To ensure that the full valve differential pressure can be accepted, it is important to adjust the positioner zero setting so that no air pressure opposes the spring force when the valve is seated. Figure 6.6.13 shows a typical positioner. Commonly, this would be known as a P to P positioner since it takes a pneumatic control signal (P) from the control system and provides a resultant pneumatic control output signal (P) to move the actuator.

Both pneumatics and hydraulics are applications of fluid power. Pneumatics uses an easily compressed gas such as air or a suitable pure gas—while hydraulics use relatively incompressible liquid media such as oil. Most industrial pneumatic applications use pressures of about 80 to 100 pounds per square inch (550 to 690 kPa). Hydraulics applications commonly use from 1,000 to 5,000 psi (6.9 to 34 MPa), but specialized applications may exceed 10,000 psi (69 MPa).