Qualication of an ultrasonic ow meter as a transfer standard formeasurements at Reynolds numbers up to 4106 betweenNMIJ and PTB
L. Cordova a,n, N. Furuichi b, T. Lederer a
a Physikalisch-Technische Bundesanstalt, Germanyb National Institute of Advanced Industrial Science and Technology, Japan
a r t i c l e i n f o
Article history:Received 19 June 2014Received in revised form7 April 2015Accepted 19 April 2015Available online 24 April 2015
Keywords:Interlaboratory comparisonUltrasonic ow meterReynolds number dependenceFlow traceabilityTransducer cavity
a b s t r a c t
The quality of any laboratory intercomparison depends to a large extent on the performance of the usedow meter. To nd a ow meter that is capable of reaching a reproducibility better than 0.05% requiresbounding all involved inuence quantities down to the required level. The present paper describes theefforts performed while qualifying a time-of-ight ultrasonic ow meter as a transfer standard. It wasdetermined that the most relevant inuence quantity besides the ow prole within the bulk ow is theeffect caused by the transducer pockets in the meter body. By taking advantage of a specially designedwindow chamber, it was possible to determine the magnitude of the errors introduced by the transducerpockets and to dene, based on the ndings, a procedure to perform a bilateral comparison between thehot water calibration facilities of the Physikalisch-Technische Bundesanstalt and the National Institute ofAdvanced Industrial Science and Technology. The results of the bilateral comparison are presented.& 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
1. Introduction and motivation
Water is used as an energy transporting medium in every type ofpower plant involving turbines; also industrial and district heatingdepend on accurate measurements of ow rate. In most cases, theactual measurement uncertainty is in the order of 1%. Consequently,every improvement of the measurement uncertainties has directconsequences for the safety and efciency of the involved systems.
Flow rate measurements in the eld are performed ideally byinstruments that have been tested at National Metrology Institutes(NMI) or at a calibration laboratory that has been accredited and/or is participating in prociency tests organized by the corre-sponding NMI as can be seen in Fig. 1. Any bias introduced by acalibration laboratory would have a direct impact on the price, onthe quality or on the competitiveness offered by its clients. Inorder for measurements to be globally consistent, it is requiredthat NMIs prove their mutual consistency periodically throughinternational comparisons. The Mutual Recognition Arrangementof the International Committee for Weights and Measures (CIPM-MRA) has established mechanisms in order to allow the NMIs toprove their mutual consistency transparently and based on thesame rules and principles. Actually there are more than 53 states
and 152 institutes, designated by the signatory bodies, participat-ing in the CIPM-MRA.
The traceability of a ow rate calibration facility is normallyassessed on a quantity-based calibration, i.e. mass, volume, time,density and temperature standards are calibrated separately. Only incases where there is a ow meter capable of delivering reproduc-ibilities much lower than the required calibration uncertainties it ispossible to provide a direct ow-rate traceability. This is possible inlow-ow hydrocarbon measurements as reported by Shimada. Highlyreproducible measurement instruments are available as seen, forexample, at the Calibration Intercomparison on Flow Meters forKerosene carried out on 1995  and the CIPM-MRA internationalkey comparison of liquid hydrocarbon ow facilities CCM-FF-K2 .Without direct ow-rate traceability, systematic errors in any systemof the calibration rig might remain undetected.
There are several relevant ow rate measurements in the eldperformed without a calibration as depicted in Fig. 1. This situation isgiven mostly in cases where the measurement conditions cannot bereproduced in a laboratory. Under these circumstances the only alter-native is to apply ow measurement technology that has a predictableworking principle that allows the use of similarity principles to infer thecalibration result and uncertainty of measurements under conditionsdifferent from those present during calibration.
The relevant ranges for energy transport through hot water varymainly between 50 1C and 250 1C. Flow rates larger than 3500 m3/hhave been reported and Reynolds numbers up to 30106. According
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Flow Measurement and Instrumentation
http://dx.doi.org/10.1016/j.owmeasinst.2015.04.0060955-5986/& 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
n Corresponding author.E-mail address: firstname.lastname@example.org (L. Cordova).
