Through an ultrasonic test to DIN EN ISO 17640, for example, full penetration welds on thermowells can be investigated with respect to irregularities (cracks, voids, insufficient bonding). To do this, the reflections of a radiated ultrasonic signal from the interfaces of irregularities are measured. To determine the position of the irregularities, the ultrasound machine is set in advance with the aid of a reference body. The ultrasonic method can also be used to measure the wall thickness of a thermowell, in order to determine the bore centrality.