Small pressure above or below atmospheric pressure is primarily measured in ____.

In measuring small pressures that are just above or below atmospheric pressure, inches of water column is a preferred unit because it can accurately represent those minor variations in pressure. The water column measurement is particularly useful for applications like HVAC systems, where precision is necessary but the pressures involved are generally low. It provides a clear and readable indication of pressure differentials, particularly in settings where low pressure readings are critical, such as ductwork or equipment pressure monitoring.

While psig (pounds per square inch gauge) and psia (pounds per square inch absolute) are common pressure units, they are typically used for higher pressures or in contexts where the pressures involved exceed what can effectively be measured using inches of water column. Inches of mercury is also often used, mainly for atmospheric pressure measurements and in different contexts such as barometry, but inches of water column aligns better with the specific requirement of measuring small pressures in HVAC applications.