See attached files to use a pressure controller (like a 7250LP or PPC4-BG15K) to control positive and negative pressures to test device (UUT) with a FPG as the reference device. The FPG measures pressure by the piston being pushed down so in positive gauge mode the positive pressure from the controller's TEST port goes to the top of the piston and pushes it down. In negative gauge mode the negative pressure from the controller's TEST port is switched (through a switch box with valves in it) to go to the bottom of the piston and pull it down.