A humidifier is provided including a first humidifier part, a second humidifier part connectable with the first humidifier part, and a sealing gasket disposed between the first and second humidifier parts. The second humidifier part is configured to hold a volume of liquid therein and the first and second humidifier parts and the sealing gasket define first and second internal passages within the humidifier. The first passage is disposed so as to be exposed to a surface of the volume of liquid and the second passage is isolated from at least one of the first passage and the surface of the volume of liquid. The second passage is configured to communicate with a pressure and/or sound sensing device.