Investigation of precision sound velocity measurement methods as reference for ultrasonic gas flow meters

Abstract

Ultrasonic gas flow meters for volumetric flow rate fiscal metering of natural gas (USMs) may possibly also be used for mass and energy flow rate measurement, partially based on velocity of sound (VOS) measurement. To establish the accuracy of the VOS measurements given by the USM, and for traceability purposes, an independent and high-accuracy VOS measurement cell may be used as reference. To include relevant effects of dispersion, the cell should preferably work in the operational frequency range of USMs, e.g. 100-200 kHz, with natural gas under high pressure. Three different transient methods are investigated, aiming to realize a VOS measurement cell, and they are seen to have several common experimental uncertainty sources. In the present work, a two-distance method is discussed in more detail as an example, and some results from measurements in an insulated chamber with air at 1 atm and ca. 25 ◦ C are presented. The relative expanded measurement uncertainty was estimated according to ISO guidelines to 282 ppm (95 % conf. level). One major source of measurement uncertainty was experienced to be small convection currents in the chamber. Without these, the expanded uncertainty would have been about 162 ppm. Such convection effects are expected to be strongly reduced in a properly designed measurement cell. The VOS measurement results were compared with predictions from a VOS model for standard air, including dispersion (J. Acoust. Soc. Amer. 93 (5), pp. 2510-2516, 1993), resulting in a mean deviation of -18 ppm with a two standard deviation spread in the data of 190 ppm over the temperature range.