Connecting device for a measuring apparatus for carrying out measurements in a space closed or sealed off by a wall, such as the space within a pipe, tube, casing or any other housing part of an appliance, characterized in that it is formed by a plate shaped part on to which a body is mounted, the diameter of which body is smaller than the diameter of the plate shaped part, while the connecting device is intended to be placed in an opening to be made in the wall, the diameter of which opening is larger than the diameter of the body but smaller than the diameter of the plate shaped part, in such a way, that the body can be sunk in the opening in the wall while the plate shaped part remains supported at its edge on the wall, whereby the plate shaped part and the body are provided with at least one measuring opening through which the space is accessible to the measuring apparatus, and whereby the outer side of the body is provided with clamping feet protruding in the direction of the bottom side of the plate shaped part for the clamping of the wall between the clamping feet and the plate shaped part.