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 (C
3
F
8
) evaporative coolant to a blend with a 20–30% admixture of hexafluoroethane (C
2
F
6
), 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 C
3
F
8
/C
2
F
6
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.