Ultrasonic Technology: Prospects for Improving Flow Measurements and Standards
Tsyh Tyan Yeh, G E. Mattingly
Ultrasonic technology is evolving rapidly. It offers prospects for improving flow measurements, for serving as transfer standards, and possibly for serving as a primary flow standard. This paper describes results from several current NIST projects that have the goal of assessing travel-time ultrasonic flow measurement techniques for their potential in improving flow measurements. These projects include a meter testing program and computer simulations of travel-time techniques in ideal conditions and in measured pipe flows for a range of metering configurations.Meter test results show that the as received performance of several commercially available, clamp-on, travel-time, ultrasonic flow meters have errors that range from 1 % to 3 % when measuring high Reynolds number water flows in nearly ideal installation conditions. These errors could be reduced if manufacturers better compensated for pipe flow profile effects using improved software or if they improved the measurement traceability of their meter calibration capabilities to NIST's flow standards. The reproducibilities of most of these units are 1 % or better, thus producing conclusions that these meters could attain accuracy levels commensurate with these reproducibilities if these software or calibration improvements are made.Results also show that these manufacturers have significantly improved: (1) the awkward requirement for a zero flow condition to attain satisfactory performance, and (2) the remove-replace variations which plagued these types of meters.Additionally, test results for an in-line, 8-path, travel-time, ultrasonic meter that was set up using only length and time standards showed uncertainties of ± 2 %, or better. These results show that this kind of meter might evolve into a primary standard for flow.The computer simulations of travel-time metering arrangements provide insight into ways that this technology can improve flow measurements. If it becomes feasible to quantitatively model all the component measurement systems that comprise the ultrasonic metering of a pipe flow using only length and time standards, this technique would be a primary standard at some specified uncertainty. This would greatly expand the capability of primary flow standards, and it would make flow measurements traceability to national standards.