Further improvements in the measurement uncertainty of PTB’s Hydrodynamic Test Field

One of the most important components of the Hydrodynamic Test Field (HTF) – and relevant for its functioning – is the flow diverter. Extensive experimental investigations of the timing error of the corresponding diverting processes and an exact, model-based functional analysis of these processes have greatly contributed to ensuring and quantitatively improving the Hydrodynamic Test Field's measurement uncertainty from 0.19% to now 0.012%.

For the Hydrodynamic Test Field as the national standard for the volume and flowrate measurement of liquids, an expanded measurement uncertainty of 0.02% within the flowrate range up to 2100 m³/h is documented in the CMC database of BIPM in Paris. The main contribution to this measurement uncertainty has so far resulted from the timing error of the flow diverter and from the measurement uncertainty of the determination of the timing error.

The measurement uncertainty of such a flowrate calibration facility is influenced by a great number of parameters and process quantities which are intrinsic to the facility. Among these, the influence of the flow diverter clearly prevails [1][2], as Fig. 2a clearly shows.

Figure 1: Flow profile in the diverter nozzle [2]
a) Principle of diverter operation
b) LDA-based investigation of the flow profile within the diverter nozzle

On the basis of a model-based analysis which – contrary to a pure "black-box" analysis – quantitatively describes the processes involved in the diverting operation by means of a cause-and-effect chain, it is possible to exactly assess the influence of each physical effect involved. Such a procedure has been applied for the first time to the functional analysis of a flow-diverting unit [2].

Fig. 1a shows the working principle of such a diverter: during the volume measurement, the diverter diverts the fluid free jet, which is conditioned by means of a nozzle having a rectangular cross-section, into a weighing tank. The diverter then reverts to its initial position so that the fluid flow can circulate again inside the measuring system without any influence or retroaction.

The uncertainty of the diverting process is caused by 3 processes: the velocity profile (Fig. 1a) in the nozzle flow and the corresponding degree of turbulence, as well as the time response of the diverting process. Instabilities or temporal fluctuations in the fluid flowrate may contribute to the measurement uncertainty of the diverter; they are, however, not characteristic of this constructional element and, thus, cannot be compensated for or corrected. As a quality-assuring measure for the operation of high-precision flowrate calibration facilities, it is therefore necessary to realise a very stable flowrate regulation [3].

Measurements were performed to determine the diverter's timing error on the basis of the model-based analysis of the diverter. Based on the result of these measurements, the diverter was adjusted, which made it possible to reduce the timing error from 20 ms to 6 ms, i.e. the expanded measurement uncertainty of the total facility was reduced from 0.019% to – currently – 0.012%.

Figure 2: Individual uncertainty contributions of the main quantities of impact to the total uncertainty of the calibration facility
a) Effective diverter timing error prior to applying the model-based analysis
b) Diverter timing error after having applied corrections based upon model-based analysis approaches

As a comparison of the relations of the timing error influence shows in Figures 2a and 2b, the prevailing influence of the timing error in the total measurement uncertainty budget of the measuring system was considerably reduced.

[1] W. Poeschel et al: A unique fluid diverter design for water flow calibration facilities, FLOMEKO 2000, Salvador, Brazil, 2000
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[2] R. Engel et al: Model-based flow diverter analysis for an improved uncertainty determination in liquid flow calibration facilities, IOP Publishing Ltd., Measurement Science and Technology, 21(2010)
http://iopscience.iop.org/0957-0233/21/2/025401?ejredirect=migration

[3] R. Engel et al: Performance improvement of liquid flow calibrators by applying special measurement and control strategies, FLOMEKO 2003, Groningen, The Netherlands, 2003
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Contact person:

Rainer Engel, Dpt. 1.5, E-mail: rainer.engel@ptb.de