TSI Instruments SIZER 3321, User Manual

Page: 49 / 101

Theory of Operation 5-9
Uncorrelated Mode
The Address PAL creates two separate addresses. One address is 10 bits
and represents the type of event (1, 3, 4), or the time-of-flight (TOF) in the
case of an event 2. And the second address is 6 bits and represents the
light-scatter intensity for an event 2. There is no second address in the
case of an event 1, 3, or 4.
Correlated Mode
The Address PAL creates one single 16-bit address. The upper 10 bits of
the address are the time-of-flight (TOF) for an event 2 or the type of event
(1, 3, 4). The lower 6 bits are the side-scatter (SS) intensity in the case of
an event 2. For events 1, 3, 4, the lower bits are zero.
Once the address(es) are created, the address PAL loads the number of
particles from that address in memory into the counting PAL. The counting
PAL then increments the number by one and stores it back to the same
address. Once the PALs are finished recording the data, the timer is reset
for the next particle.
P a r t i c l e C o i n c i d e n c e
Particle coincidence is typically defined as more than one particle in the
viewing volume of the particle counter creating a signal that causes the
counter to incorrectly classify the particles as a single, mis-sized particle.
Coincidence typically increases proportionally with particle concentration.
In the case of the Model 3321 APS
™
spectrometer; however, although
coincidence is still a problem at high concentrations, the particles are not
mis-classified. The double-crested signal processing technique allows the
processor to determine when a signal is caused by a single low scattering
particle (event 1
—see above) and when it is caused by coincidence (event
3
—see above). This means that the particle size distribution during
coincidence can be accurately measured. Coincident particles can be
detected as event 3s, but cannot be sized. Therefore they are not included
in the size distribution, but are recorded for possible concentration
corrections.