Sieving in its most elemental definition is the disjunction of fine material from tasteless material by means of a meshed or perforated surface. The technique was used as far back as the early Egyptian days as a way to size grains. These early sieves were made of woven reeds and grasses. Today the sieve test is the technique used most often for analyzing particle-size distribution.
Although at first look the sieving process appears to be elementary, in practice, there is a science and art involved in producing trustworthy and consistent results. In order to great understand sieving, there are some areas of sieve specifications that should to be explained, including:
Light Microscope Image
1. What Are Test Sieves?
Test sieves are measuring devices used to resolve the size and size distribution of particles in a material sample using wire mesh of different openings to detach particles of different sizes.
Test sieves usually consist of wire mesh held in a frame. In most laboratory applications the frame is round and is made from stainless steel or brass. The approved frame sizes are three, six, eight, ten, or twelve inch diameters and metric equivalents. The woven mesh can be made of stainless steel, brass, or bronze. For most applications stainless steel is the most tasteless material used.
2. What Are the Limitations of the Test Sieve Procedure?
The main limitation with the construction of test sieves is the inherent nature of a woven goods including control of sag when mounted and the uniformity of construction of the retention frame. It is also significant to assert consistent sizing across all the openings in a piece of mesh.
Because of the inherent variations of openings in any woven goods there are limitations to the degree of uniformity achieved in the chance size across the mesh in a sieve. This results in a practical limit to the range of openings and to the precision of results from a specific sieve.
The sieve test requires particles to pass through the sieve mesh. The practical limit for using a test sieve policy is a particle size of 20¼ (microns).
3. What Are the Test Sieve Standards?
The first sieve testing standards were developed by W.S. Tyler enterprise before 1920. This former work predated any published operation by the standards organizations and the Tyler designation is the de facto approved in many industries. In 1925, Astm International prepared the lawful approved for Test Sieve Size, Test Sieve Construction, and Test Sieve Mesh in the United States. European Standards were developed by a German university group in 1977 and are known by the designation Din 4188. These were followed by British Standards (Bs 410). The International Standards (Iso 565) were developed by the International Standards organization in Europe. This was designed to be the universal international standard. However, in practice, all of the standards are in operation.
Sieve testing standards report to the construction of the sieve frame and mesh mounting as well as the tolerances allowed in the variability of mesh openings. Basic ideas are tasteless to all of the standards and variations in terminology and in details are small. These small differences, however, can often lead to confusion. The following is a synopsis of the ideas behind these standards.
Test sieve frame standards include the following:
1. Rigid construction
2. Cloth (mesh) mounted without distortion, looseness, or waviness
3. Joint in the middle of mesh and frame to be filled or constructed so that particles will not be trapped
4. Frame will be of non-corrosive material and seamless
5. Lowest of the frame sized to of course slide into the top of same sized sieve, thus enabling stacking
6. Cloth chance to be a minimum of 0.5 inches less than nominal diameter
The wire cloth (mesh) standards include the following list of nominal size openings in inches, millimeters (microns), and sieve number. The following specific dimensional examples come from the Astm E11 Standard:
1. Allowable contrast of mean openings (depending on chance size and ranges from ± 2.9% of nominal size for 125 mm mesh to ± 15% for 20¼ mesh)
2. Not more than 5 % of the openings can exceed 1.04 times the nominal size for 125 mm mesh to 1.45 times the nominal chance for 20¼ mesh
3. Maximum private chance (for any opening) ranges from 1.0472 times the nominal size for 125 mm mesh to 1.75 times the normal mesh for 20¼ mesh
4. Wire diameters are specified and range from 8 mm for 125 mm mesh to 0.020 millimeters for 20¼ mesh
More recently, methods based on laser and vigor technologies, sedimentation techniques, image analysis, and centrifuge-type methods have gained acceptance. However, procedures using test sieves are still widely used. The sieve-test effect remains the basis or approved against which newer techniques are checked. In addition, the tool cost for the test sieve policy is significantly lower than the capital speculation needed for newer methods.
4. What Are Sieve Certifications?
Sieve certifications are statements that a test sieve meets or exceeds published criteria. It is an guarnatee that a new sieve will achieve in a predictable way. The closer the tolerance required in a manufacturing process, the higher the level of certification needed. Similarly, a devotee set of test sieves against which working sieves (sieves in everyday use) are checked for wear and incredible execution need a high level of certification. When test sieves are part of a process that is required to meet traceability prerequisites, such as a specific Iso level, a certification will document the needed traceability.
Many sieve manufacturers provide a certificate which states that the sieve was man-made in conformance with a specific approved (e.g., Astm, Iso). This Manufacturing Conformance Certificate does not reference nor does it warrant conformance of the mesh. Most manufacturers supplying a Conformance Certificate will analyze the mesh and provide a mesh certification for an extra charge.
A Mesh-Certified Sieve will be provided with a certificate that states the sieve was man-made in accordance with a specified approved and it was submitted for laboratory determination and is certified to conform to that specific specification/standard (e.g., Astm, Iso).
There is a third level of tolerance which certifies that the manufacturing approved is met and that the mesh was submitted for laboratory analysis. It also certifies that its openings fall in the middle of the specific standard/specification (e.g., Astm, Iso). This is effectively a 30% great tolerance than the mesh of a Fully-Certified sieve. This is known as a Mid-Point Sieve. These three levels of sieve certification enable the comparability of execution of one sieve to someone else of the same size.
