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

Abstract

Traceability of fluid flow measurement is a prerequisite for the accurate determination of transferred mass, heat and energy. However, considering the wide range of flow-rates, temperature, pressure, viscosity and densities, which is of interest for practical applications of fluid flow metrology, the accurate calibration of ultrasonic flow meters still represents a demanding task. Alternative to the usual gravimetric calibration, the possibility to provide traceability through using a speed of sound standard has been less explored. In this work, we describe and discuss the performance of an ultrasonic cell, and the feasibility of using this sensor to provide traceable measurements of the speed of sound. The instrument has been designed to serve as transfer standard for speed of sound and, its features make it suitable for the calibration of ultrasonic flowmeters both in the laboratory and on industrial pipelines. The sensor has been tested by performing speed of sound measurements in liquid methane between 100 K and 162 K and for pressure up to 10 MPa. The expanded relative uncertainty (k=2) of the speed of sound w is Ur(w)=0.15% for temperature below 130 K and Ur(w)=0.32% above 130 K. The obtained results are found in agreement with previously published measurements and with the predictions of the Setzmann and Wagner and GERG equations of state. Along with the results of speed of sound measurements and their expanded (k=2) uncertainty, we provide a detailed discussion of the procedures adopted to improve the stability of the sensor, and the specific corrections applied considering the cryogenic working conditions.