Vacuum Box is used to check for any leaks or faults in the welding of bottom and annular plates of the tank. It is a container made of some factory with an acrylic sheet on top with an inlet valve connecting to the Vacuum Pump.
The Vacuum Pump attached creates a vacuum in the vacuum box, which shows bubbles on the soap water applied on the weld incase of any leaks or weld defects. It is one of the most adaptive method to test for any defects or leaks in the bottom and as efficient as radiography is for shell weld tests.