Good results. Glad to know my calculations fit your results, albeit Im more "conservative" as mine are not real tests.
Im using a safety factor of 0.8 as per ABS rules.
About end caps...
The goal is achieving a pressure between disks gaskets and tube, bigger than hydrostatic pressure. A reasonable factor would be 0.8 (1.25 times bigger).
The problem is that as depth increases, tube buckling increases as well. Distortion leads to a reaction at the inner disk, wich will be the only one working as a support, making effective pressure on the gasket to decrease.
At the same time, all radial pressure is transferred to only one of the disks, the inner one, which own buckling is increased, by the way could achieve an overload situation.
As it can be seen at the pic above, a collateral effect, is that effective force on the gaskets and disks, starts to have a significative axial component (F'), and then .....
- Force is no more "normal" to disks and gaskets. Arrangement efficiency decreases (Is a cuadratical and not a linear function of hydrostatic pressure)
- Reaction force + Pressure -----> Disks distortion increases.
For depth range improvement:
A new design of "End Caps" is required. That design, would have to work against the E Tube distortion. May be by an external ring with its own gaskets working from outside.
A way to limit the E Tube distortion is also a requirement. This, from my point of view can be achieved by a thicker wall, or by a "wise" internal framing.
Finnally as we say here "There always exists a Technical indetermination, but never an echonomical one" or ... waht is the same "Among all thechnical solutions, the cheaper is the better"