With which gas, penetration and bead profile are sensitive to arc length, and long arcs for feeding filler wires are more difficult to control?

• A. Hydrogen
• B. Argon
• C. Helium
• D. Nitrogen

Answer: (C)

Helium. The shielding gas choice in GTAW directly affects the arc heat and stability. Helium creates a hotter, more energy-dense arc than argon because of its higher thermal conductivity and ionization characteristics. That means the heat input—and thus the depth of penetration and the bead shape—are more strongly influenced by how far the electrode is from the work (arc length). Small changes in arc length cause larger changes in heat, so penetration and bead profile become more sensitive. When you use longer arcs while feeding filler wire, the arc can become less stable and harder to control, making precise filler deposition and consistent bead geometry more difficult. Argon is steadier and less arc-length-sensitive, while hydrogen and nitrogen aren’t typical shielding gases for GTAW in these applications.