3I-3 A Three-Way Pulse Method for a Precision Sound Velocity Measurement Cell

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 sound velocity measurement. To establish the accuracy of the sound velocity measurements given by the USM, and for traceability purposes, an independent high-accuracy sound velocity measurement cell may serve as reference. To include dispersion effects, the cell should preferably work in the operational frequency range of USMs, i.e. 100 - 200 kHz, with natural gas under high pressure. Highly accurate sound velocity cells are available in the audio frequency range, however, less work have demonstrated sufficient accuracy in the 100 - 200 kHz range. Three transient methods are investigated as part of the sound velocity cell development [Proc. 2005 IEEE Ultrasonics Symposium, pp. 1443-1447, 2005]; the methods are seen to have several common sources of experimental uncertainty. In the present work, a three-way pulse method (3PM) is considered as a candidate for the sound velocity cell, and sound velocity results obtained in a prototype cell, containing air at about 1 atm and 27 degC are presented. The results are compared with output from a sound velocity model for air, including dispersion [J. Acoust. Soc. Amer. 93 (5), pp. 2510-2516, 1993]. The results indicate that the 3PM may have potentials to perform in line with the target specifications of the sound velocity cell, i.e. 100-200 ppm, in the frequency range of USMs, 100-200 kHz. Use of a temperature regulated bath is expected to significantly reduce the temperature induced convection flows presently limiting the accuracy of the cell, and to reduce the measurement uncertainty accordingly