Heat and water management in a fuel cell occurs due to several coupled processes, such as liquid and gas ›ows, as well as heat and mass transfers. Improper thermal and water management may cause severe mass transport limitations. In a PEMFC, the cathode is the performance limiting component due to the sluggish oxygen reduction kinetics. Other limitations in mass transport are imposed by the liquid water generated by the electrochemical reaction and brought to the cathode through electroosmotic (EO) drag. Proper water management ensures that the membrane remains fully hydrated (Su et al., 2006). A major obstacle preventing a fuel cell from realizing its theoretical current density is the reduction in ežective porosity due to the accumulation of liquid product water inside the ›ow channels, the gas diffusion layer (GDL), or the catalyst layer (CL). šerefore, information regarding the amount of liquid water inside the fuel cell is of great technical signiœcance.