Microtrac S3500, Operation And Maintenance Manual, Page: 27 / 38

Service
9/03 Microtrac S3500 Particle Size Analyzer Operation and Maintenance 21
If the background increases beyond a preset level, the software provides a “high background” warning at
the completion of the SETZERO function. In particular, when you analyze particles smaller than five
microns, you must clean the sample cell regularly. The fine particulate can raise the background level
when it adheres to the cell windows. The procedure to clean the sample cell appears in the maintenance
section of this manual.
3.4.2 Run Data Parameters
A common requirement in optical particle sizing is to provide certain basic information about the particles
being measured. With the correct information, the measuring instrument can extract the particle size
distribution from the measured optical response.
The S3500 measures particle size through the analysis of light scattered from particles into angular
measurement zones. The size range of the instrument is divided into N intervals, each represented by an
angular pattern response function. The shape of the response function determines the particle size. The
particle properties selected by the user direct the instrument to the correct set of responses to perform its
size analysis.
Once a measurement has been made, the particle properties can be changed and the measurement
recalculated if necessary. This eliminates the need to re-run samples.
Spherical
The most important of the choices is whether a particle is spherical or non-spherical. Spherical particles
have the largest variation in scattering amplitude and response shape due to the ability of the sphere to
exhibit optical resonance. Such resonance is caused by interference between the light transmitted through
and scattered around the particle. To be considered spherical, particles are usually formed by a process or
under a condition that forces spherical shape. Liquid in liquid emulsions, polystyrene spheres, and solid
glass spheres are a few examples.
Irregular
Most particles are non-spherical, including ground or crushed material, crystalline material, ceramic and
crystallized materials. The optical resonance disappears for the non-spherical particle so the variation in
scattering is less extreme than with the spherical particle. This empirically-modified variation of classical
Mie (laser diffraction) theory accounts for the non-spherical shape and the absence of the resonance.
Therefore, if a particle is irregular, but the spherical model is chosen, there will be an under-reporting of
particles in the resonance region, and the size range will be shifted. The converse is true if the irregular
model is chosen for a sphere that has a strong resonance. This will report volume of smaller size where no
particles are present. The magnitude of the problem changes as the resonance function varies greatly with
size and refractive index.
Transparent
The choice of transparent or opaque is also important. An opaque particle has no light transmitted through
it; as a result, the spherical and non-spherical distinction is not necessary. If the material is opaque, the
distinction between reflective and absorbing can be made. Metal powders are considered reflective, while
carbon black, coke, and coal are examples of absorbing particles. The choice between reflective and
absorbing is of secondary importance compared to the other choices.
For transparent particles, the selection of index of refraction of the particle and medium is also required to
complete the description of the particle properties. These parameters determine the response function and
the resonance pattern location and magnitude.
Refractive index data can be found in Microtrac documents SI-RT-01 “Fluid Reference Table” and SI-RT-
02 “Particle Refractive Index Table”, or located in the
Handbook of Chemistry and Physics published by
CRC Press, Inc., Library of Congress Card Number 13-11056, 57th edition or later.