Microtrac S3500, Operation And Maintenance Manual, Page: 26 / 38

20 Microtrac S3500 Particle Size Analyzer Operation and Maintenance 9/03
3.3.2 Dry Sample Dispersion
Among the primary reasons to use dry sample delivery methods are to avoid the need for wet sample
preparation, and where sample materials to not permit delivery in a wet state. Dry sample dispersion
therefore concentrates on the application of energy to deagglomerate the sample.
The S3500 uses air flow to an eductor to disperse dry samples. If the particle size is large or the material
inherently disperses well, air to the eductor is not required. If dispersion is necessary, first set the eductor
gap. Loosen the knurled lock nut and turn the dry delivery cup clockwise gently until it bottoms at the 0
setting. Turn counter-clockwise to position 2 or 3 and re-tighten the lock nut. Set the eductor air pressure
to approximately 15 psi while the air is flowing. If more aggressive dispersion is needed, increase the
eductor gap and/or the air pressure as necessary. See Figure 3-3.
Eductor Set at 3 Turning Up Air Pressure
Figure 3-3
Eductor Gap and Air Pressure Settings
3.4 Particle Size Measurement
A laser beam is projected through a sample cell that contains a stream of moving particles suspended in a
liquid or air. Light rays that strike particles are scattered. The scattered light forms an angular pattern
which is measured by the photodetector arrays. The photodetector arrays measure the quantity of light
(flux) at specific angles. Electrical signals proportional to the measured light flux values are then
processed by the computer to form a multichannel histogram of the particle size distribution.
3.4.1 Background Offset
All light scattered to the arrays becomes a part of the pattern. Non-sample contributions, such as
reflections and scattering from residual contamination are compensated for by executing the SETZERO
function.
The SETZERO function causes the Microtrac Software to measure and store background level data when
there is no sample in the system. The software later automatically subtracts the background from the
sample measurement data. A SETZERO function should be performed before each new sample addition.