Uncertainty Analysis for Mean Flow Velocity and Discharge Measurements using Floats based on Large-Scale Experiments

Abstract

The uncertainty of flow measurements obtained by the float method is evaluated following the international organization for standardization (ISO) 748 guideline. However, the standard uncertainty of an average flow rate has not been considered and the quantitative uncertainty has never been computed for flow measurements made using the float method. Therefore, in this study, a stream-scale experiment was performed to estimate the standard uncertainty of the mean flow velocity by considering the flow velocity uncertainty of floats. The results demonstrated that the standard uncertainty of the mean flow velocity measured by a surface float was 15.30%, while that measured by a rod float having a 50-cm draft was 11.05%. Through these results, the measurement uncertainty of discharge was evaluated according to the GUM (guide for the expression of uncertainty in measurement) method. The measurement uncertainty was then evaluated considering the standard uncertainty of the mean flow velocity. The measurement uncertainty of the discharge was increased by 3.4% as compared with that calculated without considering the standard uncertainty.