ASTM and ISO establish standards for the acceptable range of openings in a wire mesh sieve. For larger mesh sizes, the variation between the largest and smallest acceptable openings has little impact on sieve performance. However, as mesh sizes decrease, the potential for performance differences between sieves becomes significant. These discrepancies, especially when an old sieve is replaced in a master stack, can lead to problematic QC data. Mid-Point sieves can help alleviate this issue.
A mid-point sieve has had the mesh inspected after mounting. The sieve is then selected as a mid-point if the results give tolerances that are in the middle of the ASTM standard.
For example with a No. 40 sieve (425µ), key differences between ASTM standard and a Mid-Point sieve are:
The effect of these tightened tolerances is an improvement in variation of approximately 30%. These results are traceable to a NIST standard.
When a process necessitates a high degree of repeatability, mid-point sieves can be employed in all sieve tests. This is particularly crucial for smaller mesh sizes. However, the primary application for mid-point sieves is as a master stack, which is utilized to verify and confirm sieve performance used in routine testing.
Many of our customers have found they prefer using Mid-Point sieves in place of traditional Matched Sets for two reasons: the availability of Mid-Point sieves and the reliability of the results that are provided using Mid-Points. A Matched Set is obtained using all Mid-Point sieves for a designated stack. Because all Mid-Point sieves are certified and inspected to the same range of tolerances, not by using a sample standard, results are accurate and precise for every stack of sieves you use. Mid-Point sieves are also readily available, so you will not have to wait for a replacement sieve.
Although Mid-Point Sieves enhance compatibility and performance among sieves of the same nominal size, calibrating sieves with calibration beads remains the most effective method for ensuring consistent sieve performance.
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