An expiratory limb 210 suitable for use in a medical circuit comprises a first and second opening for receiving a gas and allowing it to leave the limb respectively. The limb comprises a conduit with a first end proximal the first opening and a second end proximal the second opening, a wall extending therebetween defining a lumen through which the gas passes from the first to second end, at least a portion of the wall being formed from a breathable material which allows transmission of water vapour but not liquid water therethrough. At least two heater wire sections 212 are provided along the conduit, configured to be independently controlled (e.g. by two controllers 216a, 216b) such that that a temperature drop of the gas between the first and second ends is controlled.
