Pressure decay testing, monitoring a gauge for a pressure drop over a specified period, is unacceptable as a standalone leak test under ASME B31.3 because the code explicitly mandates a 100% visual examination of all joints and connections while the system is under test pressure.
While a pressure decay test can indicate that a system is leaking, it cannot definitively prove a system is completely leak-free, nor can it tell us where a leak is occurring.
Here is a breakdown of why this method does not meet the code requirements, along with the engineering rationale.