The present invention is generally comprised of a tube adaptor, a chamber,
a chamber pressure sensor, a calibration pressure sensor, an iris valve,
an iris valve drive means, and a control and data processing means. In a
calibration mode, the calibration pressure sensor measures the purge
pressure at the abutment of two flow elements, while concurrently
measuring the corresponding pressure at the chamber pressure sensor. In
the welding mode, the calibration pressure sensor is removed, and the
corresponding pressure within the chamber is maintained throughout the
welding process to affect the geometry of the molten weld bead.