38 processing technology
Below about 20 microns however, the interactive
forces between particles increase exponentially and
particle agglomeration in air is difficult to prevent, even
with the high energy imparted by the sonic forces. The
only method of bead transportation through a filter is by
means of a liquid carrier.
The latest series of glass filter calibration standards
go down to a few microns and it is not too difficult to
measure the particle sizes in a suspension passing a
filter using a number of techniques including microscopy,
Coulter counting and light blocking methods.
High resolution sub-micron analysis
Below one micron however, it is difficult to make
narrow distribution glass microspheres and polymer
latex microspheres have to be used. However, the main
problem in challenge testing in the nanometre size range
is finding instrumentation with sufficient resolution.
Two recent developments addressing the resolution
issue are a disc centrifuge (CPS)
4
(Fig. 3) and a Brownian
Motion tracking device from NanoSight5 (Fig. 4). Strictly
speaking the former is not new in that centrifuge sizing
has been around for many years, however significant
advances in high-speed control (24 000 rpm) and data
Fig. 4. The NanoSight submicron particle size analyser.
handling have revolutionised the method.
➠
To measure filter cut point and pore size distribution,
and manufacturers of porometers. a multi-modal latex standard comprising of ten individual
Challenge testing has been perceived as being limited peaks from 0.1 to 1.5 microns has been prepared (BS-
and inaccurate, in that it could not measure pore size Partikel
6
).
distribution, only the cut point or maximum pore size. When a dilute suspension of this ‘multistandard’
The inaccuracy stems from the fact that test dusts had is drawn through a filter, the cut point can be readily
been used in the past. detected from the sizes that have failed to pass the filter,
The test dusts were wide in particle size distribution while the pore size distribution can be determined from
and irregular in shape, which led to large variations from the degree by which the individual peaks have been
lab to lab. suppressed (Fig. 5).
The uncertainties of the test dust challenge test have
been largely overcome by a new set of narrow distribution
Interpreting random trajectories
glass microspheres introduced by Whitehouse Scientific. The NanoSight technology is one of the most exciting
In conjunction with a dry supersonic fluidisation device, developments in nanoparticle measurement. The system
the cut points of filters down to 15 microns can be involves illuminating the particles with a high power laser
measured in just two minutes. light and tracking their random trajectories resulting from
By recovering the beads trapped in the pores in the collisions with the molecules of the suspending liquid (the
filters, it now also possible to measure the pore size so-called Brownian effect).
distributions. From the speed and distance travelled, a
Fig. 5. Determination
of pore size
distribution from a
‘multistandard’ latex
mixture.
www.scientistlive.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44