The characteristics of a sodium-system plugging indicator have been studied with the instrument operated in both the bare orifice and partially plugged modes. The “plugging temperature” produced when the orifice is initially bare indicates the point where nucleation of the impurity is first noted and is strongly influenced by flow rate and cooling rate. Thus, in the bare orifice mode the meter must be calibrated to produce oxygen concentration as a function of plugging temperature. In the partially plugged mode, saturation temperature is indicated each time a flow rate arrest occurs, and, therefore, no calibration is required. Also, with Na2O on the orifice, the rate of flow increase or decrease through the orifice permits the calculation of mass transfer coefficients for the dissolution or precipitation of Na2O.