The static probe was found to give similar time domain results to the wheel probe. If time-of-flight measurements are required rather than amplitude measurements as is the case in a typical thickness gauging application then the static probe (and the wheel probe if required) can operate successfully on rough surfaces. Very rough surfaces can be inspected with a hybrid system which requires a small amount of coupling liquid. In this system the rubber conforms around the larger surface roughness contours and the small amount of liquid couplant fills the remaining gaps. For example a light wipe of the rubber with couplant was sufficient to provide coupling for several measurements with the static probe.