Many samples - due to their material characteristics - require a special preparation for the measurement. For example can static charges (for example plastics), adhesion forces or cohesive forces (for example clays, soil samples, kaolins), magnetism and hydrophobic characteristics (for example pharmaceutical products, drugs, toner, graphite, titanium oxide) or coagulation (for example clays, kaolins, chalk and gypsum), complicate the dispersion and measurement.

When dispersion is necessary in order to measure the primary particles, generally two different dispersions are possible: the wet dispersion and dry dispersion.

Wet dispersion

During a wet dispersion the sample material is added to a closed liquid circuit and continuously pumped through the measurement cell. During the pumping process in the measuring circuit, ultrasound can be added in order to break up the agglomerates and obtaining single and separated particles. Unproblematic samples that submerse directly in water without significant effort and do not have large fines, are added to the dispersing unit of the ANALYSETTE 22 as a solid, portion by portion, using a spatula and can be measured reproducibly after a brief ultrasound exposure / dispersion (30 - 60 s). Prior to a measurement, especially for strongly bonded agglomerates, a treatment of the sample, with ultrasound is often helpful. For the wet dispersion, if possible should the selected liquid wet the material spontaneously and completely.

Certain materials cannot be measured in liquids at all or with great difficulty, because these dissolve, a chemical reaction occurs or they swell – like for example wheat. In these cases a dry dispersion is an alternative.

Dry dispersion

With a high-frequency feeder the sample material is continuously transported over the funnel-shaped opening of the dry measuring cell and then drops in the actual dispersion area. In an annular gap Venturi nozzle the powder is accelerated and passes with high speed through the nozzle channel and the measurement of the particle size distribution in the laser beam takes place directly behind it. Then it passes a zone with aerodynamic wave generation at the nozzle outlet. The by the high turbulent current caused strong shearing forces, as well as blows between the particles, lead to the breakdown of the agglomerates. No impact areas in the flight path of the particles exist in order to prevent soft sample materials from being ground during the dispersion process. 

FRITSCH-Know-How – For an exact measurement should:

  • Solids be  separated and free of agglomerates
  • During the entire measurement the dispersion condition should be stable, without coagulation or flocculation
  • Floating particles on the liquids surface should be avoided
  • The sample should not be destroyed respectively comminuted during the dispersion
  • Ideally, the carrier liquid should have a lower weight (density) than the solid