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|A Novel Double Weighing Tank Approach for Precision Flow Measurement
|Jaiswal, Shiv Kumar
|Double weighing tank;Precision flow;measurement;water flowmeters;traceability;mechanical designs
|Water Flow Measurement Systems (WFMSs) are widely used for the measurement of flow rate of water flowmeters and also for the traceability purposes all over world. There exists basically two methods for the liquid (water) flow measurement namely gravimetric/ weighing and volumetric. Worldwide, weighing method as described in ISO 4185 is used in WFMS for calibration of different types of flowmeters. Last two decades have witnessed significant research contributions in the development of instrumentation and techniques. During these developments, special emphasis was placed on reducing the measurement errors enabling finally improving measurement uncertainty associated with flow calibration. Nowadays, flow standards of substantially low uncertainty, in the order of 0.05 % or better, are required for high accuracy flowmeters calibration wherein error and uncertainty contributions of standards transferred to meter under test (MUT) is considered almost negligible. The need of such highly accurate flow standards opens new lines for the research and developments in this field. In a static weighing method, the main sources of measurement uncertainty are collected mass, collection time (including diverter error), and density of water. The uncertainty of the collected water mass depends upon weighing balance’s indication, drift, calibration, buoyancy correction, leaks and splashes, storage effects and evaporation of water. The collection time uncertainty depends upon timer calibration, timer actuation and diverter error. The uncertainty associated with density measurements depends upon the quality of tap water and its temperature. The associated measurement uncertainty can be improved by use of advanced instrumentation. However, it is not possible to reduce the diverter error only using better instrumentation but special mechanical designs are needed as used by several other researchers. In order to reduce the diverter error, a unique method/ technique of multiple weighing was proposed. It motivated us to work for the development of a new standard for traceability of flowmeters. The present investigations describe the design and development of such a unique water flow measurement system of DN100 size based on static weighing method. In a traditional WFMS, a single weighing tank and a bypass line are used. However, in present study two metallic weighing tanks each having capacity of 2500 kg, with conical shape at bottom were used to achieve faster draining. The drain time is directly dependent on diameter of the drain valve, therefore, drain valves of comparatively bigger size (diameter 200 mm, commercially known as DN200) were used to obtain faster draining to achieve lower drain time. The fishtail, nozzle and traditional diverter system were fabricated and used in the system. For increasing the collection time, in a multiple weighing system, a cyclic process is adopted wherein filling time (TF) of the first weighing tank is always greater than the draining time (TD) of the second tank. Further, TD also includes adequate time given for stability of the weighing system. The increased collection time thus obtained leads to improvement of measurement uncertainty.
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