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What Is A MID-Point Sieve?
A MID-point test sieve has had the mesh inspected. 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, the ASTM permissible variations for the mesh of a No. 40 sieve (425µ ) are:
1: Average Opening ± 19µ
2: Opening Dimension of 396µ Not exceeded by more than 5% of the openings
3: Maximum individual opening 502µ
The MID-point would have a variation in Average Opening of ± 9.5µ , opening dimension of 396µ Not exceeded by more than 2.5% of the openings and a maximum individual opening of 463.5µ . The effect of this tightened tolerance is an improvement in variation of approximately 30%. These results are traceable to a NIST standard.

Where Should MID-Point Sieves Be Used?
When a process requires a high level of repeatability, MID-points can be used on all sieve tests. However, the predominant application for the MID-point sieves is as master stack. A master stack is used to check and recalibrate sieves used in everyday testing.
Summary
If your process must have traceability and the precision and repeatability of your sieve testing results is critical to your process or customer requirements, MID-Point sieves should be considered for the master stacks and for every day testing of supercritical processes.