I believe that in saying "certifying fusion at 100% utilizing UT", Mr. Vanderbos was asking whether you can make sure that in a weld there was 100% fusion (and thus 0% lack of fusion) by means of ultrasonic test.
Pipewelder has correctly answered that in fact, UT is actually used to detect lack of fusion, sometimes with higher precision that radiographic test.
Giovanni S. Crisi
Sao Paulo - Brazil
You can never guarantee 100% fusion in a weld with 100% accuracy. You must establish an acceptance criteria and live with it.
Of course there are differences between UT and RT, and Marty is correct in saying indications rejected by UT may be acceptable by RT. You must live with the acceptance criteris you choose initially.
Also, when choosing UT, you must consider your bevel angle. If you are using mechanized welding with, say, a 5 degree bevel angle, then standard single element transducers will most likely miss any LOF in the fill passes. You would need to use a delta technique utilizing, say a 50 degree probe followed by a 45, depending on the depth you are looking at.
If you are concerned with minute areas of LOF, I suggest you look into the use of computerized automated UT. It is much more sensitive and reliable than manual UT.