Particle Size Distribution Testing by Sedimentation Method: Principles, Standards, and Applications

Particle size distribution testing by the sedimentation method is a well-established analytical technique that determines particle size from the settling behavior of particles in a liquid medium, governed by Stokes' Law. The method provides high-resolution size analysis for powders ranging from roughly 0.01 μm to 300 μm, depending on whether gravitational or centrifugal sedimentation is applied. Accredited third-party laboratories perform sedimentation-based particle size analysis in accordance with ISO 13317, ISO 13318, ASTM B761, and related GB standards to support quality control, research and development, and regulatory compliance across the ceramics, metal powder, pigment, abrasive, and geotechnical industries.

Technical Overview

Sedimentation-based particle size distribution testing determines the size distribution of powder particles by measuring how quickly they settle through a liquid under gravitational or centrifugal force. The technique is founded on Stokes' Law, which states that the terminal settling velocity of a spherical particle in a fluid is proportional to the square of its diameter: v = (ρs − ρl) × g × x² / (18η), where v is the settling velocity, x is the particle diameter, ρs and ρl are the densities of the solid and the liquid, g is the acceleration, and η is the liquid viscosity. Coarse particles settle rapidly, while fine particles settle slowly, so the concentration profile of a settling suspension can be mathematically converted into a cumulative particle size distribution expressed as the sedimentation (Stokes) equivalent diameter.

Two families of techniques are used. Gravitational sedimentation, standardized in the ISO 13317 series, is suitable for particles from approximately 0.5 μm to 300 μm, with the lower limit governed by Brownian motion and the upper limit by the onset of turbulent settling. Centrifugal sedimentation, standardized in the ISO 13318 series, applies centrifugal acceleration to extend the measurable range down to approximately 0.01 μm (10 nm) and dramatically shortens analysis time for fine particles. Particle concentration during settling is monitored by X-ray attenuation, optical (photometric) absorption, direct pipette extraction, or mass accumulation on a balance, depending on the instrument configuration.

The key strengths of the method are its high size resolution, its first-principles (absolute) measurement basis, and its excellent repeatability on well-dispersed suspensions. Its principal assumptions must be respected: particles are treated as smooth spheres of known true density, settling must remain in the laminar regime, and the powder must be fully dispersed into primary particles without agglomeration.

Testing Samples

Metal and Alloy Powders

Iron, stainless steel, copper, tungsten, molybdenum, nickel, cobalt, soft magnetic alloys, cemented carbide raw materials, and powders for metal injection molding (MIM) or powder metallurgy. X-ray gravitational sedimentation is particularly effective for these X-ray-absorbing materials, in line with ASTM B761.

Ceramic and Mineral Powders

Alumina, silica, zirconia, silicon carbide, silicon nitride, calcium carbonate, kaolin, talc, barite, quartz flour, and cement raw materials. These materials are routinely characterized using the X-ray gravitational technique of ISO 13317-3.

Pigments, Fillers, and Coating Raw Materials

Titanium dioxide, iron oxide and other inorganic pigments, extenders, ink raw materials, and polymer fillers, where fines content and top-cut control directly influence opacity, gloss, and dispersion stability.

Abrasives and Polishing Powders

Silicon carbide, fused and calcined alumina, cerium oxide, and micron-grade diamond powders, where tight control of D50 and D97 is essential for cutting and polishing performance.

Pharmaceutical, Food, and Cosmetic Powders

Water-insoluble active ingredients, excipients, talc, zinc oxide, and other inert powders for which sedimentation provides an orthogonal, high-resolution alternative to laser diffraction.

Soils, Sediments, and Clays

Fine-grained soils and geological sediments analyzed by hydrometer or pipette sedimentation for geotechnical gradation curves and clay-fraction determination, per ASTM D7928.

Battery, Catalyst, and Functional Materials

Graphite and carbon anode materials, lithium iron phosphate, ternary precursors, catalyst supports, and other fine or nanoscale functional powders, typically measured by centrifugal photo-sedimentation.

Testing Items

A standard sedimentation particle size distribution report typically includes the following measurable and calculated results:

• Complete cumulative particle size distribution (undersize and oversize curves) and differential (frequency) distribution
• Characteristic diameters: D10, D50 (median), D90, D97, and other percentile values
• Mass percentage finer or coarser than a specified cutoff, for example the −45 μm fraction or +75 μm residue
• Distribution width parameters such as span, uniformity coefficient, and coefficients of curvature for soils
• Calculated volume-specific surface area, based on the spherical-particle assumption
• Sedimentation (Stokes) equivalent diameter reported at a defined temperature and sedimentation medium
• Supporting true density determination by gas pycnometry (ISO 12154 or ASTM B923), which is a required input for the Stokes calculation
• Method development services: dispersant screening, ultrasonic dispersion optimization, and dispersion stability verification

Testing Methods

Sample Preparation and Dispersion

Representative samples are divided, pre-wetted, and dispersed in a suitable liquid (deionized water with a dispersing agent, or a non-aqueous medium for water-sensitive materials) following the guidance of ISO 14887. Ultrasonic energy is applied to break up soft agglomerates, the suspension is diluted to typically below 1 % by volume to avoid hindered settling, and the temperature is controlled because liquid viscosity and density directly affect the calculated diameters.

