Datum is at absolute zero line           P _abs = Absolute pressure = P +P_at

And                                                     P _abs =P_al- P _vac

P _abs =pressure measured from absolute zero pressure is called absolute pressure

P _g = Gauge pressure      =Pressure measured above the atmospheric pressure is called gauge pressure,here atmospheric pressure is taken as zero.

P _vac = Vacuum pressure = Pressure below the atmospheric pressure is the vacuum pressure.

P_at =Atmospheric Pressure