An asymmetric heat flux sensor has two sensing surfaces separately thermally coupled to the end plates of a thermoelectric module so that one is heated and the other cooled. The heated sensing surface is constrained to have a wettable area much larger than the area of the cooled sensing surface. This allows the heated sensing surface to be nearly the same temperature as the fluid ambient while providing a relatively large temperature differential. Because bubbles, which degrade a heat flux measurement, form preferentially on a heated surface, the asymmetric design avoids bubble formation and hence provides enhanced accuracy.