Field Scanning Methods of particle size measurement
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Field Scanning Methods of particle size measurement


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distribution from the measured extinction coefficient 
due to the ill-defined inversion problem. Scholtz et.al. [58] focused on the 
problem of analyzing spectra of colloidal solutions, for which the size 
distribution was known from other methods like electron microscopy and 
light scattering; they termed this 'transmission spectroscopy'. 
Turbidimeters can be used to determine the product of particle 
concentration and particle size. Small measuring zones additionally allow 
measurement of the standard deviation of the fluctuating extinction signal. 
For monodisperse particles the particle size and particle concentration can 
be calculated from the measured mean and standard deviation of the 
extinction signal [59,60]. Variation in the size of the measuring light beam 
influences the standard deviation of the extinction signal and allows the 
determination of the particle size distribution. This can be effected by the 
use of a small angle photometer with the variation in the size of the light 
beam being realized by an axially shiftable flow cell in combination with a 
convergent laser beam [61]. An alternative approach is to use two light 
beams and a flowing suspension [62]. 
10.3.3 Transient turbidity 
Transient turbidity is an optical technique for measuring the size of 
magnetic particles [63,64]. It does this by aligning particles in an electric 
field, removing the field, and following their return to random orientation 
induced by Brownian motion. Their relaxation is measured by turbidity 
and this can be related to particle size distribution if assumptions are made 
536 Powder sampling and particle size determination 
about the particle geometry and the shape of the size distribution. The 
technique is rapid (less than a second) and reproducible. Its most serious 
limitation is that the specific conductance of the sample must be less than 
100 |Limho cm"^ Transient electrical birefringence operates in a similar 
manner 
10.3.4 Holography 
Conventional methods of sampling aerosols are frequently unsatisfactory 
because they are too slow to monitor dynamic aerosols which results in the 
collection of non-representative and modified samples. Hologram systems, 
which overcome these objections, record and reconstruct large volumes of 
aerosols containing particles in the size range 3-100 |Lim. These holograms 
are called far-field or Fraunhofer holograms [65-67] because they are 
recorded at a distance from the object, effectively in its far field. The 
effective sampling depth is A9{D'^IX) that, for 50 |im particles and a ruby 
laser, gives a depth of 18 cm, which is over 3 orders of magnitude greater 
than a microscope. Prototype instruments based on the use of Fraunhofer 
holograms have been described [68-70]. 
Visual comparison of the holographically recorded radiation pattern of 
a particle with Mie theory has also been used for particle sizing [71]. 
Holography has also been used to locate sub-micron particles in a 
3-dimensional volume [72] and, in conjunction with an image analyzing 
computer, to size the droplets in sprays [73]. 
In-line holography has been used to characterize the spray produced by 
a commercial rotary device; a description of the optical system used to 
record and reconstruct the images has been given [74]. 
A simple method of laser diffractometry has been described for sizing 
droplets with radii greater than 1.5 |Lim [75]. Under partially polarized 
laser illumination, at a 90° angle to a camera receiver, well-focused 
droplets appear as small circular dots. Out of focus droplets give large 
diffraction haloes crossed by a row of dark, parallel stripes, the number of 
which is indicative of particle size. 
Yule et. al. [76] suggest these holographic techniques offer no 
significant advantage over the relatively simple two-dimensional spark 
photography technique for measuring particles in sprays. Tyler [77] has 
reassessed the application of Fraunhofer holography to particle size 
determination. 
Field scanning methods 53 7 
10.3.5 State of.polarization of the scattered radiation 
When a system, containing isotropic and monosize spherical particles of 
diameter D, is irradiated with unpolarized incident radiation of wavelength 
1^ the horizontal and vertical components of the scattered radiation are, in 
general, functions of the three parameters; refractive index m, angle of 
observation 6 and x = D/A^. The scattered intensity increases with the 
square of the particle size for vertically polarized light whereas it increases 
linearly for horizontally polarized light. If the intensities of the horizontal 
and vertical components are, respectively, H and V, then the polarization 
ratio R = H/V is a function of m, x and 6. For fixed m and x, R depends 
only on 0, hence particle size can be determined from the positions of 
maxima and minima. 
For Rayleigh scattering 7? == 0 at ^= 90°. As R increases, theory shows 
that X is a periodic function of diameter for monosize particles, and this has 
been used to measure particle size [78] specifically the size of aerosols in 
the size range 0.1 to 0.4 |im [79]. It has also been used to determine the 
sizes of sulfur solutions [80]: In this work, transmission and polarization 
methods yielded results in accord with high order Tyndall spectra (HOTS) 
for sizes in the range 0.365 to 0.62 |Lim. In the limited region where 
(0.45<J<2.8) |Lim and (1.06<m<1.12) the fluctuations in R at 90° are 
smoothed out and the following identity results [81]. 
R^\.^9{m-2f7tm^ (10.12) 
where D = ^ (10.13) 
where there are n{D) particles in the beam in the size range D to D+dD. 
Maron, Elder and Pierce [82] review and extend earlier work on R 
measurements at 90° on monodisperse polystyrene latices and found 
appreciable differences between theoretical and experimental values. They 
showed that the discrepancy is due to inherent anistropy in the latex 
particles believed to be due to non-random orientation of the polymer 
chains in the colloidal latex particles. The size range of applicability they 
give to this technique is 0.135 to 1.117 |am. 
538 Powder sampling and particle size determination 
This procedure has been used to determine droplet size in sprays. 
Oscillations in the curve relating x and D can be smoothed out by the use 
of an incident laser beam having a broad spectral bandwidth [83]. An 
accumulation of independent scattering intensities from multiple scatterers 
can be used to measure the mean droplet size of a group [84]. This 
procedure has been applied to water sprays and the experimental data 
confirmed by phase Doppler anemometry [85]. The applicability of the 
polarization ratio technique is strongly influenced by the complex 
refractive index of the dispersed media and is limited to media having a 
relative refractive index below about 1.44 [86]. 
Fig. 10.3 Design of the fiber optic FBR-sensor 
10.3.6 Forward/backward intensity ratio (FBR) 
The physical principle of FBR is the increasing lack of symmetry in the 
spatial intensity pattern of light scattered in the forward and backward 
directions which becomes significant outside the Rayleigh region. 
Depending on the sensor configuration and the light source a size range 
from 0.05 to 10 |Lim can be detected and sized in a matter of milliseconds. 
This technique is highly pertinent in processes where changing particle 
size needs to be monitored with a high time resolution. 
A prototype instrument (Figure 10.3) has been described using the ratio 
of light scattered at 60° and 120° from the direction of the incident light 
using a collecting angle of 10° [87]. For a realistic upper volume 
concentration level of 0.1% numerous particles are present in the 
measuring volume and the derived size is the mean size for these particles. 
Field scanning methods 539 
A simple calibration has been carried out with latex spheres and a practical 
application on time dependent growth of calcium carbonate [88]. A further 
experiment was carried