Simple Solutions That Work! Issue 12

62 that state the number of apertures and wire diameters measured. For a 53-micron sieve, that number is a whopping >500 measurements of the tiny holes! Surprisingly, all the three grades are made using the same raw materials and processes! Why talk about grades? Master and the Work Horses Calibration grade sieve set is very expensive. Using it for routine testing is almost criminal. A suggested practice would be to have a calibration grade sieve set properly packed and stored until needed. Let’s call this set a “master”. Earlier we discussed compliance grade or inspection grade sieve set being made from the same materials and processes. This means we can choose to have sieve sets of one of the two grades as work horses. These are relatively cheaper than the calibration grade sieve set. When they are new, compliance, inspection or calibration grade test sieves are guaranteed to meet ASTM or ISO requirements, but eventually variations in results occur when wire cloth sags, stretches, tears, or abrasive materials reduce wire diameter, all of which increase opening sizes. How do we know and replace sieves before they start dishing out bad results? It would require an elaborate in- house sieve quality control program comprising of procedures for: 1. Establishing master sieve set 2. Establishing relation between master and working sieve set and sieves 3. Discarding sieves 4. Verification of sieve sets with standard micro spheres (optional) Let’s look at each of the procedures in detail. Establishing Master Sieve Set: This is probably the simplest procedure which should ensure documentation related to calibration grade sieve set (the master). The documentation should ideally include: 1. Serial number statistical inspection data received along with sieve 2. Traceability of the inspection apparatus to national or international standards 3. Back-up records of measurements provided by manufacturer of the sieve 4. Verification program for calibration considering ultra-low usage The procedure must prescribe to not use the set for routine testing. Establishing relationship between Master and Working Sieve Set and Sieves: This procedure should compare a working sieve set with the master sieve set and establish a relation between each working sieves’ retention, to retention on a master sieve of the same size. This can be easily achieved by passing the same sample through both sieve-sets sequentially. A high frequency 3D sifter is highly recommended for use in foundry for its high sifting efficiency and repeatability in final retentions. Documentation to be maintained must include % variation in retention between master and working sieve, as well as variation in AFS number for entire sieve set. It is important to note here that sieve which belongs to one stack shouldn’t be interchanged with another. An interchanged sieve may lead to results inconsistent with the past track record of the sieve set. Important: The procedure must prescribe periodic verification of each working sieve set with master to maintain precision and accuracy of the results. Discarding Sieves: This procedure must quantitatively specify the variation of retention as well as AFS number acceptable between master and the work sieve. Typical acceptance limits vary from 0.5% to 2.5%. Once this limit is crossed by a sieve, the procedure should direct the user to discard the sieve. Example: A new 300 micron compliance grade sieve has 9.9 g retention as against 10 g of the same sample on master sieve of 300 microns, resulting in deviation of 1%. If the company using the sieve sets acceptance criteria of a work sieve to be less than 2%, this variation of 1% is acceptable and the sieve can be used. Over time when the working sieve degrades by use, the variation in retention may cross 2% in which case it should be discarded. Verification of Sieve sets with Standard Micro Spheres: For an AFS sieve set, buying standard micro spheres can be costly, however, the immense value it adds is worth it when mission critical testing is being undertaken routinely. The procedure may include verification by sifting using entire sieve set with premixed known weights of calibrated spheres for each mesh size at least once a month with standard micro spheres. Next, acceptance criteria should be put in place for variation, similar to what is discussed in the discarding sieves section. Conclusion: Following the four procedures as described above can make sure that each result of sieve analysis is usable, without worrying about sieve condition. Contact: PUSHKRAJ JANWADKAR pushkraj@versatile.in