Re: Thin Gauge Steel Tubes : I am looking for a reliable way to UT thin gauge, under 5/16", steel tube that has a circumferential joint joined utilizing a backing bar.
I'm not sure I'm properly visualizing what you're doing, but if it's a brazed or welded joint, we may be able to do something with small diameter, sharply focussed immersion transducers and appropriate fixturing. The tube diameter itself is not a problem. If it's just mechanically clamped, on the other hand, it's not a good application for ultrasonic flaw detection.
I'd be happy to look at a drawing if you can send one.
Re: Thin Gauge Steel Tubes We have been inspecting buttwelds in .056"T by 1.0" diameter buttwelds in inconnel tubing with great success using FAST ultrasonics. The backing ring should not be a problem. Not sure that this application is close to what you are doing. If you would like more information please email me.