Introduction

SEDIMENTATION is a mechanism for classifying metal powders according to their settling rate in a fluid. This measurement is based on Stokes's law of fluid dynamics, which states that, at low velocities, the frictional force on a spherical body moving through a fluid at constant velocity is proportional to the product of the velocity, the fluid viscosity, and the radius of the sphere.

The Stokes equation is a relationship between settling velocity, Ust, and particle diameter, D:

where Ps is solid density, Pt- is fluid density of viscosity '/, and g is gravitational acceleration (Ref 1). The particle size distribution can be determined by examining a sedimenting suspension of the powder. Changes with time in the concentration of density of the suspension at known depths are determined, and the size distribution is determined from these data (Ref 2).

Sedimentation is routinely used to determine particle size and size distribution. Among the number of sedimentation methods available, only a few are commonly used for metal powders--the micromerograph and light and x-ray turbidimetry. Another analyzer using the sedimentation method is the roller air analyzer; however, it is no longer manufactured, and its standards are being withdrawn.

To obtain valid results, convection currents must be avoided in the suspending fluid, and the relative rate of motion between the fluid and the powder particles must be slow enough to ensure laminar flow. This generally restricts the particle sizes that can be determined to the subsieve range.

Particles should be large compared with inhomogeneities in the fluid, which for sedimentation in air restricts the available methods to particles >5 /'m. For sedimentation in liquids, particle sizes down to 0.1 /'m can be determined.

Particles in the suspension must be perfectly dispersed, and the suspension must be diluted enough to guarantee independent motion, which translates to a maximum concentration of ~ 1 vol% of particles in the suspending medium.

Finally, wall effects should be minimized; consequently, the inside diameter of the sedimentation chamber should be sufficiently large to overcome this phenomenon.

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