Three particle sizing techniques give three different answers for the same powder, and none of them is wrong. What each one actually measures, and how to write a specification both laboratories can meet.
A cement plant and its customer's laboratory report the same consignment differently. The plant sieves and reports 9 per cent above 45 micrometres. The customer runs laser diffraction and reports 14 per cent. Both are competent laboratories, both instruments are in calibration, and the argument that follows usually ends with one side buying the other side's equipment. It should not, because the two numbers were never going to be the same.
Particle sizing does not measure a length. It measures a property and converts it into an equivalent diameter, and each technique picks a different property. Once that is understood, the disagreement stops being a fault and becomes a fact to specify around.
For genuinely spherical, narrowly distributed powder the three agree closely. For flake, needle, fibre or angular material they diverge, and the more elongated the particles the wider the gap. Milled minerals, ground coffee, fly ash, cement, pigments and most pharmaceutical actives all fall on the awkward side of that line.
That is why an argument between a sieve result and a laser result is not settled by recalibrating either instrument. It is settled by naming the technique in the specification, and where the two have to be reconciled, by an instrument such as the SYNC combined laser diffraction and image analyser that measures both on one sample.
Sieving is the oldest technique in the laboratory and it is treated as though it needs no attention. Four things decide whether one laboratory's sieve result matches another's, and only one of them is the shaker.
The motion. A vibratory shaker throws material vertically with an angular momentum so it is redistributed across the whole mesh. The AS 200 control: Vibratory Sieve Shaker holds sieve acceleration independent of mains frequency, which is precisely the reason it reproduces between laboratories, and stores 99 standard operating procedures. It also carries a test certificate and can itself be calibrated, which is the line an auditor reads. See the AS 200 control sieve shaker for the full specification. In Bangladesh, where supply frequency and voltage both wander, that independence is not a marketing line. The AS 200 digit cA: Vibratory Sieve Shaker gives electronic amplitude control so the same amplitude is held whatever the mass of the stack, and the AS 200 basic: Vibratory Sieve Shaker sets amplitude on an analogue panel for routine grading where reproducibility between sites is not the point.
The frame size. Sample mass drives this. The AS 300 control: Vibratory Sieve Shaker takes 305 mm and 315 mm sieves for aggregate and construction material, and the AS 450 control: Vibratory Sieve Shaker drives stacks up to 25 kg on 400 mm and 450 mm sieves across a range from 25 micrometres to 125 mm, with the AS 450 basic: Vibratory Sieve Shaker as the stepped amplitude alternative for high volume work. Aggregate laboratories in Bangladesh running to construction standards live in this size class rather than the 200 mm one, and the same discipline about documented calibration applies as in concrete and cement testing.
The particle shape. Needle shaped, flat, long and fibrous particles separate badly under a throwing motion because they present their long axis to the mesh at random. The AS 400 control: Horizontal Sieve Shaker moves the stack in a circular motion in the horizontal plane instead, which is the correct machine for those materials and for abrasives, up to 400 mm diameter. Where a national standard specifies tapping rather than vibration, the AS 200 tap: Tap Sieve Shaker superimposes a vertical tap on a horizontal circular motion and mechanises what would otherwise be hand sieving.
The sieves themselves. This is where most laboratories lose. A woven mesh stretches, blinds and eventually tears, and its effective aperture changes long before the damage is visible. Woven Wire Mesh Test Sieves, 200 mm / 203 mm diameter are supplied to ISO 3310-1 and ASTM E11 with inspection, calibration or certified quality options, and Perforated Plate Test Sieves to ISO 3310-2 and ISO 5223 cover the coarse fractions where woven cloth would distort. Keeping them honest is a documented activity: SieveCare: Test Sieve Cleaning, Inspection and Recalibration Service covers ultrasonic cleaning, optical inspection of the mesh and recalibration against the standard, and the UR Series: Ultrasonic Baths handle routine cleaning between samples without the mechanical damage that brushing a mesh causes.
