Page 178 - Instrumentation Reference Book 3E

162 Particle sizing

The method outlined above using a line grat- 11.7.3 Direct optical methods
icule measures the mean two-dimensional Fer&t Malvern Instruments Ltd. uses a laser and for-
diameter, the direction being fixed parallel with ward-scattering to analyze particles in suspen-
the long edge of the slide. It is equally possible sion. The parallel light from the laser passes
to measure the mean two-dimensional Martin’s through a lens, producing an intense spot at the
diameter, projected area or image shear diameter. focus. Light falling on any particle is diffracted
To obtain three-dimensional mean diameters, it
is necessary to take special steps to collect and fix and is brought to a focus in a system of Fraun-
the particles on the slide in a truly random fashion. hofer diffraction rings centered on the axis and in
the focal plane of the lens. The diameters corres-
pond to the preferred scattering angles which are
a function of the diameters of the particles. The
11.7.2.2 Sevni-uiitoi?intic niicroscope size analyzers
intensity of each ring is proportional to the total
Semi-automatic methods are those where the cross-sectional area of the particles of that size.
actual counting is still done by the analyst but The variation of intensity therefore reflects the
the task, especially the recording, is simplified or size distribution. Irregularly shaped particles pro-
speeded up. duce blurred rings. A multi-ringed sensor located
The Watson image-shearing eyepiece is a in the focal plane of the lens passes information
device that replaces the normal eyepiece of a to a computer which calculates the volume
microscope and produces a double image, the distribution. It will also assess the type of distri-
separation of which can be adjusted using a cali- bution, whether normal, log-normal, or Rosin-
brated knob along a fixed direction, which again Rammler.
can be preset. The image spacing can be cali- The position of the particles relative to the axis of
brated using a stage graticule. In the Waston eye- the lens does not affect the diffraction pattern so
piece the images are colored red and green to that movement at any velocity is of no consequence.
distinguish them. The technique in this case is to The method therefore works “on-line” and has been
divide the slide into a number of equal areas, to set used to analyze oil fuel from a spray nozzle. The
the shear spacing at one of a range of values and to claimed range is from 1 to more than 500 pm.
count the number of particles in each area with There are distinct advantages in conducting a
image-shear diameters less than or equal to each of size analysis “on-line.” Apart from obtaining the
the range. results generally more quickly, particles as they
Some methods have been developed for use occur in a process are often agglomerated, Le.,
with photomicrographs, particularly with the mechanically bound together to form much larger
electron microscope. Usually an enlargement of groups which exhibit markedly different behav-
the print, or a projection of it onto a screen, is ioral patterns. This is important, for example, in
analyzed using comparison aids. The Zeiss-End- pollution studies. Most laboratory techniques
ter analyzer uses a calibrated iris to produce a have to disperse agglomerates produced in sam-
variable-diameter light spot which can be pling and storage. This can be avoided with an
adjusted to suit each particle. The adjustment is online process.
coupled electrically via a multiple switch to eight
counters so that pressing a foot switch automat-
ically allocates the particle to one of eight preset 11.8 Analysis methods that
ranges of size. A hole is punched in the area of measure terminal velocity
each particle counted to avoid duplication.
As already discussed, the terminal velocity of a
particle is related to its size and represents a use-
11.7.2.3 Automatic microscope size amdyers ful method of analysis particularly if the area of
interest is aerodynamic. Methods can be charac-
Several systems have been developed for the auto-
matic scanning of either the microscope field or terized broadly into: sedimentation, elutriation,
and impaction.
of photomicrographs. In one type, the system is
similar to a television scanning system and indeed
the field appears on a television screen. Changes 11.8.1 Sedimentation
in intensity during the scan can be converted to A group of particles, settling, for example, under the
particle size and the information analyzed in a influence of gravity, segregates according to the
computer to yield details of size in terms of a terminal velocities of the particles. This phenom-
number of the statistical diameters. It can also enon can be used in three ways to grade the particles:
calculate shape factors. One system, the Quan-
timet, now uses digital techniques, dividing the (a) The particles and the settling medium
field into 650,000 units. are first thoroughly mixed and changes in