A device for displaying and measuring air losses in a chest drainage apparatus, having a transparent window enclosure with a connection to a tube end connected to a chest cavity to convey exuded liquids for collection a vessel and air losses, a chamber being provided within the apparatus at one of the transparent windows, that contains liquid and comprises the outlet section of a duct for air loss conveying. The device comprises a bell arranged above the outlet section, immersed in liquid at least at a lower end area, and provided, at the submerged portion, with at least one always-open air outflow port and with at least one additional port for the outflow of air.