Abstract: A measuring instrument such as a manometer adapted to be mounted in a fluid pressure system and having a rigid, generally closed instrument casing, the rear wall of which is provided with an opening in which there is inserted a blow out security element which is combined with a deformable bellow member extending into the casing so as to be able to accomodate thermal expansion and contraction of the gas or liquid enclosed within the casing, the interior of the bellow being connected to the atmosphere through a passage in the outer portion of said blow out security element. Further, as a separate sales article for mounting in an opening in the casing wall of an already existing measuring instrument, an insert unit made as a blow out element having integrally or disintegrally associated therewith a bellow member operable to be inserted through said opening when the insert unit is mounted in the opening.