In order to manufacture catalytic converters, being able to measure the exact diameter of the stainless steel housing is critical for inserting the ceramic element. This is why the diameter of circular catalytic converters is checked using a special measuring device. A confocal sensor with a special optical system is used for this task. The test item is manually inserted and centred in the holder. After starting the measurement procedure, the diameter is checked on three levels. The external diameter is directly measured, while the minimum circumscribed circle and maximum inscribed circle are calculated by evaluation software on the basis of these results.