A humidifier for humidifying a gas in a respiratory circuit (20), the humidifier comprising an evaporator (1) physically arranged outside the respiratory circuit (20) and designed to receive a defined quantity of water, vaporize said defined quantity of water and supply the vaporized water to the respiratory circuit (20) the humidifier further including a water reservoir (2) which is connected to the evaporator (1) via a dosing system (30).