Abstract: The vacuum system is the precondition to ensure high purity Ar gas in the welding chamber ot FYS-1 installation of manual TIG welding for prevetion of oxidization.The loss of liydraulic power resulting from the turbulent flow of gas inside pipeline gives rise to pressure difference be-tween inner and outer surfaces of the polymethy glass cover. The maximum of pressure difference has been analysed and calculated.The methods to redu-ce the pressure difference heve been discussed。