Types of Pressure
1. Absolute Pressure
Absolute pressure is a pressure expressed by referring to vacuum (complete) pressure as zero. It is mainly used in physical science for expressing atmospheric pressure, etc. Absolute pressure is calculated by following formula: absolute pressure = atmospheric pressure + gage pressure. KYOWA mentions absolute pressure as "abs." to differentiate absolute pressures to gage pressures.
2. Gage Pressure
Gage pressure is a pressure expressed by referring to atmospheric or ambient pressure as zero. Industrially, it is merely called pressure unless otherwise noted. Pressure higher than atmospheric or ambient pressure is called positive gage pressure and pressure lower than atmospheric or ambient pressure, negative gage pressure. Though ISO recommends to affix "Pe" or "Gauge" to gage pressure, KYOWA does not affix either of them to gage pressure.
3. Differential Pressure
Differential pressure is a difference between a specific pressure and other. Thus, it may be either positive or negative.
Relations between Pressure Units