Full vacuum pressure occurs when pounds per square inch, or PSI, reaches zero. PSI is the most common standard to measure pressure. Nonetheless, other measurements also reach zero when a full or absolute vacuum is achieved.
Torr, or millimeters of mercury, as a gauge of measure of pressure also reaches zero with the achievement of an absolute vacuum. When inches of absolute mercury are used as an indicator of pressure, the measurement also reaches zero in the presence of a full vacuum. Finally, micron or micrometre of mercury is also used to measure pressure in vacuum systems; this measurement also reaches zero when a full vacuum is achieved. The achievement of a full vacuum is useful for undertaking certain types of research experiments.