A gases measurement apparatus (20, 40) for a respiratory gases delivery apparatus, the gases measurement apparatus comprising: a gases mixing chamber (21, 41) configured to receive gases from a gases source, the gases mixing chamber comprising a gases flow path from a first end of the gases mixing chamber to a second end of the gases mixing chamber and at least one mixing element situated within the gases flow path; a gases measuring chamber configured to receive gases from the gases mixing chamber, and comprising a gases flow path from a first end of the gases measuring chamber to a second end of the gases measuring chamber, wherein a downstream direction is defined along the gases flow path from the first end to the second end and an upstream direction is defined along the gases flow path from the second end to the first end; a controller; a first ultrasonic sensor positioned at the first end of the gases measuring chamber, the first ultrasonic sensor configured to transmit a downstream acoustic pulse train in a first measurement phase, to detect an upstream acoustic pulse train in a second measurement phase, and to send a signal to the controller; a second ultrasonic sensor positioned at the second end of the gases measuring chamber, the second ultrasonic sensor configured to transmit the upstream acoustic pulse train in the second measurement phase, to detect the downstream acoustic pulse train in the first measurement phase, and to send a signal to the controller; and a humidity sensor configured to measure humidity of the gases flowing in the gases flow path; wherein the controller is configured to determine a characteristic of the gases based at least in part on a signal received from the first ultrasonic sensor, a signal received from the second ultrasonic sensor, and a signal received from the humidity sensor; wherein determining the characteristic of the gases comprises determining a flow rate; and the apparatus further comprises at least one pressure sensor configured to measure pressure of the gases flowing in the gases flow path; characterised in that the at least one mixing element comprises a vane that is configured to reduce turbulence in gases that flow from the gases flow path of the gases mixing chamber to the gases flow path of the gases measuring chamber. a gases mixing chamber (21, 41) configured to receive gases from a gases source, the gases mixing chamber comprising a gases flow path from a first end of the gases mixing chamber to a second end of the gases mixing chamber and at least one mixing element situated within the gases flow path; a gases measuring chamber configured to receive gases from the gases mixing chamber, and comprising a gases flow path from a first end of the gases measuring chamber to a second end of the gases measuring chamber, wherein a downstream direction is defined along the gases flow path from the first end to the second end and an upstream direction is defined along the gases flow path from the second end to the first end; a controller; a first ultrasonic sensor positioned at the first end of the gases measuring chamber, the first ultrasonic sensor configured to transmit a downstream acoustic pulse train in a first measurement phase, to detect an upstream acoustic pulse train in a second measurement phase, and to send a signal to the controller; a second ultrasonic sensor positioned at the second end of the gases measuring chamber, the second ultrasonic sensor configured to transmit the upstream acoustic pulse train in the second measurement phase, to detect the downstream acoustic pulse train in the first measurement phase, and to send a signal to the controller; and a humidity sensor configured to measure humidity of the gases flowing in the gases flow path; wherein the controller is configured to determine a characteristic of the gases based at least in part on a signal received from the first ultrasonic sensor, a signal received from the second ultrasonic sensor, and a signal received from the humidity sensor; wherein determining the characteristic of the gases comprises determining a flow rate; and the apparatus further comprises at least one pressure sensor configured to measure pressure of the gases flowing in the gases flow path; characterised in that the at least one mixing element comprises a vane that is configured to reduce turbulence in gases that flow from the gases flow path of the gases mixing chamber to the gases flow path of the gases measuring chamber.