Towards a new transfer standard for speed of sound measurements in liquids at cryogenic temperatures

Abstract

Measurement of fluids flows represents an unparalleled tools for the account of the transferred mass, heat and energy; however, considering the wide ranges of transported fluids flow-rates, temperatures, pressures, viscosity and densities, high accurate calibrations of ultrasonic flowmeters remain demanded.As an alternative to the usual gravimetric calibration, there is a less explored possibility to provide the traceability of the measurement through the speed of sound.In this work an ultrasonic sensor, which operates on the basis of the double pulse-echo technique is presented. This sensor is capable to provide measurements of speed of sound, fully traceable to units of length and time. Moreover, it has been optimized to operate when set both into a laboratory thermostat and into industrial plants using spools like DN50, or larger, in order to be adopted as a transfer standard for flow rate measurements. The concept behind this idea is the possibility of characterising the ultrasonic cell, namely the velocity meter, in a controlled temperature and pressure conditions in a laboratory environment. Thereafter, it is possible to connect the sensor directly to the natural gas distribution lines, nearby the industrial ultrasonic flow meters, normally used to monitor the LNG flow. This procedure would bring the significant benefit of enabling industrial flowmeters to be calibrated without the need for prior knowledge of the composition of the fluid under test, as well as avoiding the shutdown or removal of flowmeters during their calibration.The sensor has been tested and characterized performing measurements in liquid methane in the temperature range of (100 and 162) K and for pressure up to 10 MPa. The expanded relative uncertainty of the obtained speed of sound, w is U r (w) = 0.15 % (k = 2) for temperature below 130 K and U r (w) = 0.32 % (k = 2) for temperature above 130 K. The obtained values are in agreement with those available in the scientific literature and with the predictions of the Seitzmann and Wagner and GERG equation of state.Along with experimental speed of sound measurements, a deep insight of the procedures adopted to improve the stability of the sensor, the specific corrections applied considering the cryogenic working conditions and a complete analysis of the uncertainty affecting the obtained results is also discussed.