Until the development of the Mid-Point Sieve, high levels of comparability were achieved by providing sieves that were optically matched to a user's approved sieve. A time inspiring and precious policy was needed to achieve this level of comparability and the results were not significantly great than those achieved by using Mid-Point Sieves.
Mesh-Certified Sieves, Mid-Point Sieves, and sieves carrying the Manufacturing Conformance Certificate are all made with mesh that already conforms to lawful standards. However, there are three lower grade levels of sieve mesh ready when tolerance levels are not as stringent.
The first is shop Grade. These sieves have a weave that uses a larger diameter wire resulting in a high impel square-mesh cloth suitable for normal purpose screening. There are no lawful standards for shop Grade test sieves. The second, Mill Grade, is a class of woven mesh using smaller wire, which results in larger open areas in the screen mesh. There is also a Twill Weave in which the weft and warp wires alternatively run over and under two wires rater than over and under alternate wires as in approved mesh. As none of these have lawful standards against which to measure the incredible performance, none of these are provided with a mesh certificate.
5. Sieve Calibration
Quality control of the sieving process is essential, and for population involved in material processing and particle characterization, sieve calibration can be a confusing topic. It is useful to understand what sieve calibration is, why a working sieve should be calibrated, and how to calibrate a sieve.
A. What Is Sieve Calibration?
Sieve calibration is the process of checking a working sieve's performance. (A working sieve is a test sieve that is used usually to achieve a particle size analysis.)
B. Why Calibrate a Working Sieve?
Since working sieves are used daily for tests, they are also cleaned regularly. Although frequent use in itself can cause changes in mesh openings, much of the damage sustained to working sieves occurs during cleaning. Often, the operator hurries to clear the mesh of residual particles by strongly tapping the frame. This tapping can distort the mesh. Operators also use brushes to take off residual particles after a test. This process often distorts sections of the sieve mesh. These alterations of the sieve will convert the results obtained in subsequent tests, hence the need for calibration.
Excessive damage such as tears or large distortions of the mesh weave can be detected by optical inspection. Damaged sieves can be taken out of aid when the damage is observed. When the convert is small, optical consideration may not detect a contrast in the test results attributable to the sieve's change. A way to resolve if changes have occurred is to compare the sieve's execution against a known standard. This is sieve calibration.
In addition, in operations with tight particle size specifications, calibration of new test sieves is performed to construct a execution baseline for the sieve.
C. How is a Test Sieve Calibrated?
The base point of a sieve calibration process is the use of a fixed approved and there are a whole of approaches used. The most tasteless is the use of a devotee stack of sieves, a devotee sample, or calibration spheres or beads.
A devotee stack of sieves includes one of each of the sieves used in the processes. A devotee stack should consist of Mesh-Certified sieves. In the event of tight tolerances for the sieve tests it is recommended that Mid-Point sieves be used. The following steps are used for this method:
1. get ready two samples of the material prime for the calibrations process
2. Place the devotee stack of sieves on a sieve shaker
3. Load one of the samples into the top sieve
4. Run on a sieve shaker for the predetermined time
5. get ready a percent-retained determination of the result
6. Place the stack of working sieves (sieves with sizes to match devotee stack)
7. Repeat steps three through five for the second sample of the material
8. compare the results of the two analyses
9. Check variance from the devotee stack against approved tolerances
10. Replace the working sieves that are out of tolerance
Some users only calibrate one sieve at a time and compare it to one sieve from the devotee set. This policy can be done before putting new working sieves in service.
In some processes devotee samples are maintained of all material that is subject to sieve testing. The results incredible from working sieves were established through the use of a devotee sieve stack or other calibration techniques. In this method a sample from the devotee is used and the following steps are taken:
1. Place the stack of working sieves to be checked on a sieve shaker
2. Load the prime sample from the devotee sample into the top sieve
3. Run the sieve shaker for the predetermined time
4. get ready a percent retained determination of the result
5. compare the results to approved tolerances for the sieves in this stack
6. Replace the working sieves that are out of tolerance
The used sample may be returned to the former devotee sample. Depending on the type of material, deterioration may occur during the sieve test. Where this occurs the test sample is discarded after use.
As with the use of a devotee stack, some users only calibrate one sieve at a time and compare it to a execution tolerance chart for that sieve size. This policy can also be used for new working sieves before putting them into service.
Calibration spheres, in sizes for each of the sieves to be calibrated, are used to resolve the actual results obtained by each sieve tested. This method is straightforward and gives a accurate effect on the mean aperture size. The effect is traceable to Nist and Npl standards. It is a good check for standards reporting and for setting internal standards. The policy for this calibration is as
follows:
1. adopt the sieve to be calibrated
2. Empty the contents of the bottle containing the approved approved onto the sieve
3. Shake evenly over the covering for one minute
4. Theorize the percent passing through and read the mean aperture for a calibration graph
The method specified by Astm is to optically gawk a sample of the openings, measure the apertures and the wire, and compare the results with the Astm E11 Standard. Traditionally, this has been terminated visually using a microscope. However, there are new computer-based image determination systems that are beginning to have diminutive use for sieve calibration.
6. Summary
Sieves have a long history as the base for measuring and analyzing particle size in material. In spite of the arrival of new technology-based methods, procedures based on sieves continue to be the main basis for particle size determination. In order to produce trustworthy and consistent results, it is evident that sieving requires an insight of not just one, but a combination of integral factors such as test sieves, limitations of the test sieve procedure, test sieve standards, sieve certifications, and sieve calibration.
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