Dry Sieving

• Why Particle Size Test Results Change: Dispersion, Agglomeration and Sieving Conditions

  

  Oct 26, 2026

  Particle size is often treated as a fixed property of a powder. In practice, however, the reported result also depends on the physical state of the sample and the method used to measure it. A powder tested in an agglomerated dry state may behave differently after liquid dispersion. Fine particles affected by static electricity may remain on a test sieve even when their dimensions would otherwise allow them to pass. Changes in sample preparation, sieving time, dispersion conditions, or measurement principle can also affect the reported particle size distribution. This means that when two laboratories test the same material and obtain different results, neither laboratory has necessarily made an error. For QC and procurement engineers, the more useful question is not simply: “What is the particle size?” It is: “Under what sample condition and test method was that particle size measured?” Why Do Particle Size Test Results Change? Particle size results can change because the powder state and test conditions change. Agglomeration, dispersion, static electricity, wet or dry testing, sample preparation, test duration, and measurement principle can all affect the reported particle size distribution. Results should only be compared when the test method and sample conditions are controlled. When Agglomeration Makes Fine Powder Look Coarser Fine powders do not always behave as independent particles. Moisture, electrostatic attraction, surface forces, and handling conditions can cause smaller particles to associate with one another. When this happens, a test may respond to the agglomerate rather than to the individual particles inside it. In mechanical sieving, an agglomerate may remain above the sieve opening even though the individual particles would be small enough to pass. The retained fraction may therefore contain both genuinely oversized particles and clusters of smaller particles that have not been sufficiently dispersed. For supplier qualification, batch comparison, or incoming inspection, sample preparation and dispersion conditions should be controlled before numerical results are compared. When Dispersion Changes the Measured Distribution Dispersion can also change what the measurement system sees. A recent Malvern Panalytical application study used time-resolved laser diffraction to monitor spray-dried alumina granules during ultrasonic dispersion. As dispersion progressed, a coarser particle population decreased while a finer population became more pronounced. The important point is not the individual particle-size values. It is that the measured distribution changed as the physical state of the sample changed. Ultrasonic energy, liquid media, mixing conditions, and dispersion time may separate agglomerates or otherwise alter the particle population presented to the measurement system. This creates a practical laboratory question: At what point does dispersion reveal the particle state relevant to the application, and at what point does additional energy begin to alter that state? There is no universal answer. Different materials can respond differently to the same preparation procedure. For this reason, particle size methods should specify not only the instrument, but also the sample preparation and dispersion conditions. When Static Electricity Distorts Dry Sieving Dry sieve analysis introduces another important source of variation: electrostatic charge. Fine particles may adhere to the sieve mesh, sieve frame, container surfaces, or other particles. This can make an electrostatic problem look like a particle-size problem. If fine powder remains above the sieve, the immediate conclusion may be that those particles are larger than the sieve opening. But some particles may actually be small enough to pass and remain on the mesh because of static attraction or loose agglomeration. Increasing vibration does not necessarily eliminate the underlying electrostatic interaction. For laboratory sieve analysis, visible adhesion to the mesh or frame should therefore be treated as a warning that the retained fraction may contain more than genuinely oversized particles. This is especially relevant for fine polymer powders, pharmaceutical materials, chemical powders, and other materials prone to electrostatic charging. Why Wet Sieving Can Produce a Different Result Wet sieving can reduce problems associated with agglomeration, static electricity, or persistent mesh blockage. But wet sieving is not simply dry sieving with liquid added. The liquid becomes part of the test method. A suitable medium may improve dispersion, reduce static effects, and help fine particles pass through the sieve. At the same time, it may also alter the sample. Depending on the material and liquid, particles may dissolve, swell, soften, react chemically, or experience changes in surface behavior. A controlled wet-sieving method should therefore define the liquid medium, sample preparation, dispersing agents, rinsing procedure, drainage conditions, test duration, and endpoint criteria. The objective is not simply to make more material pass through the sieve. The laboratory should first determine whether high residue in