What effect draws air into the gas envelope when the shielding gas flow rate is high?

The correct answer is the Venturi effect, which refers to the phenomenon where fluids (in this case, gases) accelerate as they pass through a constricted section of a pipe, leading to a reduction in pressure. When a high flow rate of shielding gas is introduced in gas tungsten arc welding, the rapid movement of the gas can create a low-pressure area or vacuum in the vicinity of the gas nozzle. This low pressure can draw in surrounding air into the gas envelope, potentially contaminating the shielding environment.

Understanding the Venturi effect is crucial in welding as it highlights the importance of maintaining appropriate shielding gas flow rates. If the flow rate is too high, it can inadvertently pull in atmospheric air, which may contain moisture and other contaminants that negatively affect the weld quality. A balanced flow ensures that the shielding gas effectively protects the weld area from oxidation and contamination, maintaining the integrity of the weld.