There has to be some type of measurement of the filtration process; in this case where flow is not tested and the feed pressure stays constant, it would be filtration time. If the filtration time increases, one can be assured that the filter fouls and becomes blocked, meaning the risk of masking increases. Having said this, is required for terminal sterilizing grade filters, before the filling system and one would have to measure flow to feed the fill line. In addition, in the initial trial for to size a filter system for a new application, filterability trials would be performed. These trials determine how fast the filter blocks and according to these trials a filter system is sized for the specific application. The filterability trial creates a good indication whether the fluid is high foulant or not and how the filtration system requires to be configured and sized to avoid any elevated blockage of the filter.