Vacuum Box Testing:

Vacuum box bubble test is one of the safest and reliable non-destructive test methods.
The vacuum box bubble test can locate leaks in the test area that cannot be directly pressurized.
Primary application is new tank floor construction in the petrochemical field. 
Has been performed on old construction'; i.e., painted lap welds to confirm leakage conditions on tanks that are being taken out of service. The test is performed by applying a detergent solution on the test area and creating a vacuum around the test surface by external mechanism e.g. vacuum box.  The detergent solution will bubble if there is any leakage through the test area. Vacuum box testing is commonly used in tank inspections for roof and floor repairs. Special box shapes can be made to test corners and curved surfaces. 

Vacuum box testing is used to test the integrity of the seal between two plates, usually on a lap weld joint. A bubble solution is first applied to a clean, bare weld surface. The vacuum box is then placed over the joint and the compressor turned on until the gauge indicates a vacuum. Any tiny holes or cracks that penetrate through the joint will cause air to be drawn up from the opposite surface, through the solution, and will form bubbles.