Richard Jones
I'd be REAL careful with that. If those tanks were suitable for air pressure vs. hydrostatic pressure, the air compressor folks would sure be using them.

Back in another life, I used to build sewer and water plants, using miles and miles of ductile iron pipe. The DI pipe folks will tell you not to air test their product over about 7-8 psi, but you can hydrostatic test at 200+.

Be safe.

Rich