A pressure monitoring apparatus (1) for monitoring pressure of a liquid (4) in a vessel (2) comprises a housing (3) within which an airtight gas chamber (6) is formed. A pressure sensor (8) is located in the housing (3) monitors the pressure of a gas in the gas chamber (6). A conduit (12) having a communicating bore (16) of total length Lt communicates the gas chamber (6) with the vessel (2). As the pressure of the liquid in the vessel (2) increases, a liquid/gas interface meniscus (20) travels along a working length Lw of the communicating bore (16) from the second end (15) thus increasing the pressure in the gas chamber (6) to the pressure of the liquid, which is monitored by the pressure sensor (8). The total length Lt of the conduit (12) is greater than the working length Lw of the conduit (12) to avoid the liquid/gas interface meniscus (20) reaching the first end (14) of the communicating bore (16). The pressure monitoring apparatus (1) may also be mounted at the distal end (33) of a balloon catheter (30) for monitoring the pressure of a liquid inflating medium inflating the balloon (35) of the balloon catheter (30), which may be an electrically conductive liquid, with the gas isolating the pressure sensor (8) from the liquid inflating medium.L'invention concerne un appareil de surveillance de pression (1) destiné à surveiller la pression d'un liquide (4) dans un récipient (2) et comprenant un logement (3) dans lequel est formée une chambre à gaz étanche à l'air (6). Un capteur de pression (8) situé dans le logement (3) surveille la pression d'un gaz dans la chambre à gaz (6). Un conduit (12) présentant un alésage de communication (16) de longueur totale Lt fait communiquer la chambre à gaz (6) avec le récipient (2). Lorsque la pression du liquide dans le récipient (2) augmente, un ménisque d'interface liquide/gaz (20) se déplace le long d'une longueur de travail L w de l'alésage de communication (16) à partir de la seconde extrémité (15), augmentant ainsi la
