Which of the following is a primary measure of vacuum?

The primary measure of vacuum is typically expressed in inches of mercury. This unit is a standard reference in many industries for quantifying the degree of vacuum. When measuring vacuum, it is crucial to compare it to atmospheric pressure, and inches of mercury effectively convey how much pressure has been removed compared to the surrounding atmosphere.

While inches of water column and millimeters of water column can also be used to measure vacuum levels, they are less common than inches of mercury, making them secondary choices. PSI (pounds per square inch) is primarily used for measuring positive pressure rather than vacuum and doesn't accurately represent vacuum levels.

Using inches of mercury as a measurement provides a clear and relatable context, especially in applications where precise vacuum conditions are required, such as in laboratory or processing environments. Therefore, it is recognized as a primary measure of vacuum in both practical and theoretical contexts.