## Fisher Subsieve Sizer

The Fisher subsieve sizer is another common industrial method that allows particle size measurements in the range of 0.2 to 50 /'m. It is a simple method to measure particle sizes below the range of sieve analysis.

The Fisher subsieve sizer is a simple, inexpensive permeameter, which measures the flow of air through a packed powder bed. From this measurement, average particle size and porosity can be calculated from a chart that is an integral part of the instrument. The Fisher subsieve sizer is used extensively throughout industry, especially for process control purposes where only relative values are required.

Frequently, however, the results are often misinterpreted to designate an exact particle size. Consequently, analysis results refer to a Fisher "number" rather than a size. Fisher subsieve numbers should not be compared with size results from other measuring techniques, as frequently no correlation exists. Figure 3 illustrates a typical Fisher subsieve sizer, and Fig. 4 provides a schematic of the method.

Permeameters accurately and precisely measure permeability. They do not measure surface area or particle size. The flow through a porous compact depends on the structure of the pores. Flow, therefore, depends on the porosity, particle shape, particle size, particle size distribution, and method of compaction, all of which affect pore structure. Conversion of the measured permeability to a surface area is accomplished by the Kozeny-Carman equation:

E = (Fbed - Vpowder)/Vbed where K is permeability of the powder bed; E is porosity of the powder bed; V is volume, cm3; Pis true, or absolute, density of the powder material, g/cm3; Sw is surface area per unit weight of powder, cm2/g; and B is constant.

From Eq 1, it can be seen that permeability is related to three macroscopic quantities: bed porosity, surface area, and particle density. Average particle diameter (dm), also known as the Sauter mean diameter, the surface mean diameter, or the volume surface mean particle size, is calculated from the surface area by:

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