A particle-size value needs a measurement method. An FSSS result, a BET surface area and a laser Dv50 describe different aspects of a powder. Compare like-for-like results and agreed sample preparation before deciding whether a lot meets your specification.
What FSSS Measures
The Fisher Sub-Sieve Sizer uses airflow through a packed powder bed to estimate surface area and an equivalent spherical diameter. It reports one estimated size, not a particle-size distribution.
ASTM B330-20(2024)e1 describes air-permeability methods and their limits. It lists 0.5–50 μm for FSSS and 0.2–75 μm for MIC SAS. These are method/instrument ranges, not interchangeable accuracy guarantees.
The calculation assumes uniform, smooth, nonporous spheres. Results are estimates useful for relative control; their relationship to performance depends on the application. Agglomeration and whether the sample was tested as supplied or after agreed de-agglomeration matter.
The standard excludes powder mixtures and powders containing binders or lubricants. Do not apply a raw WC powder result directly to a mixed, wax-containing RTP product.
ISO 10070:2019 also covers air permeability. It explains that the calculated surface reflects paths reached by the flowing gas, omitting closed or blind pores. It is not an absolute measurement, and comparing laboratories requires matching test parameters.
| Method | Reported quantity | Information to retain |
|---|---|---|
| FSSS / air permeability | Estimated average equivalent diameter, usually μm | Instrument, procedure and sample condition |
| BET gas adsorption | Specific surface area, usually m²/g | Gas, sample preparation and analysis method |
| Laser diffraction | Size distribution, commonly volume-based | Distribution basis, optical settings and dispersion procedure |
These quantities complement one another. A surface-area-derived diameter is not automatically equal to a distribution median. Agree on which quantity determines acceptance before comparing numbers from two suppliers, and retain the measurement conditions with each result.
BET Is a Surface-Area Measurement
ISO 9277:2022 describes determining specific surface area from gas adsorption. The gas can probe external and accessible internal surfaces. Reporting BET in m²/g preserves what was measured.
A diameter calculated from BET adds a geometric model. For identical, smooth, nonporous spheres, surface area divided by mass is 6/(density × diameter). With surface area S in m²/g and density ρ in g/cm³, the corresponding diameter in μm is d = 6/(S × ρ). This follows from sphere geometry; it does not establish a conversion between BET and FSSS for real powder.
If a supplier reports a BET-derived size, request the density and assumptions as well as the measured surface area. A fixed BET range should not be inferred from a grade name alone.
Reading Laser Dv10, Dv50 and Dv90
In a volume-based distribution, Dv10, Dv50 and Dv90 are the diameters below which 10%, 50% and 90% of the measured particle volume falls. Dv50 is the volume median, not a count-based median. Malvern Panalytical's explanation of particle-size specifications shows why distribution basis and choice of statistic change the result.
Record the dispersion procedure and whether clusters remain in the tested sample. Do not use a universal multiplier to convert FSSS to laser Dv50, or assume that one method must always read higher. If a conversion is needed for a particular process, establish it from paired measurements of representative lots and document its limits.
Agree on the Measurement Before the Order
A practical request to a supplier should identify:
- The exact powder grade and lot, with the required measured quantity and units.
- The test procedure and edition, instrument, and sample preparation.
- Whether the sample is as supplied, de-agglomerated or otherwise processed.
- The acceptance limits and the agreed approach when laboratories disagree.
- Any additional surface-area or distribution measurements needed for the application.
Ask for the actual lot certificate. Agree on chemistry and impurity limits separately using the product specification; this guide does not define universal carbon, oxygen or BET acceptance limits.
Use FSSS for a requirement written in FSSS, BET for a surface-area requirement, and distribution measurements when the spread of sizes matters. A different number from a different method is a reason to inspect the method and sample condition before accepting or rejecting the powder.