Pressures lower than atmospheric are measured in which of the following?

Pressures lower than atmospheric pressure are commonly measured in inches of mercury or in microns.

Inches of mercury is a traditional unit used in various applications to express vacuum levels, where lower readings indicate pressures that are below atmospheric pressure. For instance, a vacuum gauge might indicate a level in inches of mercury, confirming how much the pressure has dropped relative to standard atmospheric pressure.

Microns, specifically microns of mercury, represent a much finer measurement of vacuum pressure. One micron is equal to one-thousandth of a millimeter of mercury, and it is frequently used in applications requiring precise measurement of low pressures, such as in refrigeration and air conditioning systems.

Thus, both inches of mercury and microns are valid units for measuring pressures that are lower than atmospheric, making the correct answer a combination of both units. Other options such as psig (pounds per square inch gauge) refer to pressure measurements that do not consider atmospheric pressure and are not suitable for indicating vacuum levels.