Abstract: For producing a tube connection for a capillary tube, the connecting end is provided with an engaged connecting cap, then on the tube end is produced a funnel-shaped widening adapted to an extension and finally it is frontally fixed with a welded joint to the connecting cap. The thus formed mouthpiece can then be so connected with a connecting unit, e.g. a diaphragm capsule, a temperature sensor, etc., that the connection, completely covered to the outside, is located in the fluid space of the duct system. All the working steps can be carried out completely automatically, e.g. on a transfer line.