100% weld efficiency is decided by the EOR in that he wants to be assured that every inch of weld is meets the NDE criteria of the applicable code. He/she may want this kind of assurance because of the risk factor involved in let's say lethal service.
Most of the pressure vessels I deal with are specced out as 85% JE to ASME Sec VIII Div 1 code therefore RT is is Spot RT according to UW-52. If the vessel is to be "U" stamped, the nameplate shall display RT3.
Complementing what Robert said, ASME VIII Div. 1 states that the welding efficiency is 100% when it has been 100% Xray tested and has met the requirements set forth in the same ASME VIII Div. 1. If Xray test shows that the weld doesn't meet the requirements, it has to be repaired, or, in the worst case, removed and remade.
For spot Xray test (and ASME VIII Div 1 indicates where the radiographs must be taken) the efficiency is 85%. When no radiographs are taken the efficiency drops to 70%.
There are some instances where 100% Xray testing is mandatory, as when the vessel will contain lethal fluids.
Giovanni S. Crisi
Sao Paulo - Brazil
Adding to the others:
"Could there be different answers depending on how the question is asked, or relating to machines, process, consumables?"
Answer, Yes.
What is the alternative to achieve 100% weld efficiency if RT is normally not carried out, for example in fillet welds.
100% dye penetrant test in this case?
By js55 
Date 06-17-2011 17:16
Edited 06-17-2011 17:19
Why is it necessary that you achieve 100% efficiency?
Keep in mind that the efficiency has nothing to do with the mechanical properties of the weld.
You could easily have a weld with 70% efficiency that is stronger, more ductile, and of greater toughness than one of 100% efficiency.
