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

We describe a combined ultrasonic instrument for gas flow metering and continuous real-time binary gas composition measurements. The combined flow measurement and mixture analysis algorithm employs sound velocity measurements in two directions in combination with measurements of the pressure and temperature of the process gas mixture. The instrument has been developed in two geometries following extensive computational fluid dynamics studies of various mechanical layouts. A version with an axial sound path has been used with binary gas flows up to 230 l.min−1, while a version with a sound path angled at 45° to the gas flow direction has been developed for use in gas flows up to 20000 l.min−1. The instrument with the axial geometry has demonstrated a flow resolution of ≤ 1 % of full scale for flows up to 230 l.min−1 and a mixture resolution of 3.10−3 for C3F8/C2F6 molar mixtures with ∼ 20 %C2F6. Higher mixture precision is possible in mixtures of gases with widely-differing molecular weight (mw): a sensitivity of < 5.10−5 to traces of C3F8 in nitrogen (mw difference 160) has been seen in a long duration ( > 1yr) continuous study. A prototype instrument with 45° crossing angle has demonstrated a flow resolution of 1.9 % of full scale for linear flow velocities up to 15 ms−1. Although this development was motivated by a requirement of the ATLAS silicon tracker evaporative fluorocarbon cooling system, the developed instrument can be used in many applications where continuous knowledge of binary gas composition is required. Applications include the analysis of hydrocarbons, vapour mixtures for semi-conductor manufacture and anaesthetic gas mixtures.