Ultrafiltration, the final filter step in ultrapure water (UPW) supply plants in advanced semiconductor fabs, removes nanoscale particles, macromolecules and pyrogens and is confirmed to be highly reliable, needing relatively low maintenance and permits long-change out cycles and even tolerating increases in particle numbers in the feed. However, on-line particle counters may reach their lower detection limit and remain undetected therefore to control these small molecules, it is crucial to maintain high integrity of the ultrafiltration membrane which requires better test methods to locate sites of membrane damage. This article expands on ultrafiltration technology for UPW and suggests a non-destructive test of ultrafiltration modules by pressure decay measurements as a necessary step in quality Metrology and Analytical Technology of UPW plants.