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Title: Ultrasonic Mass Flowmeters Using Clamp-On Or Wetted Transducers
Author: Jed Matson, Charles F. Mariano, Oleg Khrakovsky, And Larry Lynnworth
Source: 2002 International Symposium on Fluid Flow Measurement
Year Published: 2002
Abstract: The relationships among sound speed c3, temperature T, pressure P and density ? have been known for a long time for many fluids. As an example of the close connection between the first two of these, consider that in 1981, Sochajewski et al. 32 reported uncertainty in water temperature of only 0.1C when obtained from c3 in a multipath ultrasonic flowmeter using wetted transducers. In one sense, the present work may be viewed as a three-step extension of that 1981 work. Step (1) goes (when practical) to clamp-on transducers to measure sound speed step (2) obtains density from sound speed step (3) multiplies the volumetric flowrate Q by density to obtain mass flowrate MF ?Q. In the present work, we used NIST data, Lemmon et al. 14 to obtain density directly from sound speed in water and in normal alkanes. For water, pressure between 50 and 70 bar has 0.1% influence on ? at 100C, and 0.33% influence at 260C. Similar data available at the time were used in setting up or running ultrasonic clamp-on flow meters in two nuclear plants in the USA. (NC, four f24? pipes & CT, two f3? pipes) to measure flow velocity V and sound speed c. Angle-beam (wedge-type) clamp-on transducers were constructed with high-temperature plastic wedges for T up to 180C (NC), and metal buffers for T up to 282C (CT). The electronic instruments used in this work have been standard one- or two-channel flowmeters. In the former case, the flowmeter utilized a program corresponding to the ? vs c3 equation. In this case, the clamp-on flowmeters output is in terms of mass flow rate. Since density is obtained from the sound speed, the instrument is a stand-alone mass flowmeter that does not require any additional T or P sensors in order to compute the mass flow rate. The present report generalizes from water to certain other pure, well-defined liquids such as normal alkanes. It also considers air (dry or moist), steam (saturated), and flare gases. The sound speed in such fluids can yield density but one must be aware of errors when interfering variables are present, e.g., salt in water, branched (non-normal) molecules in alkane, condensate in steam, or unknown amounts of nitrogen in flare gas.

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