Capillary Flow Porometry in the Study of Filter Media

Filter

If I was to ask you to name something whose porous nature people might want to investigate, I bet that ‘filters’ would be a popular answer – if not the very top response. Today I’d like to have a look at filter media and what a porometer can tell us about them.

If you have read some of the previous posts you will know something about the underlying principles of porometry (or porosimetry). The specific porometer used to characterise a filtration medium might well be a capillary flow porometer.

Briefly, this is how it works:

Place a fully wetted sample in a sealed chamber. Introduce gas and allow pressure to increase until it is just enough to overcome the fluid’s capillary action in the largest pore. Having reached that point, increase pressure and measure flow rate as the liquid empties from the pores. Then measure gas pressure and flow rate through the dry sample. Information on various pore parameters can be computed from the pressures and flow rates measured.

Like many porous materials, the structure of filter materials doesn’t simply consist of evenly spaced, evenly sized and evenly shaped pores. It’s much more complicated than that. There may be three different types of pore. ‘Closed pores’ are totally enclosed by solid material, so the liquid and gas cannot reach them. ‘Blind pores’ are connected to the material’s surface, so they can fill with liquid or gas, but they are ‘dead ends’, so they do not allow any flow.

‘Through pores’ are the third type. These run from one surface of the material to another, allowing fluids to flow through it. The diameter of these through pores may vary considerably along their length. The minimum pore diameter – in other words the diameter of the pore’s most constricted part – is crucial, as this determines the size of the smallest particle whose passage the filter will prevent.

The largest values for constricted diameter are important, as are the mean through pore diameter and the pore distribution, as these influence the separation efficiency of the filter. The surface area of the through pores is another key factor, as small particles will often stick to that surface. Other parameters measured by the porometer include the envelope (outside) surface area and the material’s gas and liquid permeability.

Pore characteristics can be investigated in the thickness direction, in the x-y plane and even in the different layers of composite filters – without having to separate them. More than this, the porometer allows investigation of the effects of different temperatures, pressures, chemical environments, physical stresses and other variables on the material and its properties. So, as you can see, the capillary flow porometer is a very useful and versatile instrument.

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