液体大流量計校正設備の不確かさ解析

抄録

The large water flow calibration facility, the primary standard of water flow rate in Japan, is one of the largest calibration facilities for water flow meters in the world. This facility covers the flow rate range of 0.014 to 0.83m³/s (50 to 3000m³/h). Flow rates are maintained by seven pumps and an overflow head tank. A water flow meter under test is installed in one of the test lines whose inner diameter (D) ranges from 100 to 1100mm with a minimum upstream straight pipe length of 100D. The flow rate through the flow meter is measured using the gravimetric method. The uncertainty of this facility has been estimated experimentally and analytically in detail in accordance with the ‘ISO Guide to the expression of uncertainty in measurement.’ As a final result, the expanded uncertainty of the flow rate (k=2) was estimated to be 0.034%. The major error sources and their contribution to the standard uncertainty of the flow rate were the measurements of (a) the mass of the water in the weighing tank (0.0130%), (b) the time duration for which the water flow was diverted to the weighing tank (0.0085%), (c) the buoyancy for correction of the mass of the water in the tank (0.0070%) and (d) the density of the water (0.0025%). The uncertainty of the mass measurement of the water in the tank, which is the largest error source, was mainly due to the limits in the reproducibility of the weighing scale. From this result, an improvement of the calibration method of the weighing scale was proposed. It was clearly shown that this kind of uncertainty analysis is very important in assessing and improving the performance of a water flow calibration facility.