By Dr. Graham R. Rideal, Managing Director, Whitehouse Scientific Ltd., Whitehouse Scientific
Subscribe1. Introduction - The apertures of plain weave meshes can be measured by microscopy because they are open to transmitted light, figure 1. However, 3-dimensional duel layered meshed cannot be measured by microscopy because they are opaque, so a different method of pore size measurement is required, figure 2.
Pore size measurement has been traditionally performed by the Bubble Point measurement whereby the maximum aperture size present can be related to the pressure at which a bubble appears on the top side of a wetted filter medium pressurised 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 method and only gives ‘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 a 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 3.
Figure 3. The effect of particle shape on particle size measurements
Figure 4. Particle shape and aperture shape affect filter efficiency (Courtesy of G Bopp)
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 4. 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. 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 paper describes the preparation and use of narrow particle size distribution glass microsphere standards in measuring the pore sizes of some of the latest high performance filter media.
Figure 5. A 53 – 73mm filter calibration standard
2. Experimental
2.1 Preparing microsphere standards
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.
In this work, approximately 25 narrow size distribution microsphere standards have been prepared to cover pore sizes from 5 to over 600mm, figure 5.
Figure 6. Electroformed sieves are extremely uniform and are certified to NIST standards
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 3 it was seen that particle breadth or sieve size is the most relevant parameter.
Wire woven sieves have an unacceptable wide distribution in aperture sizes so NIST traceable precision electroformed sieves have been used for certification, figure 6. However, only three electroformed sieves could be used for analysis because the narrowness of the distribution, so the data was supported by microscopy to ensure a uniform distribution. Provided the results were comparable, the sieve data was then used to construct a calibration graph of the percentage passing a filter to its pore size, figure 7.
Figure 7 (a) NIST traceable Electroformed sieve and microscope results (b) Calibration graph from a 53 – 73 micron filter standard certificate
The certification of the filter standards using the NIST traceable Electroformed sieves was performed using a Gilson Sonic siever where it was possible to sieve down to 5mm in the dry state, figure 8.
In this work however, the minimum size analysed by electroformed sieves was 10mm. Below about 10mm an international team of particle size specialists were used to analyse the standards using a range of methods including sedimentation, Coulter counting and microscopy/image analysis.
The master batches of each filter standard was then subdivided into single shot bottles sufficient to test a 90mm or 50mm specimen of a filter medium.
2.2 Pore size measurement by dry sonic sifting
Figure 8. A Gilson sonic autosifter for certifying standards and testing filter media
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 often tortuous path 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 ensures that there is efficient penetration through even the most complex filter media, figures 8, 9 and 10.
To measure the pore size of filter, a 50mm or 90mm disc was clamped into the filter holder and the appropriate calibration standard fluidised on the surface. The end point corresponded to a change in weight of less than 1% passing per minute and was usually achieved in 1 – 2 minutes. The pore size was then determined from the filter standard calibration graph, figure 7(b).
The pore size measured is approximately 97% of the maximum particle passing the medium when measured by microscopy, or effectively cut point or retention quality of the filter medium.
Figure 9. A 50mm filter holder Figure 10. Sonic sieving action
As a final corroboration of the cut point of the filter meshes, single size glass microspheres could be used to confirm the maximum pore size. In this case the beads would either pass or be retained on the filter medium.
2.3 Pore size measurement using an ultrasonic wet system
Figure 11. Apparatus for measuring sub 15mm pore sizes
Although the Gilson Sonic sifter can measure particles on an Electroformed sieve down to 5mm, the restricted flow through filter media of a similar pore size makes fluidisation almost impossible and a wet, ultrasonic method must be used.
The apparatus employed was a simple split filter holder on a Buchner flask, figure 11. An ultrasonically dispersed dilute suspension of an appropriate filter standard was then drawn through the filter under test by vacuum. The particle size before and after filtration was analysed by microscope and image analysis.
