A Cryogenic Helium Mass Flowmeter for the Large Haldron Collider

This thesis is concerned with the design, development and testing of helium mass flowmeters based on different technologies to be used as diagnostic tools for the cryogenic system of the Large Hadron Collider at CERN, the European laboratory for particle physics. The flowmeters were designed and the performance assessed for liquid, supercritical and superfluid helium down to 1.8 K and with pressures up to 0.3 MPa. A testing regime and equipment to enable the performance of a range of developed flowmeters to be validated at working cryogenic temperatures and pressures is described in the thesis. In particular a novel cryogenic flow test facility with 1.5 % accuracy as well as an irradiation test station designed, constructed and used as part of the evaluation work are described in the thesis. The final validation with metrological certification down to 0.5 % accuracy was performed in collaboration with the Colonnetti metrological laboratory in Italy. Devices based on the thermal exchange and transport properties of the fluid were developed and proven to be reliable and inexpensive but with accuracy poorer than 10 %. As part of this work a new empirical heat transfer correlation for supercritical helium in the piping geometry and operating conditions of the LHC was established. Industrially available Coriolis meters which had not been employed before at such low temperatures were extensively investigated. It was found that with some minor modifications in the electronics (drive system) and in the elasticity modulus (Young’s modulus) value for the flow tubes material at low temperature, accuracy down to 0.5 % could be achieved. Additional features in the mechanical and electrical design were also introduced to improve robustness and reliability at cryogenic temperatures. They mainly consisted in purging the meters secondary containment with helium to avoid gas freezing at low temperatures and increasing the internal gauge wire to improve its fatigue strength. The elasticity modulus of 316 L stainless steel below 20 K was characterised and compared to available literature data. The successful Coriolis flowmeters were validated to meter the cryogenic power distribution in the 27-km long superfluid helium system of the future collider and are now extensively and successfully used in superconducting devices test facilities at CERN and in companion laboratories.