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ShouldYou Depend on Calibration Techniques For Sieve specification?

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ShouldYou Depend on  Calibration Techniques For Sieve specification? Powered By Docstoc
					     Change Your Approach to Determining/Establishing
                  Sieve Specifications?
History of the sieving Process

The process of sieving has been the wok horse of determining particle size distribution. It was used
over 5,000 years ago by the Egyptians to grade grain.

Advancing technologies in laser diffraction and image analysis have introduced new techniques to the
analysis of particle size, yet sieving continues to be the most widely use method. It has significant
economic advantages and is easy to execute. Sieving is sometimes called the Cinderella of particle size
analysis because it does most of the work while getting little of the credit.

At times the image of sieving                                             has been tainted by poor
quality sieves and faulty                                                 analysis. There are literally
millions of sieves in use                                                 around the world and this
technique is still the cheapest                                           way of measuring particle size
--- by a considerable margin.

Early and continuing attempts to overcome the quality problem have been the purview of standards
organizations like ASTM, ISO and other country specific groups. Part of the issue was because the
bulk of the sieves use woven wire mesh to accomplish the screening, a difficult process to maintain
precise openings throughout the sieve.

Standards were developed to assure consistency in the frame that holds the mesh, tightness of the mesh
when assembled and to fix the acceptable variation of the mesh openings.

Assessment of the suitability of the mesh is delineated by the result of optical/microscope assisted
inspection. The published criteria are stated in terms of average variation in opening size and
maximum allowable size of openings inspected. These data are processed to conclude the sieve pass or
fail status.

Problems with the Process

The process of establishing the suitability of the mesh in a sieve calls for the measurement of a
sample of the openings; in a proscribed manner over a delineated geography. The number of openings,
selected to reflect a reasonable balance between inspection time and coverage, represent less than .
001% of the total sieve openings for a given size. The acceptable variation can exceed 20% of the
nominal specified sieve mesh in the lower micron mesh sizes.

When a full certification is performed, a document is prepared that includes the details of all the
measurements, averages and deviations. The document states a pass/fail result.

Until the recent ASTM 2009 standards were adopted, there was a single pass fail criteria. A tighter
specification had been by one of the sieve manufacturers. This was know as a mid-point sieve which
was certified to fall in the middle of the published allowable deviations.
The new ASTM standard established three specification levels for the mesh. The inspections results
are subject to rigorous statistical analysis and are stated in terms of maximum standard deviation, while
retaining the maximum variance averages and maximum individual opening standards. This inspection
process, which is referred to as certification, screens out poor quality mesh.

This certification process does provide a basis for traceability and is insurance from bad quality sieves.
However, it does not give a solid predictor of a sieve's performance in real world testing. The small
sample of openings inspected and the wide acceptable variation , which increases as the opening size
decreases, present a wide framework for performance differences between any two sieves of the same
nominal size.

The process of calibration which is usually done by the users, is all about performance. In this process
the performance of a sieve is compared either to the performance of a master sieve or is compared to a
proscribe result on a master material. It is at this stage where the sieve performance is established.

New concept and approach

As noted, there is a large gap between the nominal size of a sieve 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% larger. That is significant if the application is for screening out over
sized contaminants or if the specification for the retained amount on the sieve is critical.

More helpful performance predictive intelligence on a sieve would be the “effective aperture size”.
The concept of using calibration spheres has been around for some time, but was not widely used
because of precision limitations. Whitehouse Scientific and their managing Director Dr. Graham
Rideal took on the tasks of designing an approach to using glass micro spheres to get consistent
effective sieves size information. In over three years of development, the company designed a system
to generate microspheres to within +/- 1 micron in smaller sizes.

They also developed a                                                   calibration technique, for the
most popular ranges of sieve                                            sizes, that uses a fixed weight of
sample that contains a known                                            distribution of microsphere
sizes.. This sample is placed on a                                      sieve to be calibrated and shaken
for approximately one minute.                                           By measuring the retained
weight after this process the % of                                      micro spheres passing is
calculated which is used to get a                                       mean aperture size from a
calibration chart.

