Influence of atmospheric water vapor on high-current D-C arcs

To determine the effect of water vapor on arc characteristics, arcs with direct currents in the range from 20 to 200 amperes were studied at atmospheric pressure in air containing water vapor in controlled quantities. The water vapor content of the air was varied from 0.15 to 100% (per cent) by volume and this range of values constituted conditions from extremely dry air to saturated steam. For arc electrodes, the materials employed were iron, copper, tungsten, and graphite. Data were taken relating the electrical characteristics and stability of the arc to atmospheric water vapor content, arc current, and electrode separation and material. It was found that the presence of water vapor causes an increase in arc voltage compared to the arc voltage for dry air. This increase is much greater for the electrode materials, iron and copper, than for tungsten or graphite. For a 120-ampere arc with iron electrodes at 1-centimeter separation in an atmosphere of 100% water vapor, the magnitude of the arc voltage is approximately 170% greater than for such an arc in air containing 0.15% water vapor by volume. Whereas, under similar conditions except with graphite electrodes, only a 13% voltage increase occurs. These voltage increases appear to occur in both the arc column and the electrode drop regions. From an analysis of the results, it appears that the differences between air and water vapor effects on the arc derive from the predominant roles of the large specific heats of water vapor and hydrogen and of the great energy transferring properties of hydrogen attributable to its relatively small mass.