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High reliability of quench detection system is required for the coil protection of large scale superconducting power system of SMES. The mass flow quench detection system of a forced-flow coil in SMES has been developed as the second quench detection system, while an electrical bridge detection system should be the primary one. To study the characteristics of hydraulic phenomena in the cooling path during the coil quench, the authors optimized parameters of an orifice flow meter. Three kinds of orifices were designed and fabricated. They were located at both inlets and outlets of cooling paths in the coil to measure the mass flow rates. The change of mass flow rate after quench were well observed. The response times after quench were measured. The relationship was observed between the quench detection time and the joule heat of the conductor during the quench. It was verified that this method should be applied to the second quench detection system for the 100 kWh SMES.