Absolute, Atmospheric and Gauge Pressure
The pressure exerted at sea level is 14.7 pounds per square inch (psi), or 1 bar. Pressure measured at sea level occurs due to the earth’s atmosphere, and for this reason is known as atmospheric pressure. Atmospheric pressure decreases as altitude above sea level increases.
Pressure gauges are used to measure pressure; a bourdon pressure gauge is the most commonly used pressure gauge. If a pressure gauge is lying on a table at sea level and is open to atmosphere i.e. not connected to a system, the pressure displayed will be 0 psi, or 0 bar. The pressure shown by a gauge is known as gauge pressure, it does not account for atmospheric pressure.
The sum of gauge pressure and atmospheric pressure is absolute pressure and is given by the equation:
Absolute Pressure = Atmospheric Pressure + Gauge Pressure
Pressures are often given in ‘psi’ or ‘bar’, but it is also possible that pressures may be given in ‘psia’, ‘psig’, ‘bara’ or ‘barg’. The ‘a’ and ‘g’ indicate ‘absolute’ and ‘gauge’ pressure readings. Generally, if a pressure reading is given in psi or bar, it is a gauge pressure reading i.e. psig or barg, but the ‘g’ has not been stated.
Pressure Gauge
It is important to make a distinction between absolute and gauge pressure whenever using gas charts or gas tables, as pressure relates to when a substance changes state and how much energy it carries.
Note that psia, psig, bara and barg, are also sometimes wrote psi(a), psi(g), bar(a), bar(g).
Additional Resources
https://courses.lumenlearning.com/physics/chapter/11-6-gauge-pressure-absolute-pressure-and-pressure-measurement
https://www.setra.com/blog/what-is-absolute-pressure
https://www.esi-tec.com/blog-pressure-sensors-transmitter-transducer/
https://www.britannica.com/technology/pressure-gauge
https://www.livescience.com/39315-atmospheric-pressure.html
https://en.wikipedia.org/wiki/Atmospheric_pressure