You may remember Pequeño and his family of very small 150 micron (150µ) particles being very determined to defeat sieve certification.
Episode I had Pequeño watching Brad measure a small number of mesh openings for a certified 150µ (#100) sieve and concluded that there were many places for him and his friends to sneak through.
Episode II introduced Harry theQuality Control Manager, who put Pequeño and a load of assorted-sized particles into a sieve stack for a test. As he predicted, Pequeño got through the #100 sieve. In fact, he got through the 125µ sieve as well. Given that he depended on the certification, Harry had no Idea that Pequeño and family were perched on a 106µ sieve -- two down in the stack from the certified #100 sieve.
Episode III had us observing the process of cleaning sieves after a test. Pequeño observed that Harry was very careful to gently brush away any residue in the sample. He noticed however, that Harry more aggressively pushed particles stuck in the mesh openings, he inadvertently widened them. This in turn would give Pequeño even more opportunities to get through openings that were certified to be as small or smaller than his 150µ size.
Here in Episode IV, the final episode of Pequeño’s saga, we return to Harry, the QC Manager who has been experiencing problems with his sieving results. Inconsistencies, disputes with production and customer complaints are plaguing him. He has been depending on his sieve certifications to insure that his test standards adequately assure correct and consistent results. (He had no Idea that Pequeño and family were doing their mischief).
Harry checked with other colleagues in the sieve testing game and got advice ranging from performing his own microscopic sieve inspections to setting up master stacks to compare individual sieve performance.
Appropriate visual inspection requires an optical comparator (expensive and tedious to operate) or sending the sieves out to be recertified (which removes the sieves out of service for several days). Even then, Harry was not sure that this would detect or identify his problem. (Pequeño felt that this would be good for his cause of getting through smaller than150µ mesh.)
Harry was leaning toward a master stack that would allow him to check each sieve’s actual performance against a standard he developed. Pequeño was concerned that his life may become more complicated. As Harry investigated the best way to develop the master stack, he discovered calibrated micro-spheres designed to define mesh size over the entire sieve.
These calibrating spheres would let him easily check the actual operating characteristic of an individual sieve. This check results in a mean aperture number that clearly defines how a sieve operates.
As Harry analyzed further, he concluded that he could select his master stack sieves using calibration microspheres and check his production sieves against the master results.
As he continued to ponder how this would work, Harry realized that he would not need a master stack. He could set up a program to establish the mean opening of new sieves, check them again when he seemed to have inconsistent sieving results and thus pinpoint problem sieves.
When Harry launched this calibration program, Pequeño and his family had limited opportunity to get past the #100 sieves. Further, Harry now knows if Pequeño is riding high.
I trust that these episodes about Pequeño, his family, Brad and Harry were informative, helpful and somewhat entertaining. You can see more about Calibrating Sieves by clicking on the button.
As usual I am bewildered by the vagaries of Particle size testing.
P.S. If these musings on lab test equipment are engaging, please consider forwarding them to associates and subscribing.