particle size measurement technique
There are numerous techniques of particle size measurement
which have been discussed below in detail
1) Sieving Method
Sieving is the most popular and the simplest method particle size
testing sieves are employed for classifying power different systems of
serves
are used in various, countries determination
of, sieve size
powders. In America the Tyler of U.S. standard System, in U.K mainly
the B.S system, in Geri usually the DIN system and in France the
AFNOR system widely prevalent 15th among these.
Tyler series of testing has been almost, universally. Accepted as
the standard opening of a screen is expressed in either inch or mm. or
number of meshes per linear inch. In most cases , mesh null. known by
the number of apertures per linear Inch. Woven-wire sieves arc made of
copper, brass, bronze, nickel, monel metal or stainless steel and in recent
years nylon woven sieve clothes have been employed ; each of them has
been specific use,, for example, bronze aleves are chiefly used for hard
alloy powders. Sieves vary In size of aperture and thickness of the wire
or thread front which they have been made.
Sieve size particles are usually denoted by the mesh size through
which till of the powders in tile batch will pass, for example, between
amount and size15. Particle, size distribution is also of fundamental
importance as it affects pressing and sintering behaviour as well as the
physical and mechanical properties of the sintered
material. Size
distribution is based on the percentage by weight of sample powder
which is retained on a screen of given mesh size from a given weight of
starting material after passing through the just coarser sieve. For example,
"25% minus 100 plus 150" means that 25% (by weight) of the particles
pass tile 100 mesh-screen but are retained by the 150 mesh screen. In the
similar way "50% minus 150 plus 200", "75%, minus 200 plus 250".
Most ruche powder manufacturers produce powders having wide range of
Particle size distribution. By employing a definite particle
size
distribution the interparticle voids or porosity can be reduced, i.e.,
increment inpricking density can be obtained.
Thus, with the help of its knowledge, .the properties of the green
and final products can be controlled,"Fine metal powders resultin poor
apparent density , poor flow rate and maximum sinterability while coarser
powders cause good apparent density, good flow rate but minimum
sinterability. if we use the mixed sized and uniform sized particles,
optimum packing, with optimum sinterability with lower packing density
can be achieved respectively.
2) Microscopic method
Microscopic sizing or counting technique is the most direct, simple
and well known of the many method which are employed for the
measurement of the many size and its distribution . It involves actual
counting of particles and individual examination of a large number of
particles on a slid sample of powder by an operator 12. This furbishes the
operator an indication of particles shape, size range quality of fractious
and stale of dispersion or extent of agglomeration of the sample Though
this is a long time-taking and tedious process. it is regarded as the most
reliable and standard method used for checking the accuracy of other
methods.
Optical Microscopic is employed for the determination of
particle
diameters down to about 0.3µ while the electron microscope is used for
the measurement of metal particles in the range of 10 to 0.001µ in
diameter 20.
3)Sedimentation Method
Sedimentation sizing is the classification of powder particle
according to their settling velocitics in a fluid and is the most important
method for particle size analysis in the sub-sieve range Sedimentation
involves spending the powder sample by means of proper agitation in a
fluid medium and allowing it to settle for a suitable time thereby
measuring the settling velocity in suspensions.
Sloke's law
Sloke's law establishes the relationship between the settling velocity of
falling particles and particles size which states that the settling velocity
of falling spherical particle at low velocity in a quiescent homogeneous
fluid of infinite extent is proportional to the square of the particle
diameter. The law may be expressed by
2
v = (p ρ – pt) d g
18η
where ν is the terminal velocity of the particle in cm/sec, pρ and pt the
density of the particle and fluid respectively in gm/cc, d the stoke's
diameter of spherical particle in cans, η the viscosity of the fluid in poises
and g the gravitational accelerant lion in cm/sec on this basis irregularly
shaped size is defined as the diameter of a sphere of same material having
the same settling velocity under the same controlled conditions. In order
to achieve consistent results for Irregularly shaped particles the Reynold
number which may be expressed by the formula.
R=
V.dpt
η
R : Would be less than 0.2.