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Title: The Influence Of Speed Of Sound Changes On Ultrasonic Flow Meter Calibrations
Author: William Freund, Klaus Zanker And Dale Goodson,James E. Hall
Source: 2002 International Symposium on Fluid Flow Measurement
Year Published: 2002
Abstract: The simple theory of the ultrasonic flow meter predicts that the flow measurement should be independent of the speed of sound in the meter. If this assumption is not valid in actual applications, errors may occur when a meter is calibrated at conditions different from its field installation. In order to quantify the effect of changes in the speed of sound, a series of carefully controlled calibrations have been performed. The first experiment consisted of calibrating an 8-inch and two 12-inch ultrasonic meters in a natural gas loop. The fluid was then changed to nitrogen, providing approximately a 16% change in the speed of sound. In addition, further calibrations were run with changes in the speed of sound introduced through changes in the temperature of the fluid. Natural gas calibrations were performed at 70 and 50 degrees Fahrenheit and nitrogen calibrations at 70 and 90 degrees Fahrenheit. For each series of calibrations, the average calibration curves were compared to determine the effect of the change. The resulting analysis showed that speed of sound changes introduced through temperature changes did not result in a change in the calibration curve for the ultrasonic meters outside of the expected reproducibility of the facility and the meters. Analysis of the calibrations taken with natural gas and nitrogen showed an average difference of 0.2% when referenced to SwRIs sonic nozzle standard. However, when the ultrasonic calibrations were referenced to the turbine meters, this difference disappeared. This supports the possibility for wet calibration of ultrasonic meters on air as well as natural gas as long as the effect of the density of the gas on the reference is taken into account.

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