Flow Measurement and Instrumentation 45 (2015) 2842
to the CMC tables1 there is only one facility that comes close to theserequirements: the AIST, NMIJ (hereafter, NMIJ). With temperatures ofup to 70 1C and ow rates up to 12 000 m3/h, it is able to reachReynolds numbers up to 20106; the declared expanded uncertaintyvaries depending on the ow-rate range between 0.04% and 0.08%.The next ow rate facility that can be considered for hot water owtraceability studies is the heat meter testing facility of PTB. For adeclared 0.04% expanded uncertainty it is able to measure between4 1C and 90 1C and a ow rate up to 1000m3/h. Section 2 will givemore details on both facilities.
In this sense, the ow measurement laboratories for hot waterof PTB and NMIJ cooperate in order to validate ow measurementprinciples that allow similarity conditions to be applied. And giventhat the required uncertainties to determine the inuence quan-tities acting on the ow measurement techniques are in the orderof magnitude of the uncertainties declared by the NMIs them-selves, PTB and NMIJ need to prove their mutual consistencybefore reliable experiments involving both laboratories are possi-ble. Steps towards this rst goal are described in this paper.
Firstly, an overview on the used ow measurement technologyand on the calibration facilities of PTB and NMIJ is given. In thesecond part, the results of the characterization of an ultrasonicow meter made at PTB are shown in two steps: throughconventional linearity, repeatability and reproducibility tests usingan established industrial ow meter, and through the simulta-neous measurements of the ow prole and the ow meterindication at a very carefully constructed DN200 90D long testline using a specially designed window chamber. By using thecharacterization results, a strategy is dened to apply a robustindustrial ow meter as a transfer standard in less advantageousconditions. The transfer standard is provided with a tube bundle toincrease robustness against geometry differences in the inlet pipelayouts and internal pipe diameters. The nal part of this paperpresents the comparison results and provides rst conclusions onthe application of ultrasonic ow meters under conditions outsidethe calibration ranges.
1.1. Traceability of ow meters outside calibration ranges
An established ow metering technology based on the similar-ity laws concerns orice plate ow meters. They allow a bestpossible uncertainty, in the ideal case not smaller than 0.7% asextracted from ISO5167 , in any condition where calibration isnot possible. The basis for the ISO5167 is decades of enormousresearch efforts and ten thousands of internationally coordinatedexperiments.
In the past few years, ultrasonic ow meter manufacturers havebeen introducing their products for applications where no calibra-tion is possible. Based on calibrations performed under laboratory
conditions, they propose to extrapolate the uncertainty to levelsbelow 0.7% and replace differential pressure meters. Importantsteps towards global standardization of ultrasonic ow metertechnology have been undertaken in the GERG project on ultra-sonic gas ow meters .
1.2. Ultrasonic ow meters
The type of ultrasonic ow meter used most is the parallel pathtime-of-ight ow meter (hereinafter UFM). Its simplicity makes ita good candidate for the dened purpose.
1.2.1. Ideal case integrationIn the ideal case, any path of a UFM installed at any position r=R
when exposed to a fully developed ow prole shows a curvesimilar to the one depicted in Fig. 2(a). The area under the curverepresents the ow rate; when the bulk speed is dened to be one,the area under the curve is equal to the volume of a cylinder withunity radius and unity height (). Flow measurement through theUFM can be regarded as the problem of integrating the area underthis curve.
If the ow is fully developed, any path can be used as a owmeter as can be seen in Fig. 2(b). 10 single normalized paths,referred to their own indication for Re 106, are shown as afunction of the Reynolds number. For every path position there is amonotonic relation between the indication and the real ow rate.
The following equation describes the use of multiple paths Piand weights wi:
i 1wiPi 1
The factor k of Eq. (1) is a correction factor of a semi-empirical natureintroduced to compensate for temperature and pressure variationsand to add empirical linearizing as seen, for example, in . Theintroduction of a k-factor is comparable to the determination of thedischarge coefcient at orice plates. It would be desirable to nd avalid formulation for the UFM as is the case for orice plates asproposed by Reader-Harris