Metrological Considerations on Ultrasonic Flowmeters

An ultrasonic flowmeter is a non-intrusive device that employs the transit time of an ultrasonic signal to measure gas and liquid flow rate. With no moving parts, they are extensively used to measure the flow rates of hydrocarbon gases in applications that require a wide range of pressure (e.g.: custody transfer of natural gas). But despite many technological advances, ultrasonic meters still need metrological assessment. Velocity profiles—fundamental to calculate flow rates—are constructed by making use of the well-known Gauss’ integration technique that depends, to a large extent, on a suitable choice of an orthogonal polynomial. The Gauss-Legendre’s quadrature is the most popular among manufacturers. In order to reduce uncertainties when the velocity profile has few points, the discretization error must be quantified. The paper presents simulations made with Gauss’ and Chebyshev’s quadratures for turbulent flow in smooth pipes and compares them with the theoretical profile. Some aspects of metrological reliability of ultrasonic meters are also discussed.