Measurement and control of oxygen partial pressure at elevated temperatures

In order to measure and control of the oxygen partial pressure at elevated temperatures in the range from 10−20 to 10−3 bar a standalone system is developed. One major component of the system is a potentiometric sensor which exhibits a measurement uncertainty of Δlog(pO2/bar) < 0.02. The adjustment of the oxygen partial pressure is realized using an oxygen ion pump. Here, a pulse-width modulated electric current between −10 and +10 mA is applied to yttrium-stabilized zirconia resulting in oxygen transport into or out of the system. The pump shows an accuracy in oxygen flow of 10−9 l/min and enables, therefore, the adjustment of pO2 with an uncertainty of Δlog(pO2/bar) < 0.02 even in buffers like CO/CO2 or H2/H2O close to the stoichiometric composition. Further, the impact of different temperatures at the positions of sensor and sample on the oxygen partial pressure is discussed. Finally, the application of the system in an electromagnetic levitator is presented. Here, a Ni sample is heated to 1720 °C and the surface tension of the melt is determined as a function of oxygen partial pressure.