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A helical capacitance sensor for gas fraction including void fraction and volumetric void fraction measurement of gas-liquid two-phase flow in a vertical tube of 5 mm in diameter is presented in this paper. The structure of the helical capacitance sensor is optimized using a finite element method to achieve a homogeneous sensitivity field distribution. A calibration device using the quick closing valve method is designed and used to calibrate the void fraction measurement. Experiment results show that the relative error of gas fraction is less than 10%. The reason for the nonlinearity between the sensor output and gas fraction to be measured is discussed. The linear model describing the relationship between capacitance and gas fraction is obtained by removing the wall capacitance.