What is used to measure gas pressure in a closed container?
The device you use to measure gas pressure depends on the pressure you wish to measure.
A pressure gauge measures higher pressures. The pressure-sensing element may be a tube, a diaphragm, a capsule, or a set of bellows. These change shape in response to the gas pressure. A linkage connected to a needle reads the deflection of the pressure-sensing element. Typical pressure ranges go from 0-1 bar up to 0-600 bar.
A U-tube manometer usually contains water or mercury in a U-shaped tube. One end of the U tube connects to the unknown pressure, and the other end is open to the atmosphere.
A calibrated scale behind the liquid columns measures the heights of the two levels. The different in heights Δh is proportional to the gas pressure.
A vacuum gauge measures lower pressures. Common gauges measure pressures from 1000 mbar to about 50 mbar. Some specialized devices measure pressures down to 10⁻¹⁴ bar.