Fuel cells are emerging as a viable power source for many applications. In a fuel cell, an active element referred to as a membrane electrode assembly is sandwiched between sheets of porous, gas-permeable, conductive material which serve as the primary current collectors for the MEA and provide mechanical support therefore. The MEA/primary current collector assembly is pressed between a pair of non-porous, electrically conductive separator plates or metal sheets which serve as the secondary current collectors for the primary current collectors and conduct current between adjacent cells with the fuel cell stack.
The MEAs must operate within a desired temperature range. Since the chemical reaction within the fuel cell is exothermic, a fuel cell stack often utilizes some internal cooling system to extract heat therefrom. To this end, a separate cooling layer is in thermal contact with individual fuel cells within the stack. A coolant fluid is circulated through the coolant layer to remove the heat from the adjacent cells.
As the coolant ages it collects contaminates that cause it to become electrically conductive. Some stack designs depend on plate insulation coatings for electrical isolation of the coolant fluid. If the plate coatings crack or begin to leak electrically, the stack coolant could conduct a leakage current throughout the coolant loop. Thus, it is often desirable to monitor the conductivity of the coolant to detect such conditions.
Conventional means for monitoring the conductivity of a coolant loop employ a resistor connected between two conductive points in the coolant loop with a volt meter connected across it. However, this method creates an alternate current path in the circuit and ends up loading the voltage that is being monitored.