Air is admitted through ports a and d into the central space in the valve where it applies pressure to membrane 1. Attached to the membrane are pistons 2 and 3 which are subject to the pressure of a fluid. Membrane 1 is in its central position when the pressure of the air is equal on its right and left sides. When the pressure at the left increases, membrane 1 with pistons 2 and 3 moves to the right. At this, plunger 2' opens passage b and admits fluid under pressure into the servomotor. Membrane 1 returns to its neutral position due to the difference in the effective areas of the cross sections of plungers 2' and 3'. When the pressure at the right increases, membrane 1 with pistons 2 and 3 moves to the left. Plunger 3' opens passage f and the pressure in the servomotor drops. Membrane 1 is returned to its neutral position by the pressure of fluid delivered by the pump.
$4048$EHP,Va$