Industrial experiences show clamp-on ultrasonic transit time flow meters assumptions lead to erroneous measurement results

Clamp-on (externally mounted) ultrasonic transit time flow meters (UTTFM) are portable, non-intrusive, easy to use, applies to a wide range of pipe sizes, and manufacturers claim accuracy is about 1%. However, industrial field experiences show that the instrument reading are inconsistent (results of simultaneous measurements on the same pipe, location using identical instrument can deviate up to 43%). Researches into clamp-on UTTFM implementation show the process employs various assumptions. Theoretical evaluations of these assumptions show all of them lead to erroneous measurement results; field measurements confirm the results of theoretical analysis. This article identifies and systematically examines each assumptions and the potential impacts on UTTFM measurement results. Techniques to minimize the effects of the assumptions are also discussed.