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Fluid flow measurement now encompasses a wide variety of fluids and applications. To meet this wide variety of applications the instrumentation industry has, over many years, developed a wide variety of instruments. A flow meter is a device used to measure the flowrate or quantity of a gas or liquid moving through a pipe. Flow measurement applications are very diverse and each situation has its own constraints and engineering requirements. The common types of flowmeters that find industrial applications are: i) Obstruction (differential pressure or variable area), ii) Inferential (turbine type), iii) Electromagnetic, iv) Positive displacement (integrating), v) Fluid Dynamic (Vortex shedding), vi) Anemometer, vii) Mass flowmeter (Coriolis) and viii) Ultrasonic. The present paper focuses on the Ultrasonic flowmeters. Ultrasonic flow meters are mostly used in oil and gas industries. They are also used in chemical, pharmaceutical, food and beverage, metal, mining, pulp and paper and wastewater industries. There is no single type of flowmeter available which can meet all the requirements of all applications and ultrasonic flow meter is also not an exception. Ultrasonic flowmeters use sound waves to determine the velocity of a fluid flowing in a pipe. At no flow conditions, the frequencies of an ultrasonic wave transmitted into a pipe and its reflections from the fluid are the same. Under flowing conditions, the frequency of the reflected wave is different due to the Doppler effect. When the fluid moves faster, the frequency shift increases linearly. The transmitter processes signals from the transmitted wave and its reflections to determine the flow rate. Both of them mainly utilizes clamp on flow transducers to detect flow in closed pipe. Clamp on flow meter is a unique measurement way and at present, ultrasonic flow meter is the main type with such way. The few advantages of the Clamp on ultrasonic flow meter includes no pipe cutting, no direct contact to fluid, no risk of leakage, no moving parts, no pressure loss, no future maintenance, higher accuracy, low power consumption, low cost ownership etc. However, Clamp on ultrasonic flow meters too are not completely perfect and also have several disadvantages. The main one is it has a higher requirement of pipe condition e.g. it does not measure cement/concrete pipe or pipe with such material lining; if pipe is seriously rusted, especially inner wall rusting, clamp on meter may not work well etc. Authors in this paper have attempted to review and collate all relevant information right from its principle of operation, type, accuracy, installation, advantages & disadvantages and traceability issues related to ultrasonic flowmeters. It is hoped the information presented in this paper would help the users to find relevant information on such issues in a single and concise report.
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