Electronics for the new on-line ultrasonic mixture analyzer and flowmeter for fluorocarbon mixtures in the ATLAS silicon tracker, with application to Ring Imaging Cherenkov detectors

The ATLAS silicon tracker community foresees a change from the present octafluoropropane (C₃F₈) evaporative coolant to a blend with a 20-30% admixture of hexafluoroethane (C₂F₆), allowing a lower evaporation temperature to afford the silicon a better safety margin against leakage current-induced thermal runaway as the LHC luminosity profile increases. Central to the use of this blend is a custom developed speed-of-sound instrument for simultaneous real-time measurement of the C₃F₈/C₂F₆ mixture ratio and flow. The electronics of this instrument and its functionality - which also has been employed in on-line refractive index monitoring in Ring Imaging Cherenkov detectors - is described.