A measuring device for a heat exchanger includes a heat exchanger pressure pipe having a pipe wall with a circumference, and an indentation extending over and deforming a portion of the circumference. At least one thermocouple is disposed eccentrically in the portion of the circumference deformed by the indentation. Filling material fills the indentation. A method for producing a measuring device in a pressure pipe of a heat exchanger, a method for monitoring an operating state of a heat exchanger having a pressure pipe, a heat exchanger, and a method for measuring a heat flux, are also provided. The size of the indentation can be decreased for a given size of the thermocouple due to the eccentric configuration of the thermocouple, so that the heat flux is obstructed to a comparatively small degree by the pipe wall while local overheating of the pipe wall is prevented.