Normal Metal Tanks...

If I can't get my spectra tanks to work, I need a backup plan, toward that end we are fabricating two possible tanks. A Tank made out of aluminum thinwall tubing and hydro formed end caps. I spent Saturday fabricating end caps that will be welded to the aluminum thinwall tubing.
Here is a cap being formed in the forming fixture:


After pumping about 600 PSI water into the fixture the pressure is releived and the endcap is removed and put on the mill, indicated for center and then machined out of the blank.


The result is :



My son welded up the stainless hemispheres, and we tested these...


The first test sprang a leak at 450PSI in the weld, we patched that and it sprang a leak at 590 PSI, we patched that and it sprang two leaks at 710 PSI. All leaks so far have been in the weld area. The theoretical burst for this tank should be between 800 and 1300 PSI depending on if you give any stress credit for stretching the tank and work hardening it. So 710 is close to this value. The MR of this 18" tank is about 7.6 . The aluminum tanks we will weld up Sunday have a theoretical burst of 550 and an MR of 9.6.