Calibrated glass microspheres are the logical means to establish, measure and delineate sieve quality as well as to determine performance criteria and measurements.
Advancing laser diffraction and image analysis technologies are amongst new particle size analyzing techniques, but sieving was used over 5,000 years ago by the ancient Egyptians to grade grain and continues to be the work-horse of determining particle size distribution. It is cheaper and easy to execute.
Some sieving imperfections develop because most sieves are constructed with woven wire mesh, which entail difficulties in maintaining precise openings throughout. Early and continuing attempts to increase sieve quality fall to standards organizations like ASTM, ISO and other country-specific groups.
These standards assure frame consistency, assembled mesh tightness and acceptable opening variations.
Optical/microscope-assisted inspection determines mesh suitability. The published criteria are expressed as average variation and maximum allowable opening sizes, which are processed to determine sieve pass-or-fail status.
Establishing sieve mesh suitability requires sample opening measurements in a prescribed manner over a delineated geography. The number of openings, reflecting a reasonable balance between inspection time and coverage, represent less than .001 percent of the total for a given size. The acceptable variation can exceed 20 percent of the nominal specified sieve mesh in the lower micron mesh sizes.
A full certification document includes all measurements, averages and deviations details, which then presents a pass/fail result.
Until 2009, there was a single pass/fail criterion. One of the sieve manufacturers used a tighter “Mid-Point” specification, which falls between the published allowable deviations.
A new U.S. standard (ASTM E11-09) established three specification levels for the mesh. The inspection results are subject to rigorous statistical analysis. They are expressed as maximum standard deviations, while retaining the maximum variance averages and maximum individual opening standards. This certification process eliminates poor-quality mesh.
It also provides for traceability and ensures against substandard sieves, but does not give a solid predictor of performance in real-world testing. The small sample of inspected openings and the wide acceptable variation that increases as the opening size decreases present a broad framework for performance differences between any two sieves of the same nominal size.
On the other hand, there is sieve calibration. This process is all about performance, which is compared either to that of a master sieve or to a prescribed result on a master material.
There is a large gap between a sieve’s nominal size and maximum allowable opening. For example, in the latest ASTM standard for a 63-micron sieve, the maximum individual opening can be 89 microns -- more than 40 percent larger. That is significant if the application is for screening out oversized contaminants or if the specification for the retained amount on the sieve is critical.
“Effective aperture size” is more helpful in predicting sieve performance. Calibration spheres were an option, but were not widely used because of precision limitations. Whitehouse Scientific and its Managing Director Dr. Graham Rideal successfully designed an approach utilizing glass microspheres to get consistent effective sieve-size information. Three years’ development saw the company design a system to generate microspheres to within ±1 micron in smaller sizes.
Whitehouse also developed a calibration technique that uses a sample’s fixed weight, which contains a known microsphere size distribution. The sample is placed on a sieve and shaken for about a minute. Measuring retained weight afterward provides the microspheres’ percentage passed, which is then used to get a mean aperture size from a calibration chart.
Using this method, even a 5 percent weight variation of passed spheres results in only a 1-micron aperture size difference for sieves below 100 microns. Not only does this technique produce an effective performance predictor, it also results in a tight accuracy for sieves ranging from 20 microns to 3.35 millimeters.
The calibration process is easy, quick and can be done on-site; no need to send sieves to a laboratory. It provides a result with traceability to NIST and NPL.
Changing long-standing practices and ideas takes maximum advantage of the Calibration Method of Sieve Quality Determination. Rather than relying on the certification process with attendant data-rich reports, CMSQD requires only a two-minute procedure, which begins upon delivery of a new sieve.
A calibration is done before testing and results in an “effective mean aperture.” The new sieve now has an operational performance value; changes of which will show usage effects and provide a basis for determining if the sieve should be replaced.
When a CMSQD program starts, users should check all functional sieves and record mean aperture differences. This establishes a baseline for comparing the same nominal size sieves’ real-life performance, but with different mean apertures. Variances between individual sieve test results can now be related to mean aperture calibrations and acceptable limits can be established.
Changing to CMSQD should not be more than current costs. Consider the example of a 63-micron – #230 sieve:
|Standard Compliance Sieve
Typical certification charges:
1. Old ASTM Standard Certification $ 75.00
2. New ASTM Inspection Sieves $ 85.00
3. New ASTM Calibration Sieves $135.00
Objective and traceable sieve performance number determination are among the benefits of moving to a CMSQD. This number, the mean aperture, provides a basis for evaluating sieve performance in the working test environment. This is a benchmark to compare sieve wear and provides a reference to compare multiple sieves of the same size—all at no more cost than the traditional certification charges. The calibration process is so simple; little training is required and the time to compete a calibration is measured in minutes, not hours.
Thus we see a significant improvement to current sieve certification and calibration processes. Now we can eliminate cumbersome, time-consuming and costly optical inspections, setting up and running master stacks or preparing master samples.
For more information, contact Arthur Gatenby at [email protected] or 703-876-4030.