What FSSS stands for and how it is measured
F.S.S.S. stands for Fisher Sub-Sieve Size — named after the Fisher Scientific Model 95 Sub-Sieve Sizer instrument developed in the 1940s. The measurement principle is air permeability: a known mass of powder is packed into a cylindrical tube at a standardised density, and air is forced through the powder bed at controlled pressure. The resistance of the packed bed to air flow depends on the total surface area of the particles — and from that surface area measurement, the instrument calculates an equivalent average particle diameter assuming all particles are spheres.
The method is standardised under ASTM B330 (standard test method for estimating average particle size of metal powders) and is referenced in ASTM B430 for tungsten powder and ASTM B777 for tungsten heavy alloy powder. The Fisher instrument or its modern equivalent (the Micromeritics Sub-Sieve Autosizer, which automates the same air permeability principle) is standard equipment in every tungsten powder production facility.
Why FSSS is used — and what it actually measures
FSSS is popular in the tungsten powder industry for three practical reasons: it is fast (a measurement takes 3–5 minutes), reproducible between instruments and labs if calibrated against NIST reference standards, and it has been used since the 1940s so there is a large base of historical data correlating F.S.S.S. values with sintered carbide properties.
What FSSS actually measures is not a particle size distribution — it is a single-point average equivalent sphere diameter derived from the powder bed's surface area. This means it is sensitive to the finest particles in the distribution (which contribute disproportionately to surface area) and is less sensitive to large particles or agglomerates. A powder with a broad size distribution can have the same F.S.S.S. as a narrowly distributed powder — the two will behave differently in pressing and sintering, but the F.S.S.S. number alone will not reveal this.
F.S.S.S. ranges and what they mean for tungsten powder
For tungsten metal powder (W, reduced from tungsten oxide by hydrogen): F.S.S.S. 1–3 µm is used for copper-tungsten electrical contact infiltration, where fine particles improve densification. F.S.S.S. 3–6 µm is used for tungsten heavy alloy (WHA, W-Ni-Fe) pressed and sintered components such as kinetic energy penetrators, radiation shields, and vibration damping applications. F.S.S.S. 8–15 µm is used for larger WHA counterweight and shielding components where green strength of the pressed compact is more critical than maximum sintered density.
For tungsten carbide powder (WC, carburised from W): F.S.S.S. 0.6–1.5 µm (sub-micron) is used for hardest micro-grain cemented carbide grades. F.S.S.S. 2–4 µm (fine) is the standard for cutting inserts and most tooling. F.S.S.S. 4–8 µm (medium) is used for mining and drilling grades. F.S.S.S. 8–15 µm (coarse) is used for rock drill buttons and heavy-duty wear parts.
FSSS vs. laser diffraction (PSD) — which to use?
Modern powder characterisation often uses laser diffraction particle size distribution (PSD, also called PSA) instruments — reporting D10, D50, D90 values — alongside or instead of FSSS. Laser diffraction gives a complete picture of the particle size distribution, which is important for understanding pressing behaviour and sintering shrinkage variance.
For cemented carbide powder procurement, the industry standard is still FSSS — most WC and W powder specifications in ISO, ASTM, and customer-specific documents reference FSSS rather than laser PSD. Laser PSD is increasingly used as a supplementary characterisation tool, particularly for sub-micron and ultra-fine grades where FSSS measurements become less reproducible.
When comparing powders from different suppliers, ensure you are comparing FSSS to FSSS, not FSSS to laser D50 — the two numbers can differ significantly for the same powder, particularly at fine grain sizes.
How to specify FSSS in a purchase order
A complete FSSS specification for a WC or W powder purchase order should state: target FSSS value (e.g. '4.5 µm'), acceptable tolerance (e.g. '±0.5 µm' or 'min. 4.0, max. 5.0 µm'), measurement standard (ASTM B330 or ISO 10070), and calibration requirement (instrument calibrated against NIST-traceable reference material).
For critical applications, also specify that the F.S.S.S. measurement on the CoA must be from the production lot being shipped — not a retained reference sample or a nominal production value. This is standard practice at Jayanth Tungsten: each lot is measured and reported on the CoA before shipment.