For the transport of natural gas, liquefying gas is becoming popular compared to for example transporting gas over large distance pipelines. Transport of gas is receiving much attention worldwide as for example gas is increasingly being found in remote areas and importing countries are looking for ways to diversify the resources of their gas supplies. Another trend is the increasing number of parties involved in the LNG market place. In addition, the costs of the liquefaction and re-gasification of gas and the transport of LNG are decreasing. An increase in the trade of LNG also means an increase in the custody transfer and fiscal metering points for gas in cryogenic conditions. Tank gauging has been a popular measurement method for LNG, but it has its limitations and increased demand for very accurate flow measurements can be observed. For more than 20 years ultrasonic flowmeters have been used for the fiscal measurement of gas and for more than 10 years for the fiscal metering of liquid hydrocarbons [1],[2],[8],[9],[10]. Over the past years, much experience has been gained with hundreds of custody transfer ultrasonic flowmeters operating in the field. The application of highly accurate ultrasonic flowmeters have been very successful and can be considered as a proven technology in this field [3]. Next to successful applications at 'normal' operating conditions, the ultrasonic measurement principle in itself is also very suitable for high accuracy flow measurement at cryogenic conditions. The development, construction and calibration of an ultrasonic flowmeter for custody transfer of LNG is, however, not simple taking into account the very low temperatures and the limitations regarding the calibration under reference conditions.
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