cup.

Compressibility Index and Hausner Ratio

Measures of the ability of the powder to flow and its compressibility can now be given in the form of (a) the Hausner Ratio or (b) the Compressibility Index” (see below).

In a free flowing powder, inter-particulate interaction is less significant and unsettled and tapped densities will be closer in value. In poorly flowing powders, the inverse is to be expected. It follows that the closer the Hausner ratio is to 1, the better the flow. Powders with poor flow generally have a ratio of greater than 1.25.

The Bulk Density Tester (Scott Volumeter) is described in European Pharmacopoeia chapter 2.9.34 and USP <616> Method 2 and is designed for measuring the bulk density of fine powders and similar products.