I have talked with an engineer about the pressure vs. Cubic inches of volume and the bottom line is the larger your chamber the less pressure you can put on it if it is the same gauge steel.
When you increase the thickness of the steel you also have a limit on how much pressure the rubber seal on the lid can hold.
The solution is a reverse sealing lid. The pressure needs to push the seal closed. On most paint pots clamps hold the lid and work against the pressure. In larger units you need to have a lid that folds in. That way the pressure inside pushes on the seal and makes it stronger as pressure increases.
Another aspect to larger chambers is they need to be cylinders. Making a chamber square puts to much stress on welds. This theory is proven in liquid containers. A square water tower just does not work.
If you want to construct your own large chamber, you must not use cast iron pipe. Steel is the only solution. Hit a cast iron pipe the a hammer and it will shatter. Hit a steel pipe and depending on the type of steel it will either bounce or dent, but still retain its integrity. Cast Iron pipe under pressure equals a fragmentation grenade.
I am working on a 3 foot steel tube as well and I can tell you there are large dangers involved. If not built properly say goodbye to your house.