A respiratory assistance apparatus has a gases inlet configured to receive a supply of gases, a blower unit configured to generate a pressurised gases stream from the supply of gases; a humidification unit configured to heat and humidify the pressurised gases stream; and a gases outlet for the heated and humidified gases stream. A flow path for the gases stream extends through the respiratory device from the gases inlet through the blower unit and humidification unit to the gases outlet. A sensor assembly is provided in the flow path before the humidification unit. The sensor assembly has an ultrasound gas composition sensor system for sensing one or more gas concentrations within the gases stream.