Floating or fixed?
Floating you spray diesel on the bottom and look on the top. On a fixed, all you can do is spray diesel on the roof angle. Only thing is if it is fixed and a frangible joint, you no want it welded too good to start with.
Whole idea of a frangible joint is for it to fail.
If it is a EFR pontoon, you want the diesel sprayed on the non welded side of the pontoon. Pontoons can be pressurized to 5 PSI and the welds soap tested instead of diesel tested. Keeps the pontoons cleaner.All the deck laps need diesel tested.API 650 Appendix C. A IFR, Internal Floating Roof falls under Appendix H
If this is a repair API 653 Appendix F if this is new construction API 650 Appendix T has the requirements for pen testing. 650 7.2.4.1d requires a 4 hour minimum dwell time for the corner weld and 4 hours minimum is considered accepted on Appendix C 4.2 ...with the methods described in this Standard.
Hint: Just before the tank is filled for hydro, spray the laps. Tanks that have passed a diesel test will sometimes leak as the pressure of the water and weight of the roof forces the diesel in the lap joints.
Do you have a 650 and 653? You cannot inspect BRT's without them.
Go through Appendix D for response to technical inquiries. Save you a lot of time.
Also here is a link to a AST forum.
http://www.ast-forum.com/index.aspIf you are in the BRT business, this is a great resource.