Ultrasonic flow metering – Measurement in closed conduits

The range of ultrasonic flowmeters using transit time difference has greatly improved and developed since the 2000s, and by 2022 will be one of the most widely used flowmeters in the industrial sector. This strong growth is due to the many advantages offered by this flow measurement technique: generation of zero or almost zero head losses, relevant use for a wide range of flows and fluids (fresh water, seawater, heavy water, acids, oils, liquid sodium, petroleum products, gas, air, steam...), possibility of measuring flow directly in both directions of flow, possibility of measuring flow in large-diameter pipes, technology available in non-intrusive installation. The limits of this versatility, however, lie in the accuracy of measurement, which, while remaining at the usual level of other flow measurement techniques, must be carefully assessed in the light of installation and operating conditions.

This article is designed to provide a basic understanding of this flow measurement technique, which is highly adaptable to the various flow measurement problems encountered in industrial processes. This flow measurement technique, known as "transit time difference", determines flow velocity from the difference in the path of an ultrasonic wave between an upstream and a downstream probe, and vice versa. The flow rate is then calculated by relating this velocity to the internal surface of the pipe and to the flow conditions. For the sake of completeness, the Doppler and cross-correlation measurement techniques are briefly presented as complements to the transit time difference technique.