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An upgrade to the ATLAS silicon tracker cooling control system may require achange from C3F8 (octafluoro-propane) evaporative coolant to a blend containing10-25% of C2F6 (hexafluoro-ethane). Such a change will reduce the evaporationtemperature to assure thermal stability following radiation damage accumulatedat full LHC luminosity. Central to this upgrade is a new ultrasonic instrumentin which sound transit times are continuously measured in opposite directionsin flowing gas at known temperature and pressure to deduce the C3F8/C2F6 flowrate and mixture composition. The instrument and its Supervisory, Control andData Acquisition (SCADA) software are described in this paper. Severalgeometries for the instrument are in use or under evaluation. An instrumentwith a pinched axial geometry intended for analysis and measurement of moderateflow rates has demonstrated a mixture resolution of 3.10-3 for C3F8/C2F6 molarmixtures with 20%C2F6, and a flow resolution of 2% of full scale for mass flowsup to 30gs-1. In mixtures of widely-differing molecular weight (mw), highermixture precision is possible: a sensitivity of <5.10-5 to leaks of C3F8 intopart of the ATLAS tracker nitrogen envelope (mw difference 160) has been seen.An instrument with an angled sound path geometry has been developed for use athigh fluorocarbon mass flow rates of around 1.2 kgs-1 - corresponding to fullflow in a new 60kW thermosiphon recirculator under construction for the ATLASsilicon tracker. Extensive computational fluid dynamics studies were performedto determine the preferred geometry (ultrasonic transducer spacing andplacement, together with the sound crossing angle with respect to the vapourflow direction). A prototype with 45deg crossing angle has demonstrated a flowresolution of 1.9% of full scale for linear flow velocities up to 15 ms-1. Theinstrument has many potential applications.

A combined ultrasonic flow meter and binary vapour mixture analyzer for the ATLAS silicon tracker