Dissociated ammonia carrier gas used as a reference gas is obtained from an ammonia dissociator which also provides dissociated ammonia carrier gas to a nitriding furnace whereby the source of ammonia supply gas is the same. An oxygen probe is used to regulate the nitriding potential of a nitriding furnace and atmosphere for process control and high quality nitrided parts. The method further includes correlating the probe mV output signal to a nitriding potential and adjusting the ratio of ammonia supply gas to dissociated ammonia carrier gas at the inlet of the nitriding furnace.