dry sieving represents genuinely oversized particles or poor particle dispersion. For applications affected by static electricity, agglomeration, or wet-sieving conditions, our laboratory sieve shaker selection guide explains how these factors should be considered during equipment selection. Why Sieving Time and Endpoint Criteria Matter Sieving time can also change the reported result. Consider two laboratories testing the same powder with the same nominal sieve aperture. One runs the sieve shaker for five minutes. The other runs it for fifteen minutes. If material continues to pass through the sieve after the first five minutes, the two laboratories are not performing equivalent tests. Longer sieving may give particles more opportunities to reach an opening, change orientation, or separate from weak agglomerates. For this reason, instructions such as: “Sieve until it looks finished.” are unsuitable for comparative QC. When sieve analysis is used for batch release, supplier acceptance, or interlaboratory comparison, test duration or endpoint criteria should be defined in the SOP. The objective is reproducibility, not simply reducing the amount of material left above the sieve. Why Different Particle Size Methods Do Not Always Agree If two particle size reports disagree, one of the first questions should be whether the laboratories used the same measurement principle. A result expressed in micrometers does not necessarily represent the same physical quantity across different methods. Mechanical sieving determines whether a particle can pass through an opening of defined geometry under specified mechanical conditions. Laser diffraction evaluates light-scattering behavior and calculates an equivalent particle size distribution using an optical model. Dynamic image analysis evaluates projected particle images and can provide information related to both size and shape. These differences become especially important for irregular particles. An elongated particle may remain above a sieve when lying flat but pass through the same opening when it approaches at a different angle. Laser diffraction may report an equivalent diameter, while image analysis may describe length and width separately. The key point is: The same unit, μm, does not mean that different methods are measuring exactly the same physical quantity. This is why results generated by different techniques should not be compared without considering the measurement principle. Before Comparing Two Particle Size Reports, Check These 9 Conditions Before recalibrating an instrument, rejecting a batch, or questioning a supplier’s data, first determine whether the laboratories performed comparable tests. 1. Sample SourceWere the samples taken from the same batch using comparable sampling procedures? 2. Sample ConditionWere moisture level, storage conditions, and handling history comparable? 3. Sample PreparationWere drying, mixing, dividing, or other preparation procedures controlled? 4. Agglomeration and DispersionWas the material tested in a comparable and controlled physical state? 5. Measurement MethodDid both laboratories use the same analytical principle? 6. Wet or Dry ConditionsWas the powder tested under comparable environmental and dispersion conditions? 7. Sample QuantityWas a comparable amount of material introduced into each test? 8. Test DurationWere the same sieving or dispersion times used? 9. Endpoint CriteriaDid both laboratories use the same rule to determine when the test was complete? If these conditions differ, the reported particle size values may not be directly comparable even when both laboratories are operating their equipment correctly. How to Select Equipment Without Changing the Test Method The same logic should guide laboratory equipment procurement. A laboratory sieve shaker should not be selected first and then used to define the test method afterward. A more defensible sequence is: Understand powder behavior → Define the QC objective → Select the test method → Establish test conditions → Select equipment capable of repeating that method For free-flowing materials, conventional dry sieving may be appropriate. For powders affected by static electricity or agglomeration, sample preparation may need to be reconsidered, or wet sieving may need to be evaluated. For irregular particles, the mechanical motion of the sieve shaker can become part of the method. For very fine powders or applications requiring detailed particle size distributions, another analytical technique may be more suitable than mechanical sieving alone. Particle size data become useful when the conditions behind the numbers are understood. For procurement, R&D, and QC teams, reliable comparison requires more than matching the unit “μm” on two reports. It requires comparing the sample condition, test method, and operating procedure behind those numbers. When these conditions are controlled, particle size data become far more useful for batch comparison, supplier qualification, incoming quality control, process development, and material acceptance decisions.

  THẺ HOT : Particle Size Analysis Sieve Analysis Powder Agglomeration Powder Dispersion Wet Sieving Laboratory Sieve Shaker Dry Sieving Ultrasonic screening

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