Measuring pore size to accurate standards
Graham Rideal, from Whitehouse Scientific, discusses a an alternative pore size measurement technique that uses new NIST traceable microsphere standards
Graham Rideal, from Whitehouse Scientific, discusses a an alternative pore size measurement technique that uses new NIST traceable microsphere standards
The apertures or pores of plain weave meshes can easily be measured by microscopy because they are open to transmitted light, however, 3-dimensional meshes cannot be measured by microscopy because they are opaque, figure 1, so a different method of pore size measurement is required.Pore size measurement has been traditionally performed by the bubble point test. In this method the maximum aperture size present can be related to the pressure at which a bubble appears on a wetted filter medium pressurised with air from below.
Changes in flow rates are used to estimate the pore size distribution, while the efficiency in an actual filtration process is calculated by using a 'tortuosity factor' to estimate the retention properties of the filter medium.
The limitation of this technique is that it is a second order theoretical method and gives only 'equivalent' or theoretical pore sizes often dependent on the pore structure within the filter medium.
An alternative method is the so-called 'challenge test'. In this method standard test dusts or glass beads are presented to a filter medium and the size distribution in the downstream flow analysed.
This method gives a more absolute measurement of pore size because it measures real particles but, because the size distributions involved are often broad, there is significant uncertainty in the measurement of the largest particles passing the filter medium.
In the case of irregular test dusts, there is also an ambiguity of size caused by the shape of the particles, figure 2.
Particle shape can also affect the penetration of the filter media by the challenging particles, irregular particles tending to lock into the tortuous pathways through the filter media. A simple example is the comparison of spheres and discs passing various filter media, figure 3. The optimum particles for a challenge test are therefore spherical, narrow size distribution microspheres.
The complex structures produced from the latest weaving technology make traditional testing methods - such as air permeability, bubble point measurements and challenge test methods - less reliable.
new method
Potential users of filter media are therefore demanding more accurate methods of filter pore size measurement and this requires a different approach and technical understanding of filtration efficiency. This new method of pore size measurement involves the use of narrow particle size distribution glass microsphere standards in conjunction with a sonic sifting device to transport the calibrating microspheres through the filter medium.
In the challenge method, particles of known size distribution are presented to a filter and any changes down stream measured by a particle size analyser. Traditionally test dusts have been used but as discussed above, the accuracy of the method is limited both by the shape of the irregular particles and their broad size distribution. Furthermore, elongated particles can pass through smaller pores than their equivalent spherical diameter would suggest.
This new method is based on the preparation of approximately 25 narrow size distribution microsphere standards to cover pore sizes from 5 to more than 600µm, figure 4.
relevant parameters
Preparing narrow particle size distribution microspheres is only the first stage in producing a filter calibration standard. The particle size must be certified by a method that reflects the penetration mode through a filter medium. From figure 2 it was seen that particle breadth or sieve size is the most relevant parameter.
Wire woven sieves have an unacceptably wide distribution of aperture sizes within the mesh so NIST traceable precision electroformed sieves have been used for certification, figure 5. However, only three electroformed sieves could be used for analysis because of the narrowness of the distribution, so the data was supported by microscopy to ensure a uniform distribution, figure 6. Provided the results were comparable, the sieve data was then used to construct a calibration graph of the percentage passing a filter in relation to its pore size, figure 7.
Because the particle size distribution is so narrow, comparatively large variations in the weights passing a filter have very little effect on the pore sizes, and resolutions better than 1µm are quite common.
Having a well-calibrated range of filter standards is only the first step in testing filter media. It is essential to have a means of transporting the microspheres effectively through the tortuous pathways in the complex filter structure. This problem has been solved by using a sonic sifting device that fluidises the microspheres rather than shake the filter as in traditional sieve shakers. The enormous energy imparted to the particles combined with high air velocities through the
filter pores ensure that there is efficient microsphere penetration into even the most complex filter media, figures 8 and 9.
method accuracy
To measure the pore size of a filter, a 50mm or 90mm disc is clamped into the filter holder and a known weight of the appropriate calibration standard fluidised on the surface. The end point corresponds to a change in weight of less than 1% passing per minute and is usually achieved in a few minutes. The pore size is then determined from the filter standard calibration graph, figure 7.
The definition of the pore size measured is the size at approximately 97% of the maximum microsphere passing the medium when measured by microscopy, or the effective cut point of the filter medium.
The accuracy of the method is exemplified in the test results from a stainless steel woven mesh supplied by G Bopp, which had a nominal pore size rating of 25µm. A filter calibration standard having a calibration range of 20-33µm was fluidised on the filter mesh for three minutes. Of this, 35% passed which corresponded to a pore size of 24µm, figure 10.
absolute results
The microspheres passing the mesh were collected and microscopically measured to determine the maximum size passing. It can be seen from figure 11 that the D97 value of 25µm corresponds very closely with the size determined from the sonic method result of 24µm. As a final check, very narrow particle size glass beads with a size range of only 24-28µm were fluidised on the mesh. Only 4% passed, confirming that the cut point of the filter mesh was 24µm.
This new sonic challenge test for measuring pore sizes has been shown to be a very accurate and speedy method requiring only 2-3 minutes to perform. The technique is an easy concept to understand in that it relates to filter performance; spherical particles either pass through or are retained. The parameter measured is therefore the cut point or retention performance of the filter medium.
Unlike the bubble point method, which relies on complex theories to estimate pore size, this method gives absolute results, which are traceable to NIST international standards of length.
This new technology has had a major impact in the filter media industry in that it is now possible to accurately discriminate between different products so avoiding costly duplication. It has also been extremely beneficial to the end users who can accurately optimise their production lines without the need for extensive field trials.
Bibliography
1. G R Rideal and J Storey, "A new high precision method of calibrating filters", J. Filt. Soc, 2002 2(3), 18 - 20 2. E Mayer, G Warren, "Evaluating filtration media, a comparison of polymeric membranes and non-wovens", Filt and Sep J, 1998, 33(10) 912 - 914 3. D B Purchas and K Sutherland, Handbook of Filter Media, 2nd Ed.,Elsevier Advanced Technology, ISBN 1 85617 3755, 4. Micron rating of filter media, SAE APR901, March, 1998 5. G R Rideal, E Mayer and R Lydon,"Comparative methods for the pore size calibration of filter media." Filtech 2003, Dusseldorf
