Comparison of two different methods for calibration of Cole type Pitot tubes

I. B. S. Bispo, L. C. Pacifico, W. O. Chaves, N. M. Taira, P. J. S. Jabardo

Abstract

The Cole type Pitot tubes (Pitot-Cole tubes) are widely used by water utility companies to map fluid flow velocity profile and thus measure the flow rate in pipelines. This technique is mainly used for on-site calibration of other kind of flow meters, such as electromagnetic or ultrasonic, particularly when removing the meter from the pipeline is somehow not feasible. Therefore, when using Pitot-Cole tubes for those purposes, the determination of the calibration coefficient (Cd) and its associated uncertainty contributes significantly to the results of such measurements.<BR />This paper presents the description and comparison between the results of two different methodologies for calibration of Pitot-Cole tubes: in a wind-tunnel and in a towing tank. Comparisons were also performed for two configurations of the Pitot-Cole tube, with and without a central pin between the two pressure tips, in which the inclusion of this feature increases the measured differential pressure, leading to a set of more reliable measurements.<BR />The obtained results demonstrate coherence and feasibility of the wind-tunnel calibration for normalized Reynolds numbers (RE/L) greater than 7×105. These results also show that a more precise value can be applied along distinct velocity ranges, individually for each Pitot-Cole tube, instead of employing the usual value of Cd = 0.869, used since Cole proposed this form of Pitot tube in 1896. Another major result shows that, for lower values of Re/L, a correction of the calibration coefficient is needed, in order to reduce the uncertainty associated to the measurement. This second result is especially important since the corresponding velocities are related to lower flow rates, in which their measurements are usually followed by larger uncertainty estimates.

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IMEKO TC

TC9 - Flow Measurement

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