  An upgrade to the ATLAS silicon tracker cooling control system may require a
change from C3F8 (octafluoro-propane) evaporative coolant to a blend containing
10-25% of C2F6 (hexafluoro-ethane). Such a change will reduce the evaporation
temperature to assure thermal stability following radiation damage accumulated
at full LHC luminosity. Central to this upgrade is a new ultrasonic instrument
in which sound transit times are continuously measured in opposite directions
in flowing gas at known temperature and pressure to deduce the C3F8/C2F6 flow
rate and mixture composition. The instrument and its Supervisory, Control and
Data Acquisition (SCADA) software are described in this paper. Several
geometries for the instrument are in use or under evaluation. An instrument
with a pinched axial geometry intended for analysis and measurement of moderate
flow rates has demonstrated a mixture resolution of 3.10-3 for C3F8/C2F6 molar
mixtures with 20%C2F6, and a flow resolution of 2% of full scale for mass flows
up to 30gs-1. In mixtures of widely-differing molecular weight (mw), higher
mixture precision is possible: a sensitivity of <5.10-5 to leaks of C3F8 into
part 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 at
high fluorocarbon mass flow rates of around 1.2 kgs-1 - corresponding to full
flow in a new 60kW thermosiphon recirculator under construction for the ATLAS
silicon tracker. Extensive computational fluid dynamics studies were performed
to determine the preferred geometry (ultrasonic transducer spacing and
placement, together with the sound crossing angle with respect to the vapour
flow direction). A prototype with 45deg crossing angle has demonstrated a flow
resolution of 1.9% of full scale for linear flow velocities up to 15 ms-1. The
instrument has many potential applications.
