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Sieve Testing: Standards, Certification & Calibration

ÂCertificate are all made with mesh that
already conforms to official standards.
Sieving in its most elemental definition isHowever, there are three lower grade levels
the separation of fine material from coarseof sieve mesh available when tolerance levels
material by means of a meshed or perforatedare  not  as  stringent.
surface. The technique was used as far back
as the early Egyptian days as a way to sizeThe first is Market Grade. These sieves have
grains. These early sieves were made of wovena weave that uses a larger diameter wire
reeds and grasses. Today the sieve test isresulting in a high strength square-mesh
the technique used most often for analyzingcloth suitable for general purpose screening.
particle-size  distribution.There are no official standards for Market
Grade test sieves. The second, Mill Grade, is
Although at first look the sieving processa class of woven mesh using smaller wire,
appears to be elementary, in practice, therewhich results in larger open areas in the
is a science and art involved in producingscreen mesh. There is also a Twill Weave in
reliable and consistent results. In order towhich the weft and warp wires alternatively
better understand sieving, there are severalrun over and under two wires rater than over
areas of sieve specifications that should toand under alternate wires as in standard
be  explained,  including:mesh. As none of these have official
standards against which to measure the
1.  What  Are  Test  Sieves?expected performance, none of these are
provided  with  a  mesh  certificate.
Test sieves are measuring devices used to
determine the size and size distribution of5.  Sieve  Calibration
particles in a material sample using wire
mesh of different openings to separateQuality control of the sieving process is
particles  of  different  sizes.essential, and for people involved in
material processing and particle
Test sieves usually consist of wire mesh heldcharacterization, sieve calibration can be a
in a frame. In most laboratory applicationsconfusing topic. It is beneficial to
the frame is round and is made from stainlessunderstand what sieve calibration is, why a
steel or brass. The standard frame sizes areworking sieve should be calibrated, and how
three, six, eight, ten, or twelve inchto  calibrate  a  sieve.
diameters and metric equivalents. The woven
mesh can be made of stainless steel, brass,A.  What  Is  Sieve  Calibration?
or bronze. For most applications stainless
steel  is  the  most  common  material  used.Sieve calibration is the process of checking
a working sieve's performance. (A working
2. What Are the Limitations of the Test Sievesieve is a test sieve that is used regularly
Procedure?to  perform  a  particle  size  analysis.)
The main limitation with the construction ofB.  Why  Calibrate  a  Working  Sieve?
test sieves is the inherent nature of a woven
product including control of sag when mountedSince working sieves are used daily for
and the uniformity of construction of thetests, they are also cleaned regularly.
holding frame. It is also essential toAlthough frequent use in itself can cause
maintain consistent sizing across all thechanges in mesh openings, much of the damage
openings  in  a  piece  of  mesh.sustained to working sieves occurs during
cleaning. Often, the operator hurries to
Because of the inherent variations ofclear the mesh of residual particles by
openings in any woven product there arestrongly tapping the frame. This tapping can
limitations to the degree of uniformitydistort the mesh. Operators also use brushes
achieved in the opening size across the meshto remove residual particles after a test.
in a sieve. This results in a practical limitThis process often distorts sections of the
to the range of openings and to the precisionsieve mesh. These alterations of the sieve
of  results  from  a  specific  sieve.will change the results obtained in
subsequent tests, hence the need for
The sieve test requires particles to passcalibration.
through the sieve mesh. The practical limit
for using a test sieve procedure is aExcessive damage such as tears or large
particle  size  of  20?  (microns).distortions of the mesh weave can be detected
by visual inspection. Damaged sieves can be
3.  What  Are  the  Test  Sieve  Standards?taken out of service when the damage is
observed. When the change is small, visual
The first sieve testing standards wereobservation may not detect a variation in the
developed by W.S. Tyler Company before 1920.test results attributable to the sieve's
This original work predated any publishedchange. A way to determine if changes have
activity by the standards organizations andoccurred is to compare the sieve's
the Tyler designation is the de factoperformance against a known standard. This is
standard in many industries. In 1925, ASTMsieve  calibration.
