| Â | | | | comparability and the results were not significantly |
| Sieving in its most elemental definition is the | | | | better than those achieved by using Mid-Point |
| separation of fine material from coarse material | | | | Sieves. |
| by means of a meshed or perforated surface. | | | | Mesh-Certified Sieves, Mid-Point Sieves, and sieves |
| The technique was used as far back as the early | | | | carrying the Manufacturing Conformance |
| Egyptian days as a way to size grains. These | | | | Certificate are all made with mesh that already |
| early sieves were made of woven reeds and | | | | conforms to official standards. However, there |
| grasses. Today the sieve test is the technique | | | | are three lower grade levels of sieve mesh |
| used most often for analyzing particle-size | | | | available when tolerance levels are not as |
| distribution. | | | | stringent. |
| Although at first look the sieving process appears | | | | The first is Market Grade. These sieves have a |
| to be elementary, in practice, there is a science | | | | weave that uses a larger diameter wire resulting |
| and art involved in producing reliable and | | | | in a high strength square-mesh cloth suitable for |
| consistent results. In order to better understand | | | | general purpose screening. There are no official |
| sieving, there are several areas of sieve | | | | standards for Market Grade test sieves. The |
| specifications that should to be explained, including: | | | | second, Mill Grade, is a class of woven mesh using |
| 1. What Are Test Sieves? | | | | smaller wire, which results in larger open areas in |
| Test sieves are measuring devices used to | | | | the screen mesh. There is also a Twill Weave in |
| determine the size and size distribution of particles | | | | which the weft and warp wires alternatively run |
| in a material sample using wire mesh of different | | | | over and under two wires rater than over and |
| openings to separate particles of different sizes. | | | | under alternate wires as in standard mesh. As |
| Test sieves usually consist of wire mesh held in a | | | | none of these have official standards against |
| frame. In most laboratory applications the frame | | | | which to measure the expected performance, |
| is round and is made from stainless steel or brass. | | | | none of these are provided with a mesh |
| The standard frame sizes are three, six, eight, | | | | certificate. |
| ten, or twelve inch diameters and metric | | | | 5. Sieve Calibration |
| equivalents. The woven mesh can be made of | | | | Quality control of the sieving process is essential, |
| stainless steel, brass, or bronze. For most | | | | and for people involved in material processing and |
| applications stainless steel is the most common | | | | particle characterization, sieve calibration can be a |
| material used. | | | | confusing topic. It is beneficial to understand what |
| 2. What Are the Limitations of the Test Sieve | | | | sieve calibration is, why a working sieve should be |
| Procedure? | | | | calibrated, and how to calibrate a sieve. |
| The main limitation with the construction of test | | | | A. What Is Sieve Calibration? |
| sieves is the inherent nature of a woven product | | | | Sieve calibration is the process of checking a |
| including control of sag when mounted and the | | | | working sieve's performance. (A working sieve is |
| uniformity of construction of the holding frame. It | | | | a test sieve that is used regularly to perform a |
| is also essential to maintain consistent sizing | | | | particle size analysis.) |
| across all the openings in a piece of mesh. | | | | B. Why Calibrate a Working Sieve? |
| Because of the inherent variations of openings in | | | | Since working sieves are used daily for tests, |
| any woven product there are limitations to the | | | | they are also cleaned regularly. Although frequent |
| degree of uniformity achieved in the opening size | | | | use in itself can cause changes in mesh openings, |
| across the mesh in a sieve. This results in a | | | | much of the damage sustained to working sieves |
| practical limit to the range of openings and to the | | | | occurs during cleaning. Often, the operator hurries |
| precision of results from a specific sieve. | | | | to clear the mesh of residual particles by strongly |
| The sieve test requires particles to pass through | | | | tapping the frame. This tapping can distort the |
| the sieve mesh. The practical limit for using a test | | | | mesh. Operators also use brushes to remove |
| sieve procedure is a particle size of 20? (microns). | | | | residual particles after a test. This process often |
| 3. What Are the Test Sieve Standards? | | | | distorts sections of the sieve mesh. These |
| The first sieve testing standards were developed | | | | alterations of the sieve will change the results |
| by W.S. Tyler Company before 1920. This original | | | | obtained in subsequent tests, hence the need for |
| work predated any published activity by the | | | | calibration. |
| standards organizations and the Tyler designation | | | | Excessive damage such as tears or large |
| is the de facto standard in many industries. In | | | | distortions of the mesh weave can be detected |
