Advantech VariSifter VS1000, Руководство по эксплуатации/настройке

Страница: 22 / 46

combination of methods 1 and 2, assuring
compliance with both opening size and
performance specifications.
Probably the most elementary of the
inspection methods is the use of the etched
glass slide. This procedure relies on what
is referred to as the ‘Moire Effect’, which
compares the number of wires per inch in
the wire cloth sample to the number of
lines per inch etched on the glass slide. By
microscopically measuring the wire
diameters, a rough estimate of the opening
size can be approximated. One major
short- coming of this procedure is the
assumption that all wire diameters within
the sample are the same. A slight variation
in wire diameter can translate to a
significant change in opening size.
An alternative to this measurement
approach is the use of a high-powered
optical comparator or profile projector. In
this method, powerful light sources
illuminate the mesh from both above and
below and project the image onto a glass
screen. Calibrated micrometer stages move
the mesh sample in relation to a reference
point allowing measurements with an
accuracy of 1 micrometer to be made on
both the opening and wire diameter. The
results are displayed on a numerical
readout. The broad field of view of the
comparator allows for the scanning of a
large number of sieve openings, facilitating
a more comprehensive picture of the
nature of the sieve cloth.
In the material testing of sieves,
powder samples are run on subject sieves
and the residue calculated. These values
are then compared with other sieves in
selecting what are often referred to as
'matched' sieves. There are a number of
shortcomings in this procedure also. The
first and foremost problem encountered is
that of compliance. Conceivably, it is
possible to find hundreds of sieves that will
provide the same performance data when
tested with a reference material and still
not meet ASTM standards.
While the sieves perform comparably,
they do not meet the basic criteria of ASTM
specifications, which should disqualify them
from use as a U.S. Standard Testing sieve.
Another problem encountered with material
matching is the use of reference samples
that are different in shape, size or density
than the users' products. For example, a
manufacturer of spherical steel shot would
yield significantly different results on a
sieve that had been matched with an
angular ground silica material. In this case,
both shape and density are considerably
different. The key to proper matching is
using the end-users own product or a
material that approximates the product
most closely.
The final approach is a combination of
the first two methods. First, the sieve is
inspected optically for compliance with all
applicable standards. Openings and wire
diameters are measured, not averaged.
After the sieve opening distribution has
been characterized and evaluated, actual
material testing can begin. During the
material testing, samples of the user's
product are used for the standardization
procedure. All tests are run for
repeatability and the variation between test
results calculated. This procedure yields a
testing sieve with known values in the two
most essential parameters compliance with
specifications and performance under
duplicate test conditions.
An alternative that has been used with
some success is the use of correction
factors between sieves. Once a 'master set'
of sieves has been established, a reference
sample is tested on the stack. The values
are calculated and retained. As new sieves
are acquired, the original reference sample
is tested on the new set and the values
calculated. Any variations between the
sieve stacks can be compensated for with
correction factors or multipliers. For
example, a sieve in stack 3 may retain
more or less than the comparable sieve in
the master set. A multiplier of magnitude
greater than or less than 1 is necessary to
calculate the comparable retention value
on that sieve when compared to the master
set. In this way, every sieve in use can be
compared to the master set to standardize