3. Results and discussion
Three G Bopp Betamesh samples were analysed by the Sonic challenge test: Betamesh 75, Betamesh 40 and Betamesh 25 of nominal pore size ratings of 66 - 74mm, 34 - 38mm and 22 - 26mm respectively. The microspheres passing the meshes were then microscopically examined to determine the maximum size passing. Finally, based on the results from the first two methods, monodisperse glass microspheres just above the determined cut point were sonically sifted to ensure that none passed the filter meshes.
The ultrasonic wet challenge test method was evaluated on a filter mesh of nominal size 10mm. A microsphere standard calibrated by microscopy was transported through the filter medium and the particle size of the down stream component examined to determine the pore size.
3.1 66 - 74mm wire woven filter mesh (Betamesh 75)
3.1.1 Pore size by the sonic challenge test
Approximately 0.3g of a 53 - 73mm filter calibration standard was sonically sifted on a 90mm disc of the Betamesh 75 filter mesh when 16% of the glass microspheres passed. From the Certificate calibration graph, a corresponding pore size of 56mm was determined, figure 12.
Figure 12. 53 - 73mm standard calibration graph used for Betamesh 75 Figure 13. Microscope analysis of filter standard passing Betamesh 75
3.1.2 Microscope analysis of standard passing
Figure 13 compares the particle size distribution of the 53 - 73mm filter standard before and after passing the Betamesh 75 filter. The D97 size (97% of the maximum) was 64mm.
3.1.3 MonosphereTM retention
Monodisperse glass microspheres having a peak size of 61mm were sonically fluidised on the mesh without passing through indicating that the cut point was below about 60mm.
The effective cut point of this mesh was therefore somewhere between 56 and 60mm.
3.2 34 - 38mm wire woven filter mesh (Betamesh 40)
Figure 14. A 31 - 46mm filter standard Figure 15. Microspheres passing Betamesh 40
65% of the 31 - 46mm standard passed giving a cut point of 40mm while analysis of the beads passing indicated a D97 pore size of 43mm, figures 14 and 15.
A 45mm MonosphereTM would not pass confirming a pore size/cut point of 40 - 43mm.
3.3 22 - 28mm wire woven filter mesh (Betamesh 25)
Figure 16. A 20 - 33mm filter standard Figure 17. Microspheres passing Betamesh 25
35% of a 20 - 33mm standard passed the filter mesh giving a cut point of 24mm, figure 16, this was very close to the size measured by microscopy, figure 17.
85% of a 23mm MonosphereTM passed the filter while only 4% of a 26mm MonosphereTM passed. The maximum pore size or cut point of the filter mesh was therefore very consistent at 24mm.
3.4 Ultrasonic wet challenge method for fine pore sizes
Figure 18. Ultrasonic wet challenge test for a 10mm filter mesh
A special stainless steel woven filter was designed by G Bopp to operate in a very critical application where total exclusion of particles over 10mm was required. Bubble point measurements were not acceptable because of the uncertainty of this theoretical method of calculating pore size.
In this case an 8 - 16mm standard was ultrasonically dispersed over the filter and drawn through under vacuum. The initially cloudy suspension was almost clear but the few particles that were collected all had a size below 9.5mm, figure 18.
The product was therefore within specification.
4. Conclusions
The challenge test method of measuring pore sizes using narrow particle size distribution glass microsphere standards 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.
Microscope analysis of the portion of the standards passing the filter media were in excellent agreement with the cut point of the filters determined by the sonic calibration method.
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.
5. Bibliography
G R Rideal and J Storey, ‘A new high precision method of calibrating filters’, J. Filt. Soc., Vol 2(3), 2002, p 18 - 20
E Mayer, G Warren, ‘Evaluating filtration media, a comparison of polymeric membranes and non-wovens’, Filt. and Sep. Journal, 33, (10), 1998, p 912 - 914
D B Purchas and K Sutherland, Handbook of Filter Media, 2nd Ed., Elsevier Advanced Technology, ISBN 1 85617 3755
Micron rating of filter media, SAE APR901, March 1998
G R Rideal, E Mayer and R Lydon, ’Comparative methods for the pore size calibration of filter media.’ Filtech 2003, Dusseldorf
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