In various embodiments, the present invention provides an improved apparatus and method for quantification of fluid loss from a nominally closed system using a pressure decay with mass point leak rate analysis methodology, which avoids the need for repetitive testing due to test configuration incompatibilities and allows for real time analysis and test control based upon desired results. The novel apparatus and methods of the present invention utilize a control system to maintain a desired pressure differential across the test article throughout the test by automatically raising or lowering the downstream pressure as the fluid leaks through the test article. In various embodiments, the apparatus and method of the present invention improves the efficiency of existing test methods by shortening otherwise long test durations, right-sizing the amount of data collected, providing quantifiable and controllable measurement uncertainty, reducing statistical variance, and eliminating post-process data analysis.