Microtrac S3500, Operation And Maintenance Manual, Page: 24 / 38

18 Microtrac S3500 Particle Size Analyzer Operation and Maintenance 9/03
Wet Sample Delivery
A recirculator is used to disperse the material sample uniformly in a fluid and deliver it to the analyzer. In
its most basic form, a recirculator consists of a reservoir where the sample is introduced, a fluid pump, a
valve to drain the system and the necessary tubing and connections. Flow through the analyzer sample cell
is always from bottom to top. See Figure 3-2.
Reservoir
Pump
Drain
Valve
Sample
Cell
Figure 3-2
Recirculator Flow Diagram
Most Microtrac recirculators automate the deaeration process. This clears the fluid system of any trapped
bubbles. If your recirculator does not have an automatic deaeration feature, deaearate the fluid manually
before running a SETZERO or adding sample. Turn the flow pump on for a few seconds and off for a few
seconds several times until bubbles cease to rise in the reservoir when flow is off. Deaeration is an
important step because a laser diffraction analyzer will interpret bubbles as “particles” of air.
Sample is added to the reservoir either manually or by a Microtrac autosampler. Please refer to the
appropriate manual if your system is so equipped. Precise measurement of sample quantity is unnecessary.
The Microtrac application software will guide you to add a proper amount.
Dry Sample Delivery
Dry measurements are always done in an automated fashion to ensure the correct sequence of operations.
Please refer to the Microtrac software manual for details on dry sample measurements.
The two primary methods of dry sample delivery are Vibratory and Aspiration. Vibratory delivery is
generally used for materials in the 10 µm to 3000 µm size range. Aspiration is well-suited to materials in
the 0.25 µm to 500 µm size range, and to materials that are prone to agglomeration.
In vibratory systems, rate of material flow is primarily controlled by varying the stroke amplitude. An
adjustable gate is included to provide a smooth, uniform flow of sample along the trough, but it will also
have a minor effect on flow rate. Aspiration systems control rate of flow by the depth of material in the
delivery trough and the rate of travel of the aspirator nozzle. Consult the appropriate delivery system
manual for details on operation.
Dry measurements differ from wet in that each particle in the sample is presented to the analyzer only
once. In a wet recirculator the entire sample passes continuously and uniformly through the analyzer.
Recognition of the differences in sample delivery will ensure accurate results.
Vibratory sample delivery may cause stratification in samples with a broad size distribution. That is, the
vibrating action causes the finer particles to settle more rapidly through the coarse particles and therefore
be delivered sooner. This is of no concern in terms of measurement accuracy, provided the entire sample is
consumed during a measurement. The S3500 analyzer will compile the data gathered from all particles
without regard to the order in which it sees them.