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Pressure Measurement

A bourdon gauge calibrated to read in psi is mounted on a gas tank. The gauge reads 0 when the tank is open to the atmosphere, and 75 after the tank is filled.
a) does the reading represent the absolute pressure or gauge pressure of the gas in the tank?
b) if the gauge were replaced by an open-end mercury manometer, what would be the difference (in mm) between the two mercury column levels?

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a) Since gauge reads zero when open to atmosphere, the gauge readings are not absolute. Pressure is measured relative to atmospheric pressure and 75 psi is the overpressurization above atmospheric one.
b) Convert 75 psi into mm Hg column. That would be the sought difference.