Pressure Sensors are calibrated using a pressure calibrator. A pressure calibrator is a pneumatic devices which develops a pre-set pressure. Pressure Calibrators are usually manually operated.
The Pressure Calibrator is connected to the pressure switch or transducer to be calibrated. The reference pressure is applied through the calibrator. The output of the transducer is checked for the appropriate signal.
For instance, a 1.10 bar pressure with an output of 4.20 mA should develop 12mA when a 5 bar pressure is applied. 
If the sensor output is not proper. The sensor can be adjusted using the zero and span adjustments.
Five Point Calibration
The Five point Calibration is a method usually adopted in pressure sensor calibration. The pressure sensor is tested at 0%, 25%, 50%, 75% and 100% of the range. Thus, a pressure sensor with a range of 0 to 12 bar will be tested at 0 bar, 3 bar, 6 bar, 9 bar and 12 bar.