In technology, pressure is often specified in multiples of atmospheric pressure.
It can also be defined as the thrust [compressive force acting perpendicularly to the surface of a body] acting per unit area.
Since pressure depends on both force and area, the same amount of force applied by a hand will make less pressure than when applied by something smaller, like a finger.
To measure the amount of pressure people use these units:
- Pounds per square inch - (psi, PSI, lb/in2, lb/sq in). This U.S. customary units is not elsewhere. Normal atmospheric pressure is 14.7 psi, which means that a column of air one square inch in area rising from the Earth's atmosphere to space weighs 14.7 pounds.
- Atmosphere - atm. Normal atmospheric pressure is called 1 atmosphere. 1 atm = 14.6956 psi = 760 torr.
- P=F/A, Torr - torr. This is called the Torr because it is based on the original Torricelli barometer design. One atmosphere of pressure will push the column of mercury (Hg) in a mercury barometer to a height of 760 millimeters. A pressure that causes the Hg column to rise 1 millimeter is called a torr. 1 atm = 760 torr = 14.7 psi.
- Bar - bar. The bar is almost the same as the atmosphere unit. One bar = 750.062 torr = 0.9869 atm = 100,000 Pa.
- Millibar - mb or mbar. There are 1,000 millibar in one bar. This unit is used by meteorologists who find it easier to refer to atmospheric pressures without using decimals. One millibar = 0.001 bar = 0.750 torr = 100 Pa.
- Pascal - Pa is the SI unit of pressure. It was named after the French physicist Blaise Pascal. 1 pascal = a force of 1 newton per square meter (1 newton = the force needed to accelerate 1 kilogram one meter per second per second = 1 kg.m/s2; this is actually quite logical for physicists and engineers, honest). 1 pascal = 10 dyne/cm2 = 0.01 mbar. 1 atm = 101,325 Pascals = 760 mm Hg = 760 torr = 14.7 psi.
- Kilopascal - kPa. The prefix "kilo" means "1,000", so one kilopascal = 1,000 Pa. Therefore, 101.325 kPa = 1 atm = 760 torr and 100 kPa = 1 bar = 750 torr.
- Megapascal - MPa. The prefix "mega" means "1,000,000", so one megapascal = 1,000 kPa = 1,000,000 Pa. Pressures in the megapascal range are common in high-pressure air compressors and hydraulic systems, where high pressure liquids are used to do useful work.
- Gigapascal - GPa. The prefix "giga" means "1,000,000,000", so one gigapascal = 1,000 MPa = 1,000,000 kPa = 1,000,000,000 Pa = 9,870 atm = 10,000 bar. Pressures of several gigapascals can change graphite to diamond or make hydrogen a metallic conductor!