Sieve Testing Standards - A Blessing Or A Curse

Sieving in its most elemental definition is the separation of fine material from coarse 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 reliable and consistent results. In order to better understand sieving, there are several areas of sieve specifications that should to be explained, including:

1. What Are Test Sieves?

Test sieves are measuring devices used to determine the size and size distribution of particles in a material sample using wire mesh of different openings to separate 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 standard 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 common 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 product including control of sag when mounted and the uniformity of construction of the holding frame. It is also essential to maintain consistent sizing across all the openings in a piece of mesh.

Because of the inherent variations of openings in any woven product there are limitations to the degree of uniformity achieved in the opening 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 procedure is a particle size of 20 microns.

3. What Are the Test Sieve Standards?

The first sieve testing standards were developed by W.S. Tyler Company before 1920. This original work predated any published activity by the standards organizations and the Tyler designation is the de facto standard in many industries. In 1925, ASTM International prepared the official standard 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 relate to the construction of the sieve frame and mesh mounting as well as the tolerances allowed in the variability of mesh openings. Basic principles are common 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 principles behind these standards.

Test sieve frame standards include the following:

1. Rigid construction
2. Cloth (mesh) mounted without distortion, looseness, or waviness
3. Joint between 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. Bottom of the frame sized to easily slide into the top of same sized sieve, thus enabling stacking
6. Cloth opening 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. Permissible variation of average openings (depending on opening size and ranges from /- 2.9% of nominal size for 125 mm mesh to /- 15% for 20 micron 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 opening for 20 micron mesh
3. Maximum individual opening (for any opening) ranges from 1.0472 times the nominal size for 125 mm mesh to 1.75 times the normal mesh for 20 micron mesh
4. Wire diameters are specified and range from 8 mm for 125 mm mesh to 0.020 millimeters for 20 micron mesh

More recently, methods based on laser and energy technologies, sedimentation techniques, image analysis, and centrifuge-type methods have gained acceptance. However, procedures using test sieves are still widely used. The sieve-test result remains the basis or standard against which newer techniques are checked. In addition, the equipment cost for the test sieve procedure is significantly lower than the capital investment needed for newer methods.

4. Summary

Sieves have a long history as the base for measuring and analyzing particle size in material. In spite of the advent of new technology-based methods, procedures based on sieves continue to be the main basis for particle size determination. In order to produce reliable and consistent results, it is evident that sieving requires an understanding 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.

By: Ben Needles

Article Source: http://www.directorys.uniquearticles.info

About the Author (text)

Arthur Gatenby is President of CSC Scientific Co, Inc. CSC specializes in the development, production and distribution of equipment used to measure moisture, particle size and surface tension. CSC\'s website is www.cscscientific.com Art can be reached by email at agatenby@cscscientific.com.

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