Apparatus comprising a medical device requiring a supply of a gaseous composition comprising a first gas and a second gas； a main circuit for recirculating gas through the medical device and comprising: a circulation pump for pumping gas through the main circuit, a gas outlet connected to the medical device, a gas inlet connected to the medical device, a first supply conduit for supply of gas of a first composition to the main circuit, a second supply conduit for supply of gas of a second composition different from said first composition to the main circuit, a gas outlet flow controller for controlling the flow of gas to the medical device； a first supply flow controller for controlling the flow of gas through the first supply conduit； and a second supply flow controller for controlling the flow of gas through the second supply conduit； a concentration determiner for determining the concentration of at least one gas of the gaseous composition within the main circuit； and a vent for venting gas from the main circuit, wherein at least one of the gas outlet flow controller, the first supply flow controller and the second supply flow controller comprises a regulator, a setpoint signal generator and a volumetric feedback correction system, the volumetric feedback correction system including a volumetric flow meter for determining a volumetric rate of flow of gas independently of the composition of the flow of gas.
