NOVELTY - The fluid-sensing device e.g. a quartz crystal microbalance sensor (200) for measuring a fuel concentration in a fuel cell system, comprises a main flow field having first and second ends and configured for transferring fluid to be sensed from the first end to the second end, a bypass channel fluidly connected to the main flow field and configured for diverting and returning a portion of fluid from the main flow field, a fluid sensor disposed in a middle region of the bypass channel, and a sensor region in fluid communication with a wide portion of the bypass channel.    USE - The fluid-sensing device e.g. a quartz crystal microbalance sensor is useful for measuring a fuel concentration in a fuel cell system (claimed) such as polymer electrolyte membrane fuel cells, which are useful as transportable power supplies for portable electronic equipment, power supplies for automobiles and distributed power supplies for houses and public buildings.    ADVANTAGE - The fluid-sensing device is capable of accurately and rapidly measuring the concentration of fuel in a small structure at low cost thus improving the performance of the fuel cell system.    DETAILED DESCRIPTION - The fluid-sensing device e.g. a quartz crystal microbalance sensor (200) for measuring a fuel concentration in a fuel cell system, comprises a main flow field having first and second ends and configured for transferring fluid to be sensed from the first end to the second end, a bypass channel fluidly connected to the main flow field and configured for diverting and returning a portion of fluid from the main flow field, a fluid sensor disposed in a middle region of the bypass channel, and a sensor region in fluid communication with a wide portion of the bypass channel. The bypass channel comprises a fluid inlet coupled proximal to the first end of the main flow field for receiving the fluid from the main flow field, a fluid outlet coupled proximal to the second end of the main flow field for discharging the fluid to the main flow field, and a partition wall disposed between the bypass channel and the main flow field. A cross section of middle region of the bypass channel is wider than a cross section of the fluid inlet and outlet. The bypass channel comprises a rectangular cross section or an oval cross section with respect to the direction of fluid flow, where a vertical dimension of the cross section is narrower than a horizontal dimension of the cross section. An INDEPENDENT CLAIM is included for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic structure view of a fuel cell system with a quartz crystal microbalance sensor.    Fuel cell stack (110)    Pipe (126)    Mixing tank (145)    Drive controlling unit (160)    Quartz crystal microbalance concentration sensing unit. (200)