A sensor for determining a mass flow of a fluid in a pipe includes a wall element adapted for insertion into a cutout in a wall of the pipe through which a mass flow can flow, so that at least one gap remains between the wall element and the wall of the pipe, and so that the wall element is positionable on the outside facing away from the mass flow. A sealed chamber is adapted to enclose the wall element on the outside of the pipe. At least one expandable connection is located in the sealed chamber and is clamped in between the wall element and one wall of the chamber. At least one strain gauge comprising a fiber Bragg grating sensor is located on a section of the expandable connection that is clamped in between the wall of the chamber and the wall element.