What type of gauges are calibrated in inches of mercury below atmospheric pressure?

Vacuum gauges are designed to measure pressure levels that are below atmospheric pressure. These gauges indicate the amount of vacuum present in a system, typically expressed in inches of mercury (inHg). When the pressure inside a vessel or system drops below the surrounding atmospheric pressure, it creates a vacuum.

The measurement is referenced to atmospheric pressure, meaning that a vacuum gauge will indicate how many inches of mercury are being "lifted" from the ambient atmospheric pressure by the vacuum inside the vessel. For example, a vacuum level of 10 inches of mercury signifies that the pressure inside the vessel is 10 inches of mercury less than the atmospheric pressure outside.

In contrast, pressure gauges measure positive pressures above atmospheric levels, while temperature gauges are designed to measure thermal conditions. Flow gauges, or flow meters, measure the volume or mass of fluid moving through a system and are unrelated to pressure measurements. Thus, the appropriate choice regarding which type of gauge is calibrated in inches of mercury below atmospheric pressure is vacuum gauges.