I would not like to see the film as I figure your Ug would be way out. Plus I would expect you to be using the slowest film you had. I have in the past had to shoot 3" WQT's as per Client instructions Panoramic not a nice film but was to Client instruction. If it is a Client instruction you point out the problem then shoot the film.
I expect you would be using a source, as per your Radiographic Procedure, which is Client Approved to give you acceptable films in normal Radiography, and as you say DWSI, will bring in longer exposures, you could well have taken the best possible option, by doing it SWSI, and as you say it will be what is missed, but you have to account for the Inspector, and how he views the film.
It would be good to have a calibration block like in UT where you have different depths/sizes of defects for sensitivity control but with the variety in RT its not so easy to obtain. A lot of shots may look good but you dont know whats being missed!
You might try a mock-up shot. Try to find a weld that is within reaching distance of the end of the pipe, like a short stub onto a flange or elbow. [If nothing else, RT a blank piece of pipe]
Place the proper IQI packet on the inside of the pipe - source side- tight against the pipe wall and see if you can get proper sensitivity. Ug can be ignored [for cause] if proper sensitivity can be proven.
RT of the actual material is always better than a calibration block, and is usually faster and easier. "When in doubt, ask the parts"
This is an interesting discussion, and one in which I feel we can also gain valuable insight into how RT is practiced around the world.
1. I agree with the statement that the Ug is far off to begin with, because the focal spot dimension to be used is the largest inscribed dimension of the source. In this case, a source which is essentially a cylinder made up of wafers or discs that measures 2.0mm in one dimension and 1.12mm in the other, will have a maximum inscribed dimension (the diagonal inside the cylinder) of 2.3mm. Unless you have certainty of how this cylinder is going to be oriented when facing the object and projecting onto the film, which we seldom do, it is important to use the worst case scenario which would be 2.3mm. Using this number would yield a Ug = 0.849mm
2. I would like to add that according to ASME Section V, Article 2, paragraph T-221.1 states that RT must be performed in accordance with a written procedure and lists the essential parameters which must be included in that procedure. Paragraph T221.2 then goes on to say that demonstration of the density and IQI image requirements shall be considered satisfactory evidence of compliance with the procedure.
3. Although Section V, Article 2, paragraph T-274.2 sets Ug limitations (namely 0.51 if working in mm), other sections of the code take exception to this, for example, Section I (Power Piping), paragraph PW-51, which states that the requirements of T-274 are to be used as a guide but not for the rejection of radiographs unless the geometrical unsharpness exceeds 1.8 mm. So clearly the intent of the code is that you comply with density and IQI image, with prevalence to Ug. Based on this I would contradict myself and tell you that the Ug was not way off after all, IF you can achieve both, good density, and the required IQI level.
4. The best advice was already given in this thread in succinct terms, when in doubt, ask the part. If you do a couple of test shots and qualify your procedure youll know where you stand.
I hope this helps, and thank all contributors for the healthy discussion.