The invention relates to a pressure indicating device for indicating a pressure in a first fluid system, said pressure indicating device including a first port via which it is fluidically connected to the first system, which pressure indicating device furthermore comprises a cylinder with a preloaded piston that can be displaced therein in an axial direction, during which movement fluid contained in the first system can act on the piston surface. The invention is characterized in that said piston has an opening in its piston surface, which opening can be made to communicate with a second system via a second port, said piston comprising a diaphragm which is impermeable to a first fluid but permeable to a second fluid, said diaphragm separating the first port from the second port.