For the calibration of a control valve a loop calibrator or 
a pressure calibrator can be used between 3-15psi.Check the 
valve position for 3psi which is to be 0 for a NC type 
valves.In such manner check the valve positon for various 
ranges(6psi,9psi,12psi & 15psi).the valve positon shoul be 
0 for 3psi and 50% for 9 psi and completely opened for 
15psi.
The same procedure is applicable for NO type valves.The 
valve is open for 3 psi and completely closed for 15psi.
Any errors occured may be rectified by Zero span adjustment.