A device for determining and/or monitoring at least one process variable of a medium, comprising - a measuring insert with at least one sensor element, in particular arranged in an end section of the measuring insert, for recording the process variable, and - an immersed body in the form of a protective tube for mounting the measuring insert, wherein the immersed body projects at least temporarily and/or partially into the medium and wherein the immersed body encloses the measuring insert at least in one section that faces the medium, in which the at least one sensor element is preferably located, wherein at least one coupling unit is envisaged, which can be introduced into the immersed body and is configured to fill an internal volume between an inner wall of the immersed body and an outer wall of the measuring insert at least in the section in which the sensor element is located, and to guarantee both a mechanical and thermal connection between the immersed body and the measuring insert, characterized in that there is no air gap between the outer wall of the measuring insert and the inner wall of the immersed body, wherein the coupling unit has the form of a solid body and uses graphite as a solid lubricant, and wherein the coupling unit is fixed in position inside the immersed body, and the measuring insert can be removed from the immersed body without removing the coupling unit, in particular for replacing the measuring insert. - a measuring insert with at least one sensor element, in particular arranged in an end section of the measuring insert, for recording the process variable, and - an immersed body in the form of a protective tube for mounting the measuring insert, wherein the immersed body projects at least temporarily and/or partially into the medium and wherein the immersed body encloses the measuring insert at least in one section that faces the medium, in which the at least one sensor element is preferably located, wherein at least one coupling unit is envisaged, which can be introduced into the immersed body and is configured to fill an internal volume between an inner wall of the immersed body and an outer wall of the measuring insert at least in the section in which the sensor element is located, and to guarantee both a mechanical and thermal connection between the immersed body and the measuring insert, characterized in that there is no air gap between the outer wall of the measuring insert and the inner wall of the immersed body, wherein the coupling unit has the form of a solid body and uses graphite as a solid lubricant, and wherein the coupling unit is fixed in position inside the immersed body, and the measuring insert can be removed from the immersed body without removing the coupling unit, in particular for replacing the measuring insert. encloses the measuring insert at least in one section that faces the medium, in which the at least one sensor element is preferably located, wherein at least one coupling unit is envisaged, which can be introduced into the immersed body and is configured to fill an internal volume between an inner wall of the immersed body and an outer wall of the measuring insert at least in the section in which the sensor element is located, and to guarantee both a mechanical and thermal connection between the immersed body and the measuring insert, characterized in that there is no air gap between the outer wall of the measuring insert and the inner wall of the immersed body, wherein the coupling unit has the form of a solid body and uses graphite as a solid lubricant, and wherein the coupling unit is fixed in position inside the immersed body, and the measuring insert can be removed from the immersed body without removing the coupling unit, in particular for replacing the measuring insert.