Units and scales of Pressure Measurement

Pascal (N/m²) is the unit of pressure .

Pressure is usually expressed with reference to either absolute zero pressure (a complete vacuum)or local atmospheric pressure.

• The absolute pressure: It is the difference between the value of the pressure and the absolute zero pressure.
                              

• Gauge pressure: It is the diference between the value of the pressure and the local atmospheric pressure(p_atm)
                                
  

• Vacuum Pressure: If p<p_atm then the gauge pressure becomes negative and is called the vacuum pressure.But one should always remember that hydrostatic pressure is always compressive in nature
  

Fig 4.1 The Scale of Pressure

At sea-level, the international standard atmosphere has been chosen as P_atm = 101.32 kN/m²