Two more items decide whether the number is representative at all. A sample taken badly cannot be rescued by a good sieve stack, so the PT 100: Sample Divider and its larger relatives, or the unpowered RT 6.5 to RT 75: Sample Splitter riffle splitters working to DIN 51701, belong in the same purchase. And the transcription step from balance to report is where sieve results are corrupted; EasySieve pro / EasySieve pharma: Sieve Analysis Evaluation Software reads weights straight from a connected balance, controls the shaker and produces the grading report without anyone typing a number.
Below roughly 50 micrometres, cohesion beats gravity. Fine powder agglomerates, sits on the mesh as clusters, and a vibratory shaker reports oversize that does not exist. Adding time makes it worse, because the agglomerates compact.
Air jet sieving solves it mechanically. The AS 200 jet: Air Jet Sieving Machine blows a jet of air upward through the mesh from a rotating slotted nozzle to disperse the sample, while a vacuum draws the undersize away. Because the undersize is removed continuously, each cut is made on a single sieve rather than in a stack, so an air jet analysis is a sequence of single sieve runs rather than one stack run. The AS 200 jet pro / AS 200 jet pharma: Air Jet Sieving Machines combine sieving and weighing in one instrument so the sieve is not removed between fractions, and the pharma version adds audit trail functionality for regulated laboratories.
Two consequences follow that are worth knowing before purchase. Air jet sieving needs a vacuum source of adequate capacity, and it is noisy. And because the cut is made one sieve at a time, a full distribution takes longer than a stack run, which is a fair trade for a result that is real.
Sieving stops being practical when the fraction of interest is finer than the finest usable mesh, when the sample is a suspension, or when a distribution is needed in seconds rather than in a working morning. That is the territory of the Microtrac range.
| Instrument | Principle | Range and note |
|---|---|---|
| S3500 | Laser diffraction using three positioned red laser diodes rather than one beam | 0.02 micrometres to 2.8 mm, in ten to thirty seconds, reporting volume, number and area distributions |
| BLUEWAVE | Laser diffraction adding two blue 405 nm lasers to the red source | 0.01 micrometres to 2.8 mm; the shorter wavelength improves resolution at the fine end |
| SYNC | Laser diffraction and dynamic image analysis on the same sample stream at once | 0.01 to 4000 micrometres, working to ISO 13320 and ISO 13322-2 together |
| CAMSIZER X2+ | Dynamic image analysis with air pressure, gravity and wet dispersion built in | 0.9 micrometres to 8 mm at 0.9 micrometres per pixel, shape to ISO 9276-6 |
| CAMSIZER XL | Dynamic image analysis for coarse material | 160 micrometres to 135 mm, which reaches into the range normally handled by sieving |
| NANOTRAC WAVE II | Dynamic light scattering with zeta potential | Size and charge in one benchtop measurement, for colloids and formulations |
The instrument that settles arguments is the SYNC: Combined Laser Diffraction and Dynamic Image Analyser, because it produces both a diffraction distribution and a shape distribution from one measurement, and shape is the reason the sieve and the laser disagreed. Where the material is coarse, the CAMSIZER XL: Particle Size and Shape Analyser covers 160 micrometres to 135 mm and will produce size and shape data on a few kilogrammes of aggregate or fertiliser in minutes, which is directly comparable with the sieve grading a laboratory already reports, and is fast enough to be used as a check on it.
Control and reporting sit in one place. DIMENSIONS: Measurement and Evaluation Software handles setup, calculation and reporting across the Microtrac size and shape analysers and presents diffraction and image data in one environment, which is what makes the SYNC's dual result readable rather than two files to reconcile.
A particle size clause that only names a number will be disputed. One that names the method will not.
That last step takes an afternoon and prevents the same dispute recurring for years. It is also the only honest way to reconcile a plant that sieves with a customer who does not, which is the situation most Bangladeshi cement, ceramic and fertiliser producers are actually in.