The use of a 90 metre thermosiphon cooling plant and associated custom ultrasonic instrumentation in the cooling of the ATLAS inner silicon tracker

A new 60kW thermosiphon fluorocarbon cooling plant has been commissioned to cool the silicon tracker of the ATLAS experiment at the CERN LHC. The thermosiphon operates over a height of 90 metres and is integrated into the CERN UNICOS system and the ATLAS detector control system (DCS). The cooling system uses custom ultrasonic instrumentaton to measure very high coolant vapour flow (up to 1.2 kg/second), to analyse binary gas mixtures and detect leaks. In these instruments ultrasound pulses are transmitted in opposite directions in flowing gas streams. Pulse transit time measurements are used to calculate the flow rate and the sound velocity, which - at a given temperature and pressure - is a function of the molar concentration of the two gases. Gas composition is computed from comparisons of real-time sound velocity measurements with a database of predictions, using algorithms running in the Siemens SIMATIC WinCC SCADA environment. A highly-distributed network of five instruments is currently integrated into the ATLAS DCS. Details of the thermosiphon, its recent operation and the performance of the key ultrasonic instrumentation will be presented.