The results obtained from ∼4000 h of operation of an electrochemical oxygen meter developed at General Electric Company for measuring oxygen activities in liquid sodium are described. In the present design, a small ceramic cup (ThO2 with 7 to 8 wt% Y2O3) has been brazed to a metal tube that is inserted into the liquid sodium via a standard sodium flange. The reference electrode used in the present deisgn consists of In/In2O3. Several meters were tested in a flowing sodium system, where the oxygen concentrations were controlled by a cold trap. At 500°C, the oxygen electromotive force (emf) output can be expressed as where T is the cold-trap temperature in K. The temperature coefficient of the meters can be expressed as