As the gas velocity increases, which type of zone develops behind the cup?

• A. zero-pressure
• B. high-pressure
• C. mid-pressure
• D. low-pressure

Answer: (D)

When the shielding gas speeds up, the jet accelerates and, in doing so, its static pressure drops behind the stream due to Bernoulli's principle. This creates a low-pressure zone behind the cup. That lower pressure region is a natural consequence of high-velocity gas leaving the nozzle and shaping the shielding envelope around the weld. It also explains why increasing gas velocity can improve shielding up to a point but may also invite entrainment of surrounding air if the flow becomes too strong. The other options describe higher or non-existent pressures, which don’t occur behind a fast-moving gas jet.