Pressure is most often measured in terms of gauge pressure, the pressure of a system above atmospheric pressure.
Atmospheric pressure is approximately 101,325 Pa. For most calculations, especially those involving the ideal gas law, it is necessary to use absolute pressure (gauge plus atmospheric).
Barometers are devices used to measure pressure. Hydrostatic based barometers consist of columnar devices usually made from glass and filled with a liquid of consistent density (water or mercury). Mercury is often a better choice as the density of mercury is much higher than that of water, thus allowing for higher accuracy of measurements.
The columnar section is sealed, holds a vacuum, and is partially filled with the liquid while the base section is open to the atmosphere and makes an interface with the surrounding environment.
As the atmospheric pressure changes, the pressure exerted by the atmosphere on the fluid at the base changes: increasing as the atmospheric pressure increases and decreasing as the atmospheric pressure decreases. This change in pressure causes the height of the fluid in the columnar structure to change: increase with greater pressure and decrease with lower pressure.
A hydrostatic barometer can be placed in a closed system to measure the gauge pressure of the system by subtracting the atmospheric pressure.
- Gauge pressure is the pressure of a system above atmospheric pressure, which must be converted to absolute pressure for most calculations.
- The barometer is a device which uses hydrostatic fluids (water or mercury) to directly determine atmospheric pressure and may be used to indirectly measure the gauge pressure of systems.
- Atmospheric pressure: approximately 101,325 Pa
- Gauge pressure: the pressure of a system above atmospheric pressure.
- Barometers: devices used to measure pressure