The Aerosizer (Amherst Process Instruments, Inc. Hadley MA) is a time-of-Ri
ght instrument frequently used to measure the size distribution of an aeros
ol. However, if the Aerosizer's counting efficiency, defined as the number
of particles counted divided by the total number entering the instrument, i
s not 100% or varies with particle size, the resulting size distribution wi
ll be inaccurate.
Experiments were conducted to determine the effect of particle diameter, pa
rticle concentration, photomultiplier tube (PMT) voltage, and model type on
the Aerosizer's counting efficiency. To calculate counting efficiency, the
number of particles between 0.3 and 10 mu m recorded by the Aerosizer was
divided by the number of particles of the same size collected on each stage
of a cascade impactor.
Particle diameter, aerosol concentration, Aerosizer model, PMT voltage, and
the diameter interaction terms influenced counting efficiency. Counting ef
ficiencies were less than 1 % for particles smaller than 0.45 mu m, and mor
e than 100% for particles larger than 7 mu m. Increasing the PMT voltage in
creased the counting efficiency for the smaller particles, but also created
false, larger particles. Counting efficiency decreased as count rate incre
ased for count rates greater than 20,000 particles per second. The Aerosize
r LD counted particles more efficiently than the Aerosizer Mach 2 because o
f improved laser and optics systems. Four regression models that relate cou
nting efficiency to the salient operating parameters were developed, one fo
r each combination of Aerosizer model and photomultiplier tube voltage stud
ied. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.