The most commonly used unit in vacuum measurements is Pa. 1Pa is equal to 1N/m2. The unit used in technical atmosphere is Bar. 1Bar=1kgf/cm2. The unit now used less often is torr. 1Torr is defined as 1/760atm.
While P is the symbol for pressure, the measuring unit is “pascal” or “Pa” in short. Pascal is a basic measuring unit for pressure in the international system of units, named after French physicist Blaise Pascal.
The physical significance of 1Pa: It is defined as 1N per square meter. The air density varies above the Earth’s surface where density near the ground is greater and decreases as altitude increases. While atmosphere pressure is caused by air gravity, the column in the tube falls lower at higher altitude with less air density. Higher the altitude is, less atmosphere pressure it gets. Within elevation of 3000m, the atmosphere pressure decreases by 100Pa per every 10m of rising height. Within elevation of 2000m, the atmosphere pressure decreases by 1mmHg per every 12m of rising height.