[elbrownos]

Those sound like great reasons to use something sensible like a hydraulic ram to apply 1,000,000 lbf. Force = pressure x area, and neither pressure nor area is difficult to measure precisely. I can't see the upside of using 1,000,000 lbm of stainless steel.

[kabouseng]

It's much simpler to compensate for dead weight to get as exact as possible measurement. With a hydraulic press you need to compensate for the compression and heat buildup of the liquid etc.

[adaml_623]

It's easy to measure area but how do you calibrate your pressure sensor?

I think you'll find that this device is used not as a testing tool but as a calibration tool.