Abstract: The invention relates to a method for calculating the strength and the service life of a process apparatus through which fluid flows, wherein: temperatures existing at a plurality of different points of the apparatus are measured at a first time point in order to obtain temperature measurement values (201); the temperature measurement values are used as constraints in a finite element method (203) in order to determine mechanical stresses existing at a plurality of different points in the material of the apparatus as stress values (204); the remaining service life of the material of the apparatus is determined from the obtained stress values (205); the remaining service life of the material of the apparatus is determined also in dependence on data regarding the apparatus that were determined at a second time point (207), which second time point is earlier than the first time point.