Abstract: A closed-loop control system controls introduction of either water or hydrogen into a furnace region where a part is subjected to an elevated temperature to accomplish a heat treatment process. The heat treatment process causes the part to participate in reduction and/or oxidation reactions which remain in balance at the elevated temperature so long as a hydrogen/water ratio set point is maintained. The system includes an oxygen probe in communication with the furnace region for providing (i) an oxygen output indicative of sensed oxygen concentration within furnace region, and (ii) a temperature output indicative of temperature therein. A controller determines from the oxygen output and temperature output, a measured ratio of hydrogen to water within the furnace region and compares the measured ratio with the hydrogen/water ratio set point, and provides a correction signal output in accordance with a determined difference between the measured ratio and the ratio set point.