Using this method, even a variation of 5% difference in the weight of the passed spheres results in only
a 1 micron difference in the aperture size for sieves below 100Microns. Not only does this technique
produce an effective performance predictor but it results in a tight accuracy across the range of sieves
from 20 Microns to 3.35 millimeters.

The calibration process is easy and quick and can be done on-site; no need to send it out to a laboratory.
The process provides a result with traceability to NIST and NPL, needed for most quality control
maintenance systems .

How it would work
To take maximum advantage of what we call the “Calibration Method of Sieve Quality Determination
- CMSQD” , it is necessary to change long standing practices and ideas. Rather than relying on the
certification process with attended data rich reports, the CMSQD requires only a two minute process.

The process would start when a new sieve is delivered. A calibration would be done before any testing
was performed with the sieve. This calibration results in a “effective mean aperture”. The new sieve
now has an operational performance value. Changes in this value will show the effects of usage and
provide a basis for determining if the sieve should be replaced.

At the beginning of a program of CMSQD, all functional sieves should be checked and the differences
in mean aperture recorded. This establishes a base line for comparing the real-life performance of
sieves of the same nominal size but with different mean apertures. Variances between test results of
individual sieves can now be related to the mean aperture calibrations and acceptable aperture limits
can be established.

The economics of a change to CMSQD should not be more than current costs. An example of some
costs for a 63 micron – #230 sieve shows the relationship.

                                     Certified Sieve        Mid-Point Sieve
Typical Price                         $161.30                 $322.35
Standard Compliance Sieve              $111.25                $111.25
Savings                                $ 50.05                $211.10
Calibration Spheres                    $ 37.11                $ 37.11
Net Savings                            $ 12.94                $173.99

Typical charges to perform certification service :

1. Old ASTM Standard Certification $ 75.00
2. New ASTM Inspection Sieves      $ 85.00
3. New ASTM Calibration Sieves     $135.00

The benefits of moving to a CMSQD include determination of an objective and traceable sieve
performance number. This number , mean aperture, for the first time provides a basis for evaluating the
performance of a sieve in the working test environment, a benchmark to compare the wear on a sieve
and a reference to compare multiple sieves of the same size, all at a cost no more 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.

Summary

Sieving has lasted as a widely used way of particle size analysis for centuries. In today,s version most
sieves are constructed of wire mesh, a tough thing to produce and even tougher to produce with
consistent apertures. In the past, these difficulties brought about frequent occasions of bad quality and
a tarnished reputation of this Cinderella workhorse. At least as far back as 1985, ASTM established
standards for wire mesh sieves, stating acceptable variations in sieve wire openings. A certification
that a sieve met the E11 requirements served as an insurance policy against shoddy workmanship.

Users who wanted to get a measure of how a sieve performed (calibrated against something) , certified
or not, needed to compare it to either a sieve of known performance (master sieve) or to the result
obtained using a master sample. These have been the most common methods employed to get a
performance measure for sieves used for quality control measurement.

The introduction of the tight tolerance and traceable glass micro spheres has provided a new technique
for the calibration task. Not only does this method accomplish the comparison task but it brings a
spotlight to the expected performance by defining the mean aperture.

The tools are in place to bring about a pervasive change to current processes of sieve certification and
calibration. Now in one easy, quick step the cumbersome, time consuming and costly process of
optical inspection, setting up and running master stacks, or preparing master samples can be eliminated

In conclusion, the use of calibrated glass micro spheres is the logical way to establish, measure and
delineate sieve quality and determine performance criteria and measurements.

Arthur Gatenby
********************************************************************
Arthur Gatenby is the President of CSC Scientific Company, Inc. He has worked in the field of particle
size analysis for more than 30 years. He can be reached by email agatenby@cscscientific.com or by
phone at 703-876-4030

				
DOCUMENT INFO
Description: A new approach to sieve spoecification that's not dependent on certificatiobn standards.