Micromeritics Elzone II 5390 Operator's Manual page 350

Elzone II 5390 Confirm
2. At a given concentration, a longer channel length increases the probability that more
than one particle (coincidence) will be in the orifice at the same time, thus producing
an erroneous signal. The probability of coincidence can be reduced to its original
value by diluting particle concentration, but this results in longer analysis times. The
additional analysis time is proportional to the amount of dilution, and both are
proportional to the amount that the orifice length was increased.
These contrary effects are commonly encountered in information theory applications
where one must make trade-offs between speed, precision, accuracy, and error rate.
Particles arrive at the entrance of the orifice in a random manner at a rate that depends
upon their concentration. Therefore, measuring the maximum number of particles per unit
of time by increasing the sample concentration, the more likely it is that two or more
closely spaced particles will merge to create a combined pulse. This pulse falsely indicates
a single particle of volume exceeding that of either particle alone. This combination may
indicate a particle volume that exceeds that of any particle that is actually present in the
sample. The general effect of such combined signals is to spread the particle size spectrum
upward toward larger sizes and to under-represent the amount of particle volume present
at the lower sizes.
In addition to these fully merged coincidence events, there also exists a partial coincidence
or near coincidence. Partial coincidence creates signals that can be mistaken for elongated
particles or which may be too difficult to interpret. Near coincidence signals are those
which involve a second pulse which occurs while the signal processing circuit has not yet
completed the processing of a slightly earlier pulse. The system typically does not detect
pulses which follow another pulse too closely. The coincidence correction feature in the
Elzone application enables you to compensate for the effects of full, partial, and near
coincidences.
E-2
Appendix E
539-42822-01 (Rev A) - Nov 2017