| 1925, ASTM International prepared the official | | | | by visual inspection. Damaged sieves can be taken |
| standard for Test Sieve Size, Test Sieve | | | | out of service when the damage is observed. |
| Construction, and Test Sieve Mesh in the United | | | | When the change is small, visual observation may |
| States. European Standards were developed by a | | | | not detect a variation in the test results |
| German university group in 1977 and are known | | | | attributable to the sieve's change. A way to |
| by the designation DIN 4188. These were | | | | determine if changes have occurred is to |
| followed by British Standards (BS 410). The | | | | compare the sieve's performance against a |
| International Standards (ISO 565) were developed | | | | known standard. This is sieve calibration. |
| by the International Standards Organization in | | | | In addition, in operations with tight particle size |
| Europe. This was designed to be the universal | | | | specifications, calibration of new test sieves is |
| international standard. However, in practice, all of | | | | performed to establish a performance baseline for |
| the standards are in operation. | | | | the sieve. |
| Sieve testing standards relate to the construction | | | | C. How is a Test Sieve Calibrated? |
| of the sieve frame and mesh mounting as well as | | | | The base point of a sieve calibration process is |
| the tolerances allowed in the variability of mesh | | | | the use of a fixed standard and there are a |
| openings. Basic principles are common to all of the | | | | number of approaches used. The most common |
| standards and variations in terminology and in | | | | is the use of a master stack of sieves, a master |
| details are small. These small differences, | | | | sample, or calibration spheres or beads. |
| however, can often lead to confusion. The | | | | A master stack of sieves includes one of each of |
| following is a synopsis of the principles behind | | | | the sieves used in the processes. A master stack |
| these standards. | | | | should consist of Mesh-Certified sieves. In the |
| Test sieve frame standards include the following: | | | | event of tight tolerances for the sieve tests it is |
| 1. Rigid construction | | | | recommended that Mid-Point sieves be used. The |
| 2. Cloth (mesh) mounted without distortion, | | | | following steps are used for this method: |
| looseness, or waviness | | | | 1. Prepare two samples of the material selected |
| 3. Joint between mesh and frame to be filled or | | | | for the calibrations process |
| constructed so that particles will not be trapped | | | | 2. Place the master stack of sieves on a sieve |
| 4. Frame will be of non-corrosive material and | | | | shaker |
| seamless | | | | 3. Load one of the samples into the top sieve |
| 5. Bottom of the frame sized to easily slide into | | | | 4. Run on a sieve shaker for the predetermined |
| the top of same sized sieve, thus enabling | | | | time |
| stacking | | | | 5. Prepare a percent-retained analysis of the result |
| 6. Cloth opening to be a minimum of 0.5 inches | | | | 6. Place the stack of working sieves (sieves with |
| less than nominal diameter | | | | sizes to match master stack) |
| The wire cloth (mesh) standards include the | | | | 7. Repeat steps three through five for the |
| following list of nominal size openings in inches, | | | | second sample of the material |
| millimeters (microns), and sieve number. The | | | | 8. Compare the results of the two analyses |
| following specific dimensional examples come from | | | | 9. Check variance from the master stack against |
| the ASTM E11 Standard: | | | | acceptable tolerances |
| 1. Permissible variation of average openings | | | | 10. Replace the working sieves that are out of |
| (depending on opening size and ranges from | | | | tolerance |
| ± 2.9% of nominal size for 125 mm | | | | Some users only calibrate one sieve at a time and |
| mesh to ± 15% for 20? mesh) | | | | compare it to one sieve from the master set. |
| 2. Not more than 5 % of the openings can | | | | This procedure can be done before putting new |
| exceed 1.04 times the nominal size for 125 mm | | | | working sieves in service. |
| mesh to 1.45 times the nominal opening for 20? | | | | In some processes master samples are |
| mesh | | | | maintained of all material that is subject to sieve |
| 3. Maximum individual opening (for any opening) | | | | testing. The results expected from working sieves |
| ranges from 1.0472 times the nominal size for 125 | | | | were established through the use of a master |
| mm mesh to 1.75 times the normal mesh for 20? | | | | sieve stack or other calibration techniques. In this |
| mesh | | | | method a sample from the master is used and |
| 4. Wire diameters are specified and range from 8 | | | | the following steps are taken: |
| mm for 125 mm mesh to 0.020 millimeters for | | | | 1. Place the stack of working sieves to be |
| 20? mesh | | | | checked on a sieve shaker |
| More recently, methods based on laser and | | | | 2. Load the selected sample from the master |
| energy technologies, sedimentation techniques, | | | | sample into the top sieve |
