What is the meaning of the letter “g” or “a” after pressure measuring units?

These letters indicate either the “manometric/ instrumental measuring mode” or the “absolute measuring mode”, where the gauge-mode pressure (g from the English gauge) is one measured normally by observing the surrounding pressure and the absolute-mode pressure (a from the Engl. absolute) is measured by observing the zero pressure.

In spite of their inscription on many instruments, abbreviations like barg, psig, kPaA, kPaG, bara, psia, etc. are not legitimate and create conflicts with the international standards – they should not be used.

In order not to engage in details it is essential that you know the following:
Absolute pressure (…a) = manometric pressure (…g) + atmospheric pressure.

We remind you briefly a conversion relationship between pressure measuring units:
1 bar = 100 000 N/m2 = 1.01972 x 10000 kgf/m2 = 1.01972 at (kgf/cm2) = 0.98692 atm = 750.06 torr (mm Hg)