The invention relates to the area of pressure monitoring devices for blood lines. In such devices a pressure transducer line (12) branches off a blood line (11) to enable the connection of a pressure transducer (21). To avoid any contamination of the pressure transducer a first pressure transducer protector filter (13) is used that is dividing the pressure transducer line into a blood line section (12A) between the blood line and the filter and a pressure transducer section (12B) between the filter and the pressure transducer. It is an object of the present invention to provide a system (40) by which an unnoticed failure of the first pressure transducer protector filter in a pressure transducer line is avoided. The system according to the present invention solves this problem by providing a sensor (24； 24") for detecting the presence of the liquid in the pressure transducer section (12B) of the pressure transducer line. The invention also provides a device (20) to be used in such a system by which an unnoticed failure of the first pressure transducer protector filter in the pressure transducer line is avoided and wherein the device itself can easily be re-used.
