pressure vessel testing

huangyhg

pressure vessel testing
old tester- diaphram pump, pressurizes 70 psi / stroke then regens until vessel breaks.
new tester- i designed a feed back loop analog control to regulate the pressure gradient at 80 - 100 psi / sec. using a booster cyl., the pressure is gained linearly and adjusted 100 times a sec.
both mediums are water.
both sample lots used in the testers are from the same materila batch ( polypropelene ) from the same process, at the same manufacture time and from the same press.
problem: the old tester breaks vessels at 700 psi + -
the new tester breaks them at 500 psi + - .
could there be a stress re-allignment going on in the old test that is not present in the new, due to the rest between pulses. ( rest time is approx. .2 sec., pulse is approx. .3 sec. ) which is allowing the vessel to fail at a higher pressure ?
other factor - new uses threaded nipples and the old uses a air cylinder to clamp.