Bubble Point is often used to measure the largest opening in a filter medium, but it can give variable results, firstly because the area of the sample required for testing is infinitesimally small compared to the total area of a filter medium on a roll and so may not be representative of the whole. Secondly, errors may occur in the theory relating to converting the observation of a bubble to a maximum pore size or Maximum Penetrating Particle (MPP).

An alternative method is Challenge Testing, where particles are presented to the filter and those passing measured. In this case, particle shape is an important factor as irregular particles must be aligned within pores in order to pass through what could be a tortuous path. The results are then dependent on the method used for particle size analysis.

To minimise measurement uncertainties associated with particle shape therefore, spherical glass beads should be used in Challenge Testing to determine the Maximum Penetrating Particle (MPP).In this work, analyses of beads passing sand screen filters were performed ...