What is the primary physical law governing this test method?

Stokes' Law, which relates the terminal settling velocity of a spherical particle to its diameter, fluid viscosity, and density difference.

What is the effective particle size range for sedimentation methods?

It is most effective for particles between 1 and 75 microns. Particles larger than this may fall too fast (causing turbulent flow), and smaller particles are affected by Brownian motion.

Why must the concentration of the suspension be kept low?

To ensure free settling conditions. High concentrations lead to hindered settling, where particles interact and fall at rates not predicted by Stokes' Law.

IS 5282

: 1969

Liquid sedimentation methods for determination of particle size of powders

CurrentSpecializedTesting MethodMaterials Science · Sieves, Sieving and Other Sizing Methods

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This standard prescribes liquid sedimentation methods for determining the particle size distribution of fine powders in the sub-sieve range. It covers procedures such as the pipette method and the sedimentation balance method, which utilize Stokes' Law to calculate particle size based on settling velocity in a viscous fluid.

Liquid sedimentation methods for determination of particle size of powders

Quick Reference Values

Typical particle size range1 to 75 micrometers

Maximum Reynolds number for Stokes' law validity0.2

Recommended suspension concentration0.2% to 2.0% by volume

Key Formulas

D = sqrt((18 * η * v) / (g * (ρ_s - ρ_l))) — Stokes' Law for calculating the equivalent spherical diameter of a settling particle

Practical Notes

Proper and complete dispersion of the powder in the liquid is critical; failure to break up agglomerates will artificially increase the apparent particle size.

Strict temperature control (usually via a water bath) is necessary because the viscosity of the suspending liquid changes significantly with temperature, directly affecting sedimentation velocity.

The test calculates an 'equivalent spherical diameter'. For highly irregular, flaky, or needle-like particles, results may differ from actual physical dimensions obtained via microscopy.