Various shapes of catalysts require that the stainless steel casing has a precise dimensional stability. For round catalysts, stainless steel tubes are therefore subjected to a diameter check in a special measuring system. A confocal sensor measures the distance to the tube, which is rotated through 360°. The outer diameter, the minimum circumscribed circle, and the maximum inscribed circle are directly determined by software calculations. The sensor moves in a horizontal direction, depending on the size of the catalyst.