5


Figure 4. Narrow distribution microspheres give higher resolution results than broad distribution irregular particles


1. Several tests must be performed until the smallest size failing to pass the filter is found. 2. Single size monospheres have a limited range of sizes.


3. Filter pores are very rarely all the same size, so there will always be some ‘bleeding’ through the larger pores.


4. Single size monospheres are very expensive.


The optimum challenge test particles are therefore spherical in shape with a distribution somewhat broader than monodisperse, but narrower than the Arizona Test dust particles.


The only disadvantage of narrow size distribution challenge particles is that a wide range of standards must be prepared to cover all pore sizes (Table 1).


When the filter standards in Table 1 are used in the Challenge Test, it is possible to test with measurement uncertainties of less than 1 micron [6].


Figure 6. Raw data of particles passing a filter indicates a maximum aperture size of approximately 59 microns (Number average from the Whitehouse Image Analyser)


Figure 7. Removing non-spherical particles from a Challenge Test (Figure 6) reduces the maximum aperture size of a filter from 59 to 49 microns


When the non-spherical particles are electronically removed from the analysis, the maximum aperture size of the filter is reduced from 59 microns to 49 microns (Figure 7).


Table 1. Range of NIST traceable glass microsphere filter standards (microns) 2 - 6


16 – 25 45 - 62


106 - 147 252 - 346


Conclusion


An understanding of the basics of particle size analysis is essential when it comes to interpreting the results, whether it be in the analysis of a product or the use of particles in a calibration application.


Large errors can occur if for example, number averaged data is compared with volume or weight averaged data. It is essential to understand the parameters produced from the various particle sizing methods. For example, sieve analysis measures the width of particles, whilst most other methods generate the equivalent spherical diameters of particles.


Figure 5. Challenge Test apparatus for filter calibration Accounting for Misshapes


In the preparation of the calibration microspheres, crushed glass is spherulised in a melt process. There is therefore always the possibility of some particles not melting into spheres and others colliding and fusing together. Although most misshapes are removed during the refining process to produce the filter standards, the few that remain must be accounted for in the particle size analysis of the particles passing the filter.


The Challenge Test apparatus consists of a simple split filter holder for the filter under test, gravity or a vacuum to draw the suspension of challenge particles through the filter and an ultrasonic probe to prevent particle build-up on the surface of the filter, (Figure 5).


After the Challenge Test, microscopy and image analysis is used to measure the particles passing the filter (Whitehouse ShapeSizer [5]). The raw data averaged on a number basis is shown in Figure 6.


It can be seen that the largest particles present have a shape factor (maximum/minimum) greater than 1, showing they are non-spherical, which would lead to an overestimation of the filter aperture size (as illustrated in Figure 2).


In the filter Challenge Test method, failure to recognise the influence of particle shape can cause significant errors in the results. Microscopy and Image Analysis is unique in being able to measure particle shape and eliminate non-spherical particles from the analysis, producing highly accurate results.


It is hardly surprising therefore, that one of the biggest growth areas in particle size instrumentation is in the Image Analysis sector.


References 1. Dictionary of Filtration and Separation, S Tarleton and R Wakeman, Filtration Solutions, 2008, ISBN:978-0-9559346-0-5


2. Coulter Counter, www.beckmancoulter.com 3. Elzone Electrical Zone Sensing, www.micromeritics.com 4. Arizona Test Dusts, www.particletechnology.com 5. Whitehouse Scientific Image Analyser, the ShapeSizer, www.WhitehouseScientific.com 6. ‘Challenge Testing Filters Using Certified Microspheres’, Graham Rideal, FILTRATION, 11 (3), 2011, p 172


5 - 9


20 - 34 53 - 73


127 - 175 304 - 417


9 - 11


26 - 36 63 - 86


151 - 209 360 - 498


10 - 14 31 - 46 75 - 103 180 - 248 383 - 591


12 - 18 36 - 55 80 - 103 214 - 295 484 - 700


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