19Recall from basic physics that friction force always opposes the direction of motion. Thus, when the valve is opening, friction works against the actuator’s air pressure (assuming an air-to-open valve), requiring additional air pressure to maintain motion. When the valve is closing, though, packing friction works in the same direction as the actuator’s air pressure “helping” the valve stay more open than it should. This is why the positioner must maintain less actuator air pressure for any given position while moving closed than while the valve moves open. The difference in air pressure moving open versus moving closed at any given stem position is proportional to twice the dynamic packing friction. Stated mathematically, Fpacking = 1 2(Popening Pclosing)A.