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We describe a combined ultrasonic instrument for continuous gas flow measurement and simultaneous real-time binary gas mixture analysis. In the instrument, sound bursts are transmitted in opposite directions, which may be aligned with the gas flow path or at an angle to it, the latter configuration being the best adapted to high flow rates. Custom electronics based on Microchip® dsPIC and ADuC847 microcontrollers transmits 50kHz ultrasound pulses and measures transit times in the two directions together with the process gas temperature and pressure. The combined flow measurement and mixture analysis algorithm exploits the phenomenon whereby the sound velocity in a binary gas mixture at known temperature and pressure is a unique function of the molar concentration of the two components. The instrument is central to a possible upgrade to the present ATLAS silicon tracker cooling system in which octafluoropropane (C3F8) evaporative cooling fluid would be replaced by a blend containing up to 25% hexafluoroethane (C2F6). Such a blend will allow a lower evaporation temperature and will afford the tracker silicon substrates a better safety margin against leakage current-induced thermal runaway caused by cumulative radiation damage as the luminosity profile at the CERN Large Hadron Collider (LHC) increases. The instrument has been developed in two geometries following computational fluid dynamics studies of various mechanical layouts. An instrument with 45° crossing angle has been built in stainless steel and installed for commissioning in the ATLAS silicon tracker evaporative fluorocarbon cooling system. It can be used in gas flows up to 20000 l.min−1, and has demonstrated a flow resolution of 2.3% of full scale for linear flow velocities up to 10 m.s−1 in preliminary studies with air. Other instruments are currently used to detect low levels of C3F8 vapour leaking into the N2 environmental gas surrounding the ATLAS silicon tracker. Gas from several parts of the tracker is aspirated through two instruments and analyzed. A long duration continuous study of more than a year has demonstrated a sensitivity to mixture variation of better than 5.10−5. The developed instrument has many applications where continuous knowledge of binary gas composition is required. Such applications include anaesthesia, the analysis of hydrocarbon mixtures, and vapour mixtures for semiconductor manufacture. [ABSTRACT FROM PUBLISHER]