Liquid sedimentation methods for determination of particle size of powders
1969 Edition

Incremental sedimentation focuses on measuring particle concentration at specific depths within the sedimentation vessel, allowing determination of particle size distribution at discrete points. Conversely, cumulative sedimentation assesses the average concentration or total sediment accumulation over a defined depth or the entire column, providing an integrated particle size profile. Incremental methods involve sampling at set depths and calculating particle sizes from settling velocities, while cumulative methods measure total sediment or average concentration. Incremental techniques require stable suspensions with minimal flocculation for accuracy, whereas cumulative approaches average fluctuations over the sediment column.

To calibrate the pipette, first thoroughly clean and dry the sedimentation tube and pipette. Fill the sedimentation vessel with distilled water up to approximately 20 cm. Using the stop cock, draw water into the pipette bulb until it reaches the designated graduation mark, typically 10 ml. Then, switch the stop cock to discharge position and collect the water in a pre-weighed container. Apply positive pressure to expel any residual water from the bulb and discharge tube into the container. Weigh the collected water and calculate the pipette volume by dividing the water weight by its density at the test temperature. This process ensures precise volume measurement essential for accurate sedimentation analyses.

IS 5282 recommends selecting dispersing agents through empirical evaluation, often by trial and error, to prevent particle agglomeration and flocculation. The standard's Appendix B provides a list of suitable liquids and dispersing agents tailored to different materials. Generally, a small amount of dispersing agent is incorporated into the suspension medium, mixed thoroughly to create a uniform paste or suspension. Stability is assessed qualitatively by observing flow characteristics and sediment rigidity. Effective dispersing agents enable suspensions that flow freely, form compact sediments with minimal volume, and show no sediment displacement upon tilting. Mechanical stirring or air bubbling can aid dispersion, with entrapped air removed by applying reduced pressure.

The standard sets particle size boundaries based on material density and a Reynolds number threshold of 0.2 to ensure measurement errors remain below 5%. The Reynolds number is calculated using particle settling velocity, diameter, fluid viscosity, and fluid density. Upper limits correspond to the maximum particle size where laminar flow (Re ≤ 0.2) is maintained, while lower limits are established to avoid interference from convective or diffusive effects. Appendix A tabulates these limits for various densities (e.g., 2 g/cc, 4 g/cc, 8 g/cc) under standard fluid conditions (viscosity 0.01 poise, density 1 g/cc). This framework guarantees reliable sedimentation-based particle size measurements within specified ranges.

IS 5282's Appendix F lists multiplicative correlation factors to convert particle diameters among sieve, projected, and Stokes (sedimentation) measurements. For example, converting sieve diameter to projected diameter involves multiplying by 1.40, whereas sieve to Stokes diameter uses a factor of 0.94. The reverse conversions apply respective reciprocal factors, such as projected to sieve diameter at 0.71 and Stokes to sieve diameter at 1.07. These factors facilitate combining data from different measurement techniques, though caution is advised when particles exhibit irregular shapes. Ideally, specific correlation factors should be established experimentally for the powder in question to enhance accuracy.