Question

Define the terms: (i) absolute pressure (ii) Gauge Pressure (iii) vacuum pressure.

Answer 1

i) Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. 
ii) Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. 
iii) Vacuum pressure is the difference between the atmospheric pressure and the absolute pressure.
Pressure(vac) = pressure(atm) - pressure(abs)