The tube to tube through transmissions technique for inspecting finned heat exchanger tubes

Finned tubes are used in chemical processing and air conditioner heat exchangers. Conventional electromagnetic methods cannot be used to inspect the finned tubes for flaws because of the interference caused by the fins. We describe here tests of a recently developed method, the Tube to Tube Through Transmission (T4) technique which uses coplanar exciter and detector coils in neighboring tubes. As the method is new there is, as yet, little theoretical or experimental explanation for it. We report first, therefore, studies of the magnetic fields and the induced eddy currents. Since the coupling path between exciter and detector coils is primarily diffusive, skin effect equations can be applied to give useful approximations for signal analysis. Signals from machined full circumferential grooves in finned heat exchanger tubes were analyzed using these equations, and it was found that the depth of the grooves or general thinning could be estimated to within ±10%.