Studies have been made to predict the discharge coefficient of critical flow Venturi nozzle theoretically and experimentally. However, the studies about the influence of the inlet curvature to the discharge coefficient in the laminar–turbulent transition boundary layer region are very difficult because of the lack the accuracy of the critical flow Venturi nozzle shape. To solve these problems, our group suggested the technique with High-Precision Nozzle and already confirmed the effectiveness. In this research, we clarify that the effect of the inlet curvature R on the discharge coefficient Cd of critical-flow Venturi nozzle with a toroidal throat is investigated in the laminar boundary layer region, the laminar-turbulent transition in the boundary layer and turbulent boundary layer region. R ranged from 2.0D to 1.0D, which were obtained by shaving the inlet of a high precision nozzle that was originally complying with ISO 9300. At first, we measure the discharge coefficient of critical flow Venturi nozzle experimentally and discuss the phenomena of the laminar-turbulent transition by the inlet diameter. Here, the calibrations were performed by a constant volume tank system developed for the primary standard in Japan. Moreover, we try to propose and discuss the hyperbolic tangent equation as the revised Universal Curve to combine the Cd’s for the laminar and turbulent boundary layers.

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