International prepared the official standard
for Test Sieve Size, Test Sieve Construction,In addition, in operations with tight
and Test Sieve Mesh in the United States.particle size specifications, calibration of
European Standards were developed by a Germannew test sieves is performed to establish a
university group in 1977 and are known by theperformance  baseline  for  the  sieve.
designation DIN 4188. These were followed by
British Standards (BS 410). The InternationalC.  How  is  a  Test  Sieve  Calibrated?
Standards (ISO 565) were developed by the
International Standards Organization inThe base point of a sieve calibration process
Europe. This was designed to be the universalis the use of a fixed standard and there are
international standard. However, in practice,a number of approaches used. The most common
all  of  the  standards  are  in  operation.is the use of a master stack of sieves, a
master sample, or calibration spheres or
Sieve testing standards relate to thebeads.
construction of the sieve frame and mesh
mounting as well as the tolerances allowed inA master stack of sieves includes one of each
the variability of mesh openings. Basicof the sieves used in the processes. A master
principles are common to all of the standardsstack should consist of Mesh-Certified
and variations in terminology and in detailssieves. In the event of tight tolerances for
are small. These small differences, however,the sieve tests it is recommended that
can often lead to confusion. The following isMid-Point sieves be used. The following steps
a synopsis of the principles behind theseare  used  for  this  method:
standards.
1. Prepare two samples of the material
Test sieve frame standards include theselected  for  the  calibrations  process
following:
2. Place the master stack of sieves on a
1.  Rigid  constructionsieve  shaker
2. Cloth (mesh) mounted without distortion,3. Load one of the samples into the top sieve
looseness,  or  waviness
4. Run on a sieve shaker for the
3. Joint between mesh and frame to be filledpredetermined  time
or constructed so that particles will not be
trapped5. Prepare a percent-retained analysis of the
result
4. Frame will be of non-corrosive material
and  seamless6. Place the stack of working sieves (sieves
with  sizes  to  match  master  stack)
5. Bottom of the frame sized to easily slide
into the top of same sized sieve, thus7. Repeat steps three through five for the
enabling  stackingsecond  sample  of  the  material
6. Cloth opening to be a minimum of 0.58.  Compare  the  results of the two analyses
inches  less  than  nominal  diameter
9. Check variance from the master stack
The wire cloth (mesh) standards include theagainst  acceptable  tolerances
following list of nominal size openings in
inches, millimeters (microns), and sieve10. Replace the working sieves that are out
number. The following specific dimensionalof  tolerance
examples  come  from  the  ASTM E11 Standard:
Some users only calibrate one sieve at a time
1. Permissible variation of average openingsand compare it to one sieve from the master
(depending on opening size and ranges fromset. This procedure can be done before
± 2.9% of nominal size for 125 mmputting  new  working  sieves  in  service.
mesh  to  Ã‚±  15%  for  20?  mesh)
In some processes master samples are
2. Not more than 5 % of the openings canmaintained of all material that is subject to
exceed 1.04 times the nominal size for 125 mmsieve testing. The results expected from
mesh to 1.45 times the nominal opening forworking sieves were established through the
20?  meshuse of a master sieve stack or other
calibration techniques. In this method a
3. Maximum individual opening (for anysample from the master is used and the
opening) ranges from 1.0472 times the nominalfollowing  steps  are  taken:
size for 125 mm mesh to 1.75 times the normal
mesh  for  20?  mesh1. Place the stack of working sieves to be
checked  on  a  sieve  shaker
4. Wire diameters are specified and range
from 8 mm for 125 mm mesh to 0.0202. Load the selected sample from the master
millimeters  for  20?  meshsample  into  the  top  sieve
More recently, methods based on laser and3. Run the sieve shaker for the predetermined
energy technologies, sedimentationtime
techniques, image analysis, and
centrifuge-type methods have gained4. Prepare a percent retained analysis of the
acceptance. However, procedures using testresult
sieves are still widely used. The sieve-test
result remains the basis or standard against5. Compare the results to acceptable
which newer techniques are checked. Intolerances  for  the  sieves  in  this  stack
addition, the equipment cost for the test
sieve procedure is significantly lower than6. Replace the working sieves that are out of
the capital investment needed for newertolerance
methods.
