Page 184 - Instrumentation Reference Book 3E

168 Particle sizing

11.9 Analysis methods that infer bles can cause false signals. Aqueous or organic
size from some other property electrolytes can be used. It is usual to calibrate the
instrument using a standard sample of latex poly-
The methods discussed so far either measure size mer particles. The technique permits several runs
directly or measure a fluid-dynamic effect depend- on each sample/electrolyte mix and it is easy to test
ent on the particle terminal velocity which, the effect of changes of concentration, which
although dependent on size, is also affected by should be negligible if dilution is adequate.
density. shape, and surface structure. The follow- Models are available with 16-channel reso-
ing methods do not measure size directly nor are lution, and the output can be in tabular form
they dependent on terminal velocity. giving particle frequency volume or particle
cumulative volume or can be in the form of an
automatic plot.
11.9.1 Coulter counter
11.9.2 Hiac automatic particle sizer
The Coulter counter uses the principle that the
electrical resistance of a conducting liquid is The Pacific Scientific Hiac-Royco analyzer, now
increased by the addition of an insulating mater- manufactured by a Danaher division, can be con-
ial. Particles are assessed individually. To obtain sidered to be the optical equivalent of the Coul-
adequate sensitivity the volume of liquid meas- ter. In this, the cylindrical orifice is replaced by a
ured must be similar to the volume of the particle. two-dimensional funnel-shaped orifice which
These criteria are achieved by containing the guides the particle stream through a very narrow
electrolyte in two chambers separated by a nar- light beam located at its throat. A sensor meas-
row channel containing an orifice, the dimensions ures the obscuration caused by each particle as it
of which are accurately known. An electric current passes through. The responses are proportional to
from a constant-current source passes through the the particle cross-sectional areas and are sorted by a
orifice from one chamber to the other. The voltage pulse-height analyzer. A range of orifices is avail-
across the orifice is therefore directly proportional able to suit particle sizes from 2 pm up to 9 mm.
to the resistance of the orifice. each covering a size range in the ratio 3O:l.
The sample, suitably dispersed, is placed in one Although the measurement on a given particle is
of the chambers. An accurately controlled along one axis, an irregularly shaped particle will
volume of the well-agitated electrolyte is then not tend to be orientated in any particular way so
passed through the orifice. The concentration of that statistically the area measured will be a mean
the sample (of the order of 0.1 percent) is such cross-sectional area. The optical method has an
that particles pass through individually. Each advantage over the conductivity method in that it
particle causes a voltage pulse, and a pulse-height can operate in any liquid or gas provided it is
analyzer increments one of a set of counters, each translucent over the very short optical path
representing a size maximum. length (typically 2-3 mm of fluid). Scattering-
The theory of the Coulter counter is compli- coefficient problems are reduced by calibration
cated, particularly for randomly shaped particles, using standard samples. The instrument has been
but it has been shown that, to a first approxima- used “on-line” to measure the contamination of
tion, the pulse height is directly proportional to hydraulic fluid. The number of particles in a fixed
particle volume, errors being less than 6 percent volume is found by counting while timing at a
when the particle size is less than 40 percent of the constant flow rate.
orifice diameter. This size limitation also repre-
sents a reasonable practical limitation to avoid 11.9.3 Climet
blockage of the orifice. Although the resistivity of
the particles should affect the result, in practice, The Climet method involves measuring the light
surface-film effects make this insignificant. The scattered from individual particles which are
method also works with conducting particles. directed accurately through one focus of an ellip-
The lower limit on particle size is set by the tical mirror. Light is focused onto the particles
electronic noise in the circuit and in practical terms and it is claimed that about 90 percent of the
is usually taken to be about 4 percent of the orifice scattered light is detected by a photomultiplier
diameter. Orifices are available ranging in size at the other focus of the ellipse. Direct light is
from 10 pm up to 1 mm giving a particle-size range masked. The response is pulse-height analyzed.
(using different orifices) from 0.4 LLm to 400 pm.
Samples containing a wide range of sizes need to be 11.9.4 Adsorption methods
wet-sieved to remove those larger than 40 percent
orifice size. With small sizes, as always, care In some processes, a knowledge of the surface area
has to be exercised to avoid contamination. Bub- of particles is more important than the actual size.