A small portable spectrometer was developed to monitor oxygen and hydrogen as the free or dissociation products in the tungsten arc-inert gas welding process. The hydrogen (6562.8 Å) and oxygen (7771.9 Å) lines were monitored and the argon (8521.4 Å) line was used for the internal standard. The instrument, a fixed Eagle mount, used a 76X50 mm, 60 cm focal length, concave grating with 610 grooves/mm blazed in the second order. The reciprocal dispersion at 6562 Å was 12 Å/mm. The oxygen triplet was resolved. Optical design requirements included the use of very short arc lengths in the order of 0.5 mm and less, an optical plane of 65° to 75° with a large flat plate instead of the conventional 90° with respect to the electrodes, and an arc that does not remain in a fixed optical position in space. The focusing lens–limiting aperture assembly was mounted on the torch head with a fiber optic bundle transmitting the light to the spectrometer. The instrument recorded only the signal above background, including the very high background produced by introducing the electrode tip into the optical path. This was accomplished by the use of an oscillating optical block in the entrance beam which yields an ac signal above the dc signal from background. The direct ratio readout system used a Devar ratio computer in each analytical channel and was essentially instantaneous.
W. A. Loseke, E. L. Grove, and E. S. Gordon, "The Design of a Portable Polychromator to Monitor Contaminants in Tungsten Arc-Inert Gas Shields," Appl. Spectrosc. 25, 64-70 (1971)