What does gauge pressure measure?

Gauge pressure specifically measures the difference between the absolute pressure in a system and the atmospheric pressure. This means that gauge pressure provides information about how much greater the pressure is compared to the pressure of the surrounding atmosphere. It does not account for atmospheric pressure, which is why gauge measurements can be negative if the measured pressure is lower than atmospheric pressure.

For example, if a tire has a gauge pressure of 30 psi, it means that the pressure inside the tire is 30 psi higher than the air pressure outside the tire. This measure is particularly useful in applications like diving, where differences in pressure relative to the surface (atmospheric pressure) need to be considered to assess the risks and necessary adjustments during descent and ascent.

Other options describe different types of pressure measurements: total pressure considers all pressures acting on a point (such as gauge and atmospheric pressure combined), average pressure of a liquid refers to a mean value over a volume rather than a differential measure, and the pressure at which a liquid turns to gas pertains to vapor pressure, which is a separate concept entirely.