Abstract: A device for determining the rate of flow of air in the intake duct of an internal combustion engine comprises a detector duct of generally rectangular cross section housing two ultrasonic detectors one at each end, lying adjacent the detector duct is an intake duct having an inlet air filter which also serves as an ultrasonic acoustic filter. Adjacent ends of the intake and detector ducts are joined by a deflector which turns the air through 180.degree. as it passes from the intake duct to the detector duct. Within the deflector are fins which guide the air and turn it with the minimum of pressure loss and turbulence; adjacent the deflector in the intake duct is located a set of fins serving to diffuse the air as it passes, to reduce any temperature or velocity gradient across its width, and adjacent the deflector in the detector duct is located a set of fins serving to guide the air stream and reduce turbulence maintaining the even pressure and temperature across the stream.

Abstract: In a device for measuring ultrasonically the flow of gas, for example the air flow in the induction duct of an internal combustion engine, an upstream piezo-electric transducer is located in a plenum chamber upstream of a measurement duct and a downstream piezoelectric transducer is located in a seating in the measurement duct. To reduce vorticity in the gas flow the upstream transducer is seated at the apex of a frusto-conical element in the inlet chamber and the seating of the downstream transducer is tapered externally.

Abstract: An ultrasonic device for determining the air flow rate in a conduit, comprising a duct, which may form part of the conduit or may be connectable thereto, at each end of which is a respective enlarged cylindrical chamber communicating axially with the duct and housing a respective piezoelectric transducer. In use of the device the piezoelectric transducers both transmit ultrasonic signals towards each other along the duct and the air speed in the duct is determined by the difference in transit times of the ultrasonic signals in the two directions. The air flow through the chambers is largely radial and the piezoelectric transducers are located axially at positions where the air speed is very low to minimize disturbances thereto.