What does a draft gauge measure?

A draft gauge is specifically designed to measure the pressure difference, usually in terms of a low-pressure range, and is typically calibrated in inches of water column (inches of H2O). This type of measurement is crucial in HVAC systems, combustion processes, and other applications where air or gas flow pressures need to be monitored.

Inches of H2O are a common unit for measuring low pressure because they can accurately reflect small changes in pressure that can influence system performance and efficiency. This makes the use of inches of water column particularly suited to applications involving airflow and gas venting, as opposed to other pressure measurement units that may be more appropriate for high-pressure environments.

The other options represent different measurement standards that are not typically associated with the function of a draft gauge. PSIG (pounds per square inch gauge) and PSIA (pounds per square inch absolute) measure pressures that are generally higher than what a draft gauge would be used to monitor, while inches of mercury (inches of Hg) is used in different contexts, such as barometric pressure measurements. Thus, inches of water column is the correct and relevant measure that a draft gauge is intended to provide.