The present study carries out numerical analysis of the coupled natural convection and oxygen transfer of low-Prandtl-number (similar to 0.02) liquid lead and lead-bismuth eutectic (LBE) for testing and calibrating low concentration level oxygen sensors. The analysis is performed on the two-dimensional coordinates in a rectangular container, where the fluid movement is laminar for the purpose of sensor test and calibration. The oxygen supply is from the cover gas at the top of the container. Natural convection and oxygen transfer are examined under three temperature boundary conditions: (a) heated from the lower part and cooled from the upper part of the sidewalls of the container; (b) heated from the sidewalls and cooled from the top of the container; (c) one sidewall heated and the opposing wall cooled. It is found that there are four, two and one convective circulation cells under conditions (a), (b), and (c), respectively. All these flows induced by the natural convection greatly enhance the oxygen transfer in the liquid metal. The most efficient one is under condition (b), in which it takes about 1000 s for the oxygen concentration in the whole field to reach similar to 90% of the input oxygen concentration from the top, instead of similar to 10(6) s by the pure diffusion. (c) 2005 Elsevier Ltd. All rights reserved.