The used sample may be returned to the
4.  What  Are  Sieve  Certifications?original master sample. Depending on the type
of material, deterioration may occur during
Sieve certifications are statements that athe sieve test. Where this occurs the test
test sieve meets or exceeds publishedsample  is  discarded  after  use.
criteria. It is an assurance that a new sieve
will perform in a predictable way. The closerAs with the use of a master stack, some users
the tolerance required in a manufacturingonly calibrate one sieve at a time and
process, the higher the level ofcompare it to a performance tolerance chart
certification needed. Similarly, a master setfor that sieve size. This procedure can also
of test sieves against which working sievesbe used for new working sieves before putting
(sieves in everyday use) are checked for wearthem  into  service.
and predicted performance need a high level
of certification. When test sieves are partCalibration spheres, in sizes for each of the
of a process that is required to meetsieves to be calibrated, are used to
traceability prerequisites, such as adetermine the actual results obtained by each
specific ISO level, a certification willsieve tested. This method is simple and gives
document  the  needed  traceability.a precise result on the mean aperture size.
The result is traceable to NIST and NPL
Many sieve manufacturers provide astandards. It is a good check for standards
certificate which states that the sieve wasreporting and for setting internal standards.
manufactured in conformance with a specificThe procedure for this calibration is as
standard (e.g., ASTM, ISO). Thisfollows:
Manufacturing Conformance Certificate does
not reference nor does it certify conformance1.  Select  the  sieve  to  be  calibrated
of the mesh. Most manufacturers supplying a
Conformance Certificate will analyze the mesh2. Empty the contents of the bottle
and provide a mesh certification for an extracontaining the appropriate standard onto the
charge.sieve
A Mesh-Certified Sieve will be provided with3. Shake evenly over the surface for one
a certificate that states the sieve wasminute
manufactured in accordance with a specified
standard and it was submitted for laboratory4. Calculate the percent passing through and
analysis and is certified to conform to thatread the mean aperture for a calibration
specific specification/standard (e.g., ASTM,graph
ISO).
The method specified by ASTM is to optically
There is a third level of tolerance whichinspect a sample of the openings, measure the
certifies that the manufacturing standard isapertures and the wire, and compare the
met and that the mesh was submitted forresults with the ASTM E11 Standard.
laboratory analysis. It also certifies thatTraditionally, this has been accomplished
its openings fall in the middle of thevisually using a microscope. However, there
specific standard/specification (e.g., ASTM,are new computer-based image analysis systems
ISO). This is effectively a 30% betterthat are beginning to have limited use for
tolerance than the mesh of a Fully-Certifiedsieve  calibration.
sieve. This is known as a Mid-Point Sieve.
These three levels of sieve certification6.  Summary
enable the comparability of performance of
one  sieve  to  another  of  the  same  size.Sieves have a long history as the base for
measuring and analyzing particle size in
Until the development of the Mid-Point Sieve,material. In spite of the advent of new
high levels of comparability were achieved bytechnology-based methods, procedures based on
providing sieves that were optically matchedsieves continue to be the main basis for
to a user's standard sieve. A time consumingparticle size determination. In order to
and costly procedure was needed to accomplishproduce reliable and consistent results, it
this level of comparability and the resultsis evident that sieving requires an
were not significantly better than thoseunderstanding of not just one, but a
achieved  by  using  Mid-Point  Sieves.combination of integral factors such as test
sieves, limitations of the test sieve
Mesh-Certified Sieves, Mid-Point Sieves, andprocedure, test sieve standards, sieve
sieves carrying the Manufacturing Conformancecertifications, and sieve calibration.



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