A saliva treatment system (1, 5, 6) of an oral interface appliance (100, 200, 202, 500, 600, 600a, 600b) and a method for measuring liquid capacity in a reservoir (14, 55, 64) of the saliva treatment system (1, 5, 6). The saliva treatment system (1, 5, 6) comprises an oral interface appliance (100, 200, 202, 500, 600, 600a, 600b), a fluid adjuster (50) and a communicating tube (12, 52, 62). The oral interface appliance (100, 200, 202, 500, 600, 600a, 600b) is placed within the oral cavity of a user, the fluid adjuster (50) is communicated with the oral interface appliance (100, 200, 202, 500, 600, 600a, 600b) and the communicating tube (12, 52, 62). The communicating tube (12, 52, 62) is communicated with a negative pressure source (15, 56, 65) to create a negative pressure path between the oral interface appliance (100, 200, 202, 500, 600, 600a, 600b) and the negative pressure source (15, 56, 65), and saliva in the oral cavity of the user is collected into the reservoir (14, 55, 64) through the negative pressure path. The fluid adjuster (50) serves to restrict all fluids from flowing towards the communicating tube (12, 52, 62), so that air is fed, either constantly or as needed, into lines of the saliva treatment system (1, 5, 6) in the case that the negative pressure environment within the oral cavity of the user is maintained, contributing to removal of the saliva from the oral interface appliance (100, 200, 202, 500, 600, 600a, 600b) and the communicating tube (12, 52, 62).
