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Gauge pressure is the pressure measured relative to atmospheric pressure and reads zero when inside pressure equals atmospheric pressure. Absolute pressure is the total pressure, including both gauge pressure and atmospheric pressure combined. Most pressure gauges, like those on scuba tanks, are calibrated to display gauge pressure rather than absolute pressure.

A manometer is a U-shaped glass tube containing water or mercury with one end exposed to atmospheric pressure and the other connected to a container of unknown pressure. By comparing the pressure at the bottom of both ends, the gauge pressure is estimated. The height difference between the two liquid columns directly indicates the pressure difference being measured.

A vacuum chamber is a rigid chamber from which air is pumped out, creating pressure lower than ambient atmospheric pressure. Since gauge pressure is measured relative to atmospheric pressure, the pressure inside the vacuum chamber reads as a negative value. The absolute pressure in fluids cannot be negative, but gauge pressure can when measured against atmospheric reference.

A pressure gauge reads zero when the pressure inside a container equals atmospheric pressure. Most gauges are calibrated to measure zero at atmospheric pressure, which is why a deflating soccer ball shows zero when internal pressure matches the outside air pressure. This zero reading represents gauge pressure, not absolute pressure.

A mercury barometer consists of a single long tube closed at one end, filled with mercury and inverted over a mercury dish. The mercury level in the tube rises and falls depending on the atmospheric pressure outside the tube. From the height of the mercury column, atmospheric pressure is estimated directly.

Common pressure gauges include manometers for measuring gauge pressure and barometers for measuring atmospheric pressure. Other types include strain gauges, capacitance pressure gauges, piezoelectric pressure gauges, and ion gauges, which measure pressure by ionizing molecules in highly evacuated chambers. Different gauges are useful in various pressure ranges and physical situations.

Absolute pressure is calculated by adding gauge pressure and atmospheric pressure together. When a gauge displays a positive value, that gauge pressure is added to standard atmospheric pressure to find the total absolute pressure. Understanding this relationship is essential for interpreting pressure measurements in fluid systems and pressure of fluids applications.