In Situ optical spectroscopy has been used on plasma arc welded 2219 aluminum to measure both the average temperature of and the amount of hydrogen and oxygen in the welding arc. Hydrogen and oxygen levels of less than 75 ppm can be readily detected. It is shown that below a critical shield gas flow rate, the rapid invasion of atmosphere can be readily detected by this technique, and that this critical flow rate is dependent on the temperature of the arc.

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