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

Traceability of fluid flow measurement is a prerequisite for the accurate determination of transferred mass, heatand 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 ultrasonicflow meters still represents a demanding task. Alternative to the usual gravimetric calibration, the possibility toprovide traceability through using a speed of sound standard has been less explored. In this work, we describeand discuss the performance of an ultrasonic cell, and the feasibility of using this sensor to provide traceablemeasurements of the speed of sound. The instrument has been designed to serve as transfer standard for speedof sound and, its features make it suitable for the calibration of ultrasonic flowmeters both in the laboratoryand on industrial pipelines. The sensor has been tested by performing speed of sound measurements in liquidmethane between 100 K and 162 K and for pressure up to 10 MPa. The expanded relative uncertainty(= 2)of the speed of soundisr() = 0.15% for temperature below 130 K andr() = 0.32% above 130 K. Theobtained results are found in agreement with previously published measurements and with the predictions ofthe Setzmann and Wagner and GERG equations of state. Along with the results of speed of sound measurementsand their expanded(= 2)uncertainty, we provide a detailed discussion of the procedures adopted to improvethe stability of the sensor, and the specific corrections applied considering the cryogenic working conditions.