Definition and principle
The bubble point method is the most widely used for pore size determination. It is based on the fact that, for a given fluid and pore size with a constant wetting, the pressure required to force an air bubble through the pore is inverse proportion to the size of the hole.
The procedure for bubble-point test is described in American Society for Testing and Materials Standard (ASMT) Method F316.
D = (4g x cos q) / P
Since no pores in a practical filter element are likely to be shaped like capillary tubes it is necessary to introduce a shape correction factor K into the formula.
D = K1 / P
D is again the maximum average diameter of the pores in mm.
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Appearance of bubbles
One of the great advantages of the bubble point test is that it can be performed of filters under actual use conditions and with any filter. It is a non-destructive test, thus it does not contaminate the filter and so can be used to determine the integrity of a filter at any time, as well as establishing the absolute rating.