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

Abstract

An upgrade to the ATLAS silicon tracker cooling control system may require a\nchange from C3F8 (octafluoro-propane) evaporative coolant to a blend containing\n10-25% of C2F6 (hexafluoro-ethane). Such a change will reduce the evaporation\ntemperature to assure thermal stability following radiation damage accumulated\nat full LHC luminosity. Central to this upgrade is a new ultrasonic instrument\nin which sound transit times are continuously measured in opposite directions\nin flowing gas at known temperature and pressure to deduce the C3F8/C2F6 flow\nrate and mixture composition. The instrument and its Supervisory, Control and\nData Acquisition (SCADA) software are described in this paper. Several\ngeometries for the instrument are in use or under evaluation. An instrument\nwith a pinched axial geometry intended for analysis and measurement of moderate\nflow rates has demonstrated a mixture resolution of 3.10-3 for C3F8/C2F6 molar\nmixtures with 20%C2F6, and a flow resolution of 2% of full scale for mass flows\nup to 30gs-1. In mixtures of widely-differing molecular weight (mw), higher\nmixture precision is possible: a sensitivity of <5.10-5 to leaks of C3F8 into\npart of the ATLAS tracker nitrogen envelope (mw difference 160) has been seen.\nAn instrument with an angled sound path geometry has been developed for use at\nhigh fluorocarbon mass flow rates of around 1.2 kgs-1 - corresponding to full\nflow in a new 60kW thermosiphon recirculator under construction for the ATLAS\nsilicon tracker. Extensive computational fluid dynamics studies were performed\nto determine the preferred geometry (ultrasonic transducer spacing and\nplacement, together with the sound crossing angle with respect to the vapour\nflow direction). A prototype with 45deg crossing angle has demonstrated a flow\nresolution of 1.9% of full scale for linear flow velocities up to 15 ms-1. The\ninstrument has many potential applications.\n