The surface in the area of the spot weld has, more or less, a strongly formed or deformed indentation. Dispite the unfavourable topography, a good coupling is still achieved by using a water delay path. This flexible arrangement matches itself to the surface conditions. The diameter of the weld nugget is determined by using probes having different crystal diameters. Preparing the probe As opposed to conventional probes, the special probe in this case requires preparation.
Accurate results depend on the condition of the membrane and therefore the water delay path.
To achieve optimum coupling it must be stretched tight so that about 4 mm to 7 mm protrudes. If this cannot be done then the membrane must be replaced and the probe refilled with water. In doing this, special attention must be given to avoiding air bubbles which cause interference echoes. Normal couplant is used between the probe and the surface of the test object. The probe is positioned, using slight pressure, vertically and centrically onto the surface of the spot weld.