Pressure below atmospheric pressure is primarily measured in ____.

Pressure below atmospheric pressure is primarily measured in inches of mercury. This unit is commonly used in various applications, particularly in meteorology and vacuum systems, to indicate how much pressure is below the ambient atmospheric pressure.

Inches of mercury provide a clear reference point based on the weight of a column of mercury, and it’s commonly used to measure low-pressure systems, making it practical for applications like determining vacuum levels.

Other units, such as psig (pounds per square inch gauge), are used to measure pressure relative to atmospheric pressure but are not exclusive to below-atmospheric measurements, as they can indicate both positive and negative pressures relative to atmospheric pressure. psia (pounds per square inch absolute) measures total pressure including atmospheric pressure and is not suitable for measuring pressures below atmospheric levels. Inches of water column is also a valid unit used for low pressure, but inches of mercury is more standard for significant pressure differences in many applications.