A fuel cell system comprising a fuel cell stack and at least one hydration sensor apparatus for measuring membrane hydration in the fuel cell stack is disclosed. The hydration sensor apparatus comprises (1) an electrically insulated sensor comprising a polymer electrolyte membrane, (2) a power supply, and (3) a load resistor. The sensor, the power supply and the load resistor of the hydration sensor apparatus are electrically connected and the sensor of the hydration sensor apparatus is fluidly connected to the fuel cell stack such that, during operation of the fuel cell system, the polymer electrolyte membrane of the hydration sensor apparatus is exposed to a fuel fluid stream of the fuel cell stack.