An independent mechanical device designed for accurate mixing of oxygen (O2) and air or O2 and nitrous oxide (N2O) in pre-set concentrations appropriated for breathing. The mixer receives all three gases from either the hospital gas pipeline or from compressed gas cylinders; it may include a pressure-regulating mechanism. It usually delivers the gases with adjustable N2O or air concentrations and ensures a minimum O2 level to the patient through a face mask, mouthpiece, or endotracheal (ET) tube. It is used to improve O2 uptake and/or relieve pain throughout a hospital, during dental procedures, or in obstetrics during labour. It may also be attached to ventilators or anaesthesia machines.