A modified Fowler-Milne method for monochromatic image analysis in multi-element arc plasma welding

The monochromatic images of 794.8 nm Ar I and 487.98 nm Ar II spectrums are obtained by high speed camera, and these images are normalized with respect to their relative maximum emission coefficient in a pure argon (single element) arc. The relative maximum emission coefficient of 487.98 nm Ar II is calculated based on Olsen-Richter diagram. Unlike other spectroscopic methods, the presented method does not require spectra scan or the maximum emission coefficient of non-argon elements. This technique is applied to the measurement of radial profile of temperature and gas composition of argon-helium arc plasma in gas tungsten arc welding (GTAW), significant demixing occurs in the argon-helium arc plasma. Helium concentration in arc plasma is higher than that in original gas mixture, and it decreases as the point moves away from the axis along radial direction and increases as the point gets closer to the cathode. The calculated arc temperature and gas composition are compared with the data obtained using conventional method where a spectrometer is used for spectral line intensity measurement and two-line relative intensity method for calculation. Very good agreements have been observed. That means, the developed technique measures arc temperature and gas composition as accurate as the conventional spectroscopic methods. With the usage of a high speed camera, the developed technique enables measurement of arc temperature and gas composition distribution in a dynamic arc such as arc ignition, pulsing arc.