Hydraulic bulge forming has been used as a method to determine the properties of sheet metal alloys in biaxial stretching at room temperature. Gas-pressure bulge forming alleviates the issues of using hydraulic fluids when the tests are conducted at high temperatures (above 200°C). Testing a sheet metal alloy by gas-pressure blow-forming (GPBF) under controlled temperature and pressure conditions requires an accurate and reliable mechanism that delivers repeatable results. It was the purpose of this work to design and implement such an instrument. This instrument should deliver real-time data for material displacement during forming, which can then be used to better understand material plastic response and formability. Four different subsystems within this mechanism must interact, but also have enough independence for analysis and for assembly purposes. The combined sub-systems produced a GPBF apparatus capable of forming a sheet aluminum alloy AA5182 with a thickness of 1.5 mm into a dome with a height nearly equal to its radius under a constant gas pressure as low as 40 psi at 450°C. This GPBF apparatus produced, for the first time, in-situ data for dome peak displacement during gas-pressure bulge forming of AA5182 sheet at 450°C. / text