Two suppliers quote the same powder as "200 mesh". The first feeds cleanly through a loss-in-weight screw; the second bridges in the hopper every second shift. Nothing in the quotation explains the difference: it never stated the size. Mesh is a nickname for an aperture, a particle size report is a distribution, and the figure most suppliers print is one point cut from that distribution. Once a buyer knows which of the three numbers is in front of them, most powder complaints turn into specification gaps rather than supplier failures.

One Word, Three Different Sizes

A molecular sieve (CAS 1318-02-1) carries three sizes on one datasheet. The 4A grade names a pore opening near 0.4 nm, and that is the number deciding whether water is adsorbed and ethanol is not. The bead diameter, typically 1.6 to 3.2 mm, decides the pressure drop across the bed. Ask for "small beads" and you get more pressure drop for the same separation; ask for "4A powder" and the chemistry is right but the bed blinds. For a sieve, the pore is the product and the granule is the packaging.

What a Mesh Number Actually States

Mesh counts openings per linear inch of wire cloth, so the number is a shortcut to an aperture table: 100 mesh is about 149 microns, 140 about 105, 200 about 74, 325 about 44. ASTM E11, ISO 565 and the older Tyler series assign slightly different apertures to the same number, which is why a serious specification writes the micron figure next to the mesh and names the standard.

More important, "passing 200 mesh" is a ceiling, not a centre. It says the lot went through a 74 micron opening; it says nothing about how much sits below 10 microns. A classified grade with a median near 55 microns and a jet-milled grade near 18 microns can both claim 100 percent through 200 mesh, and they behave differently in every feeder, blender and press. Below roughly 45 microns dry sieving also stops being reliable: fines agglomerate, static pins them to the cloth, and screens blind.

Laser Diffraction Measures a Different Body

Laser diffraction reports the volume-weighted distribution of spheres that scatter light the way the particles do. It is fast, covers 0.1 to 3,000 microns and supplies almost every modern D50 figure, but refractive index, obscuration, dispersion pressure and sonication time all move the result. For an agglomerated powder extra sonication breaks clusters and pulls the median down by microns. When nano calcium carbonate (CAS 471-34-1) is quoted at 20 to 100 nm with 15 to 50 square metres per gram, what ships is a cluster of primary particles, and the dispersion step does as much work as the mill did.

Sieve analysis is mass-based and orientation-sensitive, so it disagrees with laser in a predictable direction. A flake passes only if it lands edge-on, a needle if its second-largest dimension is small enough, so platelets and acicular crystals read coarser by sieve than their volume distribution suggests. Kaolin clay (CAS 1332-58-7), mica and most pigments sit in that category. When two certificates disagree by a factor of two, the question is not who is wrong but which method produced each figure.

D50 Is One Point; Span Is the Shape

A median without a spread is half a specification. Report D10, D50 and D90, then compute span as D90 minus D10 divided by D50. Two lots of microcrystalline cellulose (CAS 9004-34-6) with the same 40 micron median behave differently at a span of 0.9 and 2.2: the tight lot flows and compresses predictably, the wide lot segregates and dusts at the feed port. Our MCC spans 20 to 200 microns because direct compression, granulation and cosmetic dusting want different windows from one chemistry. The same logic explains why crospovidone (CAS 25249-54-1) comes as a coarse and a fine type: identical cross-linked polymer, identical disintegrant mechanism, different tablet surface and mouthfeel, chosen by particle size rather than assay.

Surface Area Is Not Particle Size

The most expensive misreading in powder buying is treating a surface area figure as a size. Silica gel (CAS 112926-00-8) is quoted at 300 to 800 square metres per gram. It is a 3 to 7 mm granule whose area lives inside a pore network of 2 to 50 nm, and that internal area is the point of a desiccant. Read the figure as a particle size and you order a powder that dusts, cakes and channels instead of a bead that dries a drum of solvent.

Pigments sit in between. Pigment Yellow 42 (CAS 51274-00-1) is a synthetic iron oxide whose primary crystals are submicron, while the powder on the pallet is an agglomerate several microns across. Colour strength depends on how much dispersion energy breaks those agglomerates, not on the D50 printed on the bag.

Bulk Density, Flow and the Freight Bill

Skeletal and bulk density are different numbers, and buyers pay for the confusion. Barium sulfate (CAS 7727-43-7) has a true density near 4.3 grams per cubic centimetre, yet a precipitated grade shovels at roughly half that in bulk. Light powders hurt twice: microcrystalline cellulose at 0.26 to 0.31 puts a 25 kilogram bag at 80 to 95 litres, so a hopper sized for a tonne holds a third of it and the freight bill is mostly air.

Flow follows from the same data. The Hausner ratio, tapped density divided by bulk density, separates free-flowing powders below about 1.25 from cohesive ones above 1.4, and an angle of repose past 40 degrees predicts bridging. Fines are the usual cause, and they matter beyond flow: particles under 10 microns are the respirable fraction, and combustible organic fines from hydroxypropyl starch (CAS 9049-76-7), cellulose or activated carbon (CAS 64365-11-3) bring dust-explosion duties a coarse grade avoids. Where fines are genuinely needed, sodium bicarbonate (CAS 144-55-8) for flue-gas injection or lactose (CAS 63-42-3) for blend uniformity, buy a classified cut with a stated top size.

Seven Lines for a Powder Enquiry

First, the method and its conditions: laser dry or wet, dispersion pressure, sonication time, or sieve standard and duration. Second, D10, D50 and D90 with the span. Third, the top cut as a percentage through a named sieve, plus a fines limit under 10 microns. Fourth, bulk and tapped density, with the Hausner ratio if flow is critical. Fifth, moisture or loss on drying measured at the same time, because agglomerates grow with water and a damp lot sieves coarse. Sixth, surface area with its method, only where area drives performance. Seventh, form and history: milled, jet-milled, spray-dried, classified or coated, since two powders with one median differ in roundness and porosity, and so in PMMA bead flow or dusting. A supplier who answers these seven lines is quoting a product; one who answers with a mesh number is quoting a habit.