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Is gauge pressure always zero-referenced against ambient air pressure? Or is it referenced against the sum of all pressures acting on a fluid, which usually just happens to be ambient air pressure?

To clarify, here is a potential question: Imagine a container with two immiscible fluids of different densities. 1atm of atmospheric pressure is acting at the surface of the first fluid. An object at the deepest point of the first fluid has a gauge pressure of 3atm, so the absolute pressure at this depth is 4atm. If the absolute pressure of the deepest point of the second fluid is 8atm, what would the gauge pressure of the object be if it was submerged to the deepest point of the second fluid?

If gauge pressure is always zero-referenced to air pressure, the gauge pressure would be 8atm - 1atm = 7atm. However, if it accounts for the sum of the pressure from all the fluids above it, then the gauge pressure would be 8atm - 1atm - 3atm = 4atm. Which is the correct gauge pressure?

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As that wikipedia link suggests, gauge pressure is compared to the surrounding air pressure. It really is intended to be used for "what a pressure gauge reads" in "everyday life". The all-surrounding presence of air on the Earth makes it a convenient zero point, so it is immediately obvious when things are not in equilibrium with the environment.

Your example illustrates why gauge pressure is not always the most reasonable way to present the pressure of a fluid. Where confusion is possible, it's always safest to use absolute pressure.

Of course, you're only ever incorrect by 1 atm... :-)

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