The relative value of a sieve certification process vs a sieve calibration has perplexed me for a long time.
Eventually it dawned on me that the entire certification process is an insurance policy of sorts. Firstly, it confirms that the mesh is within the ASTM or ISO spec -- even though the spec allows for significant variation. Secondly, it meets the traceability demands of ISO mandates.
However, the inspection reports only minimally predict a sieve's performance. I recall a situation in which a customer with a high-powered QC program had trouble matching the performance of a new shipment of mid-point sieves. [Mid-point sieves must conform to a much tighter tolerance than the standard spec.] After an exhaustive investigation, the old batch of sieves was determined to be at the low end of the spec while the new ones were at the high end. The mid-point certification reports did not indicate this discrepancy.
The customer then used a procedure that compared the performance between the two batches. That process finally pinpointed the problem. This is what I think calibration is all about -- ensuring predictable performance in an operating environment.
Calibration techniques vary from comparing a sieve result with a master set of sieves (Master Stack) to comparing result with a known sample (Master Sample). Both of these techniques are application-specific.
Another approach to calibration involves utilizing calibration spheres or beads, which compares a specific sieve's performance to a traceable high-precision standard. It provides a result as a single specific quantitative measure of the expected sieve performance. The result is a mean sieve opening size.
Given that the high-precision beads are traceable to an ISO-recognized standard, this calibration method serves the insurance purpose of meeting ISO or internal company QC audit demands. Unlike the certification report, a calibration using this technique also yields a single performance indicator. In most circumstances, using the calibration bead method eliminates the need to use Master Stacks or Master Samples.
Whitehouse Scientific has a process that provides a sieve calibration that results in a mean sieve size to approximately +/- one micron. It took them approximately three years to develop their traceable calibration process. For more on this see Sieve Calibration
I hope this rant stimulates some questions and discussion.
Thanks for reading this.
A still perplexed,