A humidification system comprises a first sensor and a second sensor. The first and second sensors are adapted to sense flow characteristics within the system. The first and second sensors are isolated from the flow by barriers formed by respective first and second sealing members. The sealing members extend through apertures formed in the system and have a portion that contacts the sensing elements of the respective first and second sensors. A cartridge can hold the sensors and provide repeatable penetration depths into a flow passage of the system. A medical tube has a composite structure made of two or more distinct components that are spirally wound to form an elongate tube. One component can be a spirally wound elongate hollow body; the other component can be an elongate structural component spirally wound between turns of the spirally wound hollow body.