I designed and built a burst tester to pressurize at a rate of 80-100 PSI/Second using water. The system is controlled by an analog feedback loop that regulates the pressure gradient. The old tester, uses a diaphram pump and pulses pressure until vessel failure.

Problem:
On the old tester, I am getting failures at 700+- PSI.
The new tester is getting failures at 500+- PSI.

Both units are testing vessels from the same batch of PolyPro plastic and the same press at the same time, from the same process. Why the difference in failure pressures?
I would have thought that the pulsing would have failed at a lower rather than higher. I was wrong. Can anyone assist me.