Vacuum (Or Vacuum pressure) (P_vacc):

It is pressure of fluid, which is always less than atmospheric pressure. Pressure in a steam condenser is one such example. It can be measured by a pressure gauge but the gauge reads on negative side of atmospheric pressure on dial. The vacuum signifies difference between absolute pressure and atmospheric pressures.

Absolute Pressure (P _abs):

It is pressure of fluid, which is measured with respect to the absolute zero pressure as the reference.

Absolute zero pressure can occur only if molecular momentum is zero, and this condition occurs when there is perfect vacuum. Absolute pressure of fluid can be more or less than atmospheric depending upon, whether gauge pressure is expressed as absolute pressure or vacuum pressure.

Inter relation between various types of pressure representations. It is shown in Figure given, which can be expressed as follows.

p_abs = p_atm  + p_gauge

p_abs = p_atm  - p_vace