Diaphragm seals are used to protect the pressure measuring instrument from aggressive, adhesive, crystallising, corrosive, highly viscous, environmentally hazardous or toxic media. A diaphragm made of the appropriate material provides for the separation from the medium to be measured. Thus even the most difficult measuring requirements can be met by combining measuring instruments with diaphragm seals. A fluid inside the system, which can be chosen to suit the particular application, hydraulically transmits the pressure to the measuring instrument. The diaphragm is welded to the top housing and leak proof tested, to guarantee a separation between process fluid and fill transmission fluid. This construction feature and its compact design suits many application that require frequent maintenance.