Diffusion flow test comes from the phenomena. That is, when the bubble point is under certain pressure, the pressure difference on the two sides of filter membrane because air or nitrogen push the liquid across the holes of filter membrane. On the small size filter, diffusion flux is very slow so that it can not be measured. However, as for large size filter, diffusion flow is a very important and sensitive test method.
When the filter cartridge is soaked, certain volume and pressure of air in upside of filter will be isolated. When the pressure of input air is approximate to the value of bubble point, a large amount of gas has not yet across the holes.
For the specific diffusion flow measurement, the 80 percent or so of bubble point value is generally recommended as the initial pressure value of diffusion flow measurement, and the signal for the diffusion flow value in the section is an amplified, calibrated and converted pressure attenuation signal and is capable of sensitively reflecting the integrity condition of the gas-liquid phase on the two sides of the pore of the filter membrane. In the circumstance that the inflection point cannot be found easily in case of unobvious pressure attenuation tested at the early stage, the application of diffusion flow method is proved to be reliable compared with other methods especially for the measurement of PES or other asymmetric membrane filters.