Gravitational Sedimentation – Pipette Method (ISO 13317-2)

A fixed-position pipette (Andreasen-type) extracts small volumes of suspension from a defined depth at timed intervals. The extracted solids are dried and weighed, and the mass-versus-time data are converted into the cumulative size distribution. The method is robust, low-cost, and traceable, and is well suited to the 1–250 μm range.

Gravitational Sedimentation – X-Ray Method (ISO 13317-3)

A collimated X-ray beam passes through the settling suspension at a known height, and the attenuation of the beam continuously measures the local particle concentration. Because the X-ray signal responds directly to mass concentration, the technique offers excellent precision for X-ray-absorbing materials such as metals, oxides, and minerals across a typical range of 0.1–300 μm, making it the industry reference for ceramic and metal powder specifications.

Sedimentation Balance Method (ISO 13317-4)

A balance pan records the cumulative mass of particles settling out of suspension. This cumulative approach yields the oversize distribution directly and is advantageous for coarser, faster-settling powders.

Hydrometer Method for Soils (ASTM D7928)

A calibrated hydrometer immersed in a settling soil suspension reads the liquid density, which decreases as particles settle. Successive readings generate the fine-grained portion of the geotechnical gradation curve, normally combined with sieve analysis for the coarse fraction.

Centrifugal Sedimentation (ISO 13318)

Disc-centrifuge instruments rotate the suspension so that centrifugal acceleration hundreds of times greater than gravity drives fine particles outward past an optical (Part 2) or X-ray (Part 3) detector. This extends the measurement range down to approximately 0.01 μm, resolves narrow or multimodal distributions with very high resolution, and is the preferred approach for nanopowders and sub-micron quality control.

Data Treatment and Reporting

Raw concentration–time or mass–time data are converted to Stokes diameters and reported in accordance with ISO 9276 and GB/T 15445. Reports state the sedimentation medium, dispersant, temperature, true density, instrument type, all D-values, the full distribution table and curves, and instrument verification results obtained with certified reference materials.

Applications

Sedimentation particle size distribution testing is used throughout industry for purposes that include:

• Quality control of milling, classification, and air-separation processes, including endpoint determination
• Incoming inspection of purchased powders against supplier specifications
• Research and development of ceramics, pigments, coatings, and powder metallurgy materials, where fines content and top-cut strongly affect sintering behavior, opacity, and packing density
• Geotechnical site characterization, where the sedimentation-derived clay and silt fractions govern soil classification, permeability, and compaction properties
• Regulatory and contractual compliance testing against national and international specifications
• Failure analysis, contamination investigation, and cross-method verification of laser diffraction or image analysis results
• Nanomaterial characterization for battery, catalyst, and polishing applications requiring resolution of sub-micron populations

FAQ

Below are answers to frequently asked questions about sedimentation-based particle size distribution testing:

• Question: What particle size range can the sedimentation method measure? Answer: Gravitational sedimentation typically covers approximately 0.5–300 μm depending on particle density, while centrifugal sedimentation extends down to roughly 0.01 μm. The exact working range depends on the density difference between the particles and the liquid.
• Question: How does sedimentation compare with laser diffraction? Answer: The two methods are complementary. Sedimentation offers superior resolution of closely spaced or multimodal populations and provides an absolute, first-principles measurement, whereas laser diffraction is faster and covers a broader dynamic range. Because sedimentation yields a Stokes (hydrodynamic) equivalent diameter and laser diffraction yields a volume-equivalent sphere diameter, slight differences between results on non-spherical particles are expected and normal.
• Question: What information must be provided with a sample? Answer: The client should state the material identity, expected size range, chemical compatibility or solubility constraints, and the purpose of the test. The true particle density is required for calculation; if unknown, it can be measured by gas pycnometry as part of the service.
• Question: How much sample is needed and how long does an analysis take? Answer: Typically 1–10 g of powder is sufficient. A single gravitational analysis requires from about 30 minutes to several hours depending on fineness, while centrifugal measurements of sub-micron powders are usually completed in 15–60 minutes, excluding sample preparation and method validation.
• Question: Can irregular, flaky, or acicular particles be tested? Answer: Yes. The reported result is the Stokes equivalent sphere diameter, meaning particles of any shape are characterized by the diameter of a sphere that settles at the same velocity. For highly anisometric particles, sedimentation data may differ systematically from image-based or diffraction-based results, and this should be considered when comparing methods.
• Question: Which standards govern sedimentation particle size testing? Answer: The principal standards are ISO 13317 (gravitational liquid sedimentation, including the pipette, X-ray, and balance methods), ISO 13318 (centrifugal liquid sedimentation), ASTM B761 (metal powders by X-ray gravitational sedimentation), ASTM D7928 (hydrometer analysis of fine-grained soils), ISO 14887 (dispersion preparation), and ISO 9276 and GB/T 15445 for the presentation of results.
• Question: Can water-sensitive or water-soluble powders be analyzed? Answer: Yes. Ethanol, isopropanol, glycol, or other non-aqueous media can be selected, with the liquid density and viscosity entered into the calculation. Method development is performed to confirm dispersion quality and settling stability in the chosen medium.
• Question: How is result quality assured? Answer: Accredited third-party laboratories verify instrument performance with certified reference materials, control suspension temperature and concentration, verify dispersion by repeated measurement, and report full measurement conditions so that results are reproducible between laboratories.