Just how effective is your sieve?

Even though the weaving process is descended from the industrial revolution, knowing the exact size of the wire spacing in your sieve can be make or break in the pharmaceutical industry

Sieve cloth is sold both by mesh number (wires per inch) and by its wire spacing.  Of the two, the wire spacing is a more accurate specification because it reflects the performance of a sieve. This can be clearly seen in figure 1 where two meshes of the same mesh number are compared showing that, if the wires are not evenly spaced, there are large differences in the aperture size. This is a real problem both for the manufacturers of quality meshes and the end users as, on the basis of mesh number alone, second rate material can be sold meaning any cost advantage would soon be negated by performance or replacement.

Even when a mesh is manufactured to an ISO specification, there is still an ambiguity. This is because the current standard is based on an independent measurement of wire spacing in the X and Y direction - warp and weft, as it is known. If there is a significant difference between the X and Y measurements, which one reflects the performance of the mesh? In addition, there are wire diameter tolerances. The problem is, the more parameters that are measured for a given specification, the more difficult it is to match sieves. Indeed multi-parameter specifications have posed real challenges in sieve matching in the pharmaceutical industry, and some test sieve manufactures have scanned thousands of sieves in order to find only a few with matching parameters.                                    Figure 1. Specifying sieves by mesh 
                                                             number masks poor quality

Understanding the standard
A common misconception is that all the apertures of a sieve mesh are the same size. Considering that the manufacturing process is based on simple weaving technology, it is remarkable that the apertures are so uniform, but it is important to know and understand the limitations in order to optimise accurately the performance of a mesh.

The specification of a 250 micron sieve according to ISO 3320-1:2000 is summarised in figure 2.
Figure 2. ISO tolerances

The first important observation is that the mean aperture size of a 250 micron sieve is allowed to vary from 240 microns to 260 microns while still remaining within specification. Secondly, the standard deviation tolerance allows for apertures up to 272 microns. Finally, the maximum permissible aperture is approximately 310 microns. Therefore such a sieve could not be used with any confidence in scalping oversize particles of 300 microns from a product.

A second misconception is that a sieve will cut a particle size distribution according to the average aperture size. In practice, the cut point occurs very close to the upper standard deviation. It is therefore better to use a sieve slightly smaller than the nominal size if it is being used in precision separation processes.

Aperture description
Unfortunately, because it takes so long to agree new ISO standards, technology may have already overtaken even the latest ISO revision. For example, with the rapid advance of microscopy and image analysis, it is now possible to rapidly scan a mesh and analyse the geometry of each individual aperture. The aperture can then be measured in the X or Y direction or even the diagonal, which may be important if the mesh is being used with platelets such as aluminium flake.

In describing an aperture size, the three dimensions above all have their advocates but if an aperture is described merely by one dimension then, strictly speaking it would be circular, as only a circle is mono-dimensional. The equivalent circle diameter of an aperture is an easy concept to understand in that it relates to the smallest sphere that will be trapped by the mesh.

Sieve calibration microspheres
As the parameter retaining a sphere is the minimum aperture size, the use of certified glass beads for calibrating sieves is a very accurate and speedy method.

Although glass microspheres have been used in the past to calibrate sieves, they were supplied in broad particle size distributions for calibrating sieve stacks. The resolution on individual sieves was therefore poor.

To increase the precision of microsphere standards, they should be narrow in particle size distribution (figure 3) and certified using precision electroformed sieves where apertures vary by less than 3 microns.

A range of NIST traceable sieve standards (for example from Whitehouse Scientific) is now available to calibrate individual sieves from 20 microns up to several millimetres. From a calibration graph on the test certificate, the percentage of beads passing the mesh is used to determine the aperture size (equivalent spherical diameter) of woven wire sieves.

Sieve aperture comparisons
In table 1 the certificate results for an ISO calibrated sieve are compared to an image analysis method where individual apertures were examined using the Whitehouse ShapeSizer. The results from the glass bead calibration method are also shown.    

If the sieve is used in a particle size measurement, which parameter should be used?

When each individual aperture is measured, there are significant differences between the average sizes depending on whether the minimum or maximum dimension is selected. However as expected, the results from the glass bead test are almost identical to the minimum aperture average obtained from microscopy and image analysis.
 
The highest precision
Notwithstanding all that has been said above, the stainless steel weaving process for producing test sieves is remarkably accurate. Considering that the technology is a direct descendant of the industrial revolution weaving looms in woollen mills, it is an outstanding achievement to take 10,000 x 18 micron wires, pull each of them manually through a spacing comb with 20 micron gaps (the weft wires) and then fire another 18 micron wire across the loom to create a 20 micron aperture - all this with an accuracy of +/-4 microns.

Careful as you go
Given the precision in the weaving process, it is of paramount importance that the sieves are well cared for. The cardinal sin is to poke the sieve surface with a metal object or wire brush to encourage the passage of product. Surface damage may not be visible with the naked eye but can have disastrous consequences for both particle size analysis and separation processes. For the finer sieves, ultrasonic baths are the only cleaning method that should be used.

Although ISO standards are useful in being able to distinguish good and bad meshes, there is still an ambiguity in interpreting the results. For most applications, this may not be an issue, but in high precision pharmaceutical applications, a better method is required. This is provided for either by the latest microscopy and image analysis methods or, more conveniently, by using traceable glass bead standards.

By, Dr Graham Rideal, Managing Director, Whitehouse Scientific, Chester