| image analysis, and centrifuge-type methods have | | | | 3. Run the sieve shaker for the predetermined |
| gained acceptance. However, procedures using | | | | time |
| test sieves are still widely used. The sieve-test | | | | 4. Prepare a percent retained analysis of the |
| result remains the basis or standard against which | | | | result |
| newer techniques are checked. In addition, the | | | | 5. Compare the results to acceptable tolerances |
| equipment cost for the test sieve procedure is | | | | for the sieves in this stack |
| significantly lower than the capital investment | | | | 6. Replace the working sieves that are out of |
| needed for newer methods. | | | | tolerance |
| 4. What Are Sieve Certifications? | | | | The used sample may be returned to the original |
| Sieve certifications are statements that a test | | | | master sample. Depending on the type of |
| sieve meets or exceeds published criteria. It is an | | | | material, deterioration may occur during the sieve |
| assurance that a new sieve will perform in a | | | | test. Where this occurs the test sample is |
| predictable way. The closer the tolerance required | | | | discarded after use. |
| in a manufacturing process, the higher the level of | | | | As with the use of a master stack, some users |
| certification needed. Similarly, a master set of test | | | | only calibrate one sieve at a time and compare it |
| sieves against which working sieves (sieves in | | | | to a performance tolerance chart for that sieve |
| everyday use) are checked for wear and | | | | size. This procedure can also be used for new |
| predicted performance need a high level of | | | | working sieves before putting them into service. |
| certification. When test sieves are part of a | | | | Calibration spheres, in sizes for each of the sieves |
| process that is required to meet traceability | | | | to be calibrated, are used to determine the actual |
| prerequisites, such as a specific ISO level, a | | | | results obtained by each sieve tested. This |
| certification will document the needed traceability. | | | | method is simple and gives a precise result on the |
| Many sieve manufacturers provide a certificate | | | | mean aperture size. The result is traceable to |
| which states that the sieve was manufactured in | | | | NIST and NPL standards. It is a good check for |
| conformance with a specific standard (e.g., ASTM, | | | | standards reporting and for setting internal |
| ISO). This Manufacturing Conformance Certificate | | | | standards. The procedure for this calibration is as |
| does not reference nor does it certify | | | | follows: |
| conformance of the mesh. Most manufacturers | | | | 1. Select the sieve to be calibrated |
| supplying a Conformance Certificate will analyze | | | | 2. Empty the contents of the bottle containing the |
| the mesh and provide a mesh certification for an | | | | appropriate standard onto the sieve |
| extra charge. | | | | 3. Shake evenly over the surface for one minute |
| A Mesh-Certified Sieve will be provided with a | | | | 4. Calculate the percent passing through and read |
| certificate that states the sieve was | | | | the mean aperture for a calibration graph |
| manufactured in accordance with a specified | | | | The method specified by ASTM is to optically |
| standard and it was submitted for laboratory | | | | inspect a sample of the openings, measure the |
| analysis and is certified to conform to that | | | | apertures and the wire, and compare the results |
| specific specification/standard (e.g., ASTM, ISO). | | | | with the ASTM E11 Standard. Traditionally, this has |
| There is a third level of tolerance which certifies | | | | been accomplished visually using a microscope. |
| that the manufacturing standard is met and that | | | | However, there are new computer-based image |
| the mesh was submitted for laboratory analysis. | | | | analysis systems that are beginning to have |
| It also certifies that its openings fall in the middle | | | | limited use for sieve calibration. |
| of the specific standard/specification (e.g., ASTM, | | | | 6. Summary |
| ISO). This is effectively a 30% better tolerance | | | | Sieves have a long history as the base for |
| than the mesh of a Fully-Certified sieve. This is | | | | measuring and analyzing particle size in material. In |
| known as a Mid-Point Sieve. These three levels of | | | | spite of the advent of new technology-based |
| sieve certification enable the comparability of | | | | methods, procedures based on sieves continue to |
| performance of one sieve to another of the | | | | be the main basis for particle size determination. |
| same size. | | | | In order to produce reliable and consistent results, |
| Until the development of the Mid-Point Sieve, high | | | | it is evident that sieving requires an understanding |
| levels of comparability were achieved by providing | | | | of not just one, but a combination of integral |
| sieves that were optically matched to a user's | | | | factors such as test sieves, limitations of the test |
| standard sieve. A time consuming and costly | | | | sieve procedure, test sieve standards, sieve |
| procedure was needed to accomplish this level of | | | | certifications, and sieve calibration. |