Bill planned to utilize a master stack of mid-point sieves for this purpose. He wanted to know how he could calibrate the stack.
The tolerances for these mid-point sieves are better than ASTM or ISO standards. In fact, they are selected because they fall in the middle of the standard (hence the name mid-point sieves.) They include a detailed report of the measurements that serve as the baseline calibration.
Bill's next concern was how to maintain assurance that the sieves in the master stack maintained the original tolerances. In common practice, the master stack is only used to check production or working sieves after many production tests or if aberrant results are detected. Bill recognized this, but wanted to know when and how the master stack should be checked.
The following rules of thumb for checking sieve calibrations can serve as a guideline:
|Number of Tests||Number of Tests||Time|
|425 Micron (#40) to 12.5 millimeter||60 to 80||2 to 3 Years|
|106 microns (#140) to 355 microns (#45)||50||18 Months|
|45 microns (#325) to 90 microns (#170)||30||12 Months|
|20 microns (#625) to 38 microns (#400)||20||6 Month|
Based on the production rates, the timing of master stack calibration checks may be estimates. Similarly, the need for checking the master stack sieves can be determined.
When a master stack sieve needs to be checked, a recertification using Sieve Calibration Standards can be performed.
Additionally, sieve manufacturers and several sieve distributors conduct this process using optical comparators. A new recertification service is available using video and image analysis technology. It costs $50 to $60 per sieve.
Considering location and security, Bill asked if the equipment needed to apply these techniques was readily available. The answer is yes. There are small optical comparators costing a few hundred dollars that can measure a small number of sieve openings at a time. Bench top comparators costing $3,000 to $10,000 are suitable for viewing and measuring a larger number of sieve openings. Video/image analysis systems start at $20,000 and automate the test sieve recalibration process.
In summary, the calibration cycle of master stack mid-point sieves should be established based on the quality control precision standards of your company. Further, the number of working sieve calibration checks made against the master sieve stack is the basic determinate of when master stack sieves should be checked. Recalibration services are readily available in most industrialized areas. As an alternative, calibration standards and a range of equipment are available for in-house recertification and calibration.
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P.P.S. Get info on Mid Point Sieves.