When you are sick, would you rather treat the sickness by searching for the symptoms to find the cause or find the direct cause of the sickness to treat it. I think we all can agree that we would want to treat the actual cause of the sickness. In a sense when you use a pressure decay test method for hydrostatic testing your are searching for the indirect result of the actual leak rate. When you test for the leak rate, you are testing for the direct result of the test which not only makes much more sense but eliminates additional variables that can be associated with standard pressure decay tests.
Pressure decay is an older technology that has was developed for over a century ago. It is an indirect measurement of leak rate. In pressure decay, the BOP under test is pressurized, then isolated from the pressure supply. The pressure within the BOP is then monitored using a pressure sensor, seeking for a pressure drop. Pressure decay testing can be subject to many variables such as thermal effects, trapped air, and the compressibility of the BOP. Pressure decay can be easily manipulated by these variables.
A more precise method is to directly measure the leak rate. In this test the BOP is also pressurized, but in this case the BOP remains connected to the pressure supply. Once an equilibrium is reached and the pressure inside the BOP is stabilized, the amount of fluid add to maintain equilibrium equals the amount leaking out. Leak rate is a direct measurement. Measuring leak rate is relatively insensitive to changes the BOP volume, compressibility, trapped air, or other variables and does not need to be recalibrated if a larger, or smaller, BOP is tested.