Pressure
Pressure is the force exerted on an object divided by the area over which it is exerted, and a barometer is a device used to measure atmospheric pressure. A barometer can be made by filling a tube with a liquid, often mercury, and inverting the tube in a dish containing the same liquid (Figure 11.2). If the air has been removed completely from the vertical tube, the liquid in the tube assumes a level such that the pressure exerted by the mass of the column of liquid in the tube is balanced by the pressure of the atmosphere pressing down on the surface of the liquid in the dish. Pressure is often reported in units of millimeters of mercury (mm Hg), the height (in mm) of the mercury column in a mercury barometer above the surface of the mercury in the dish. At sea level, this height is about 760 mm. Pressures are also reported as standard atmospheres (atm), a unit defined as follows:
1 standard atmosphere (1 atm) = 760 mm Hg (exactly)
The SI unit of pressure is the pascal (Pa).
1 pascal (Pa) = 1 newton/m2
Because the pascal is a very small unit compared with ordinary pressures, the unit kilopascal (kPa) is more often used. Another unit used for gas pressures is the bar, where
1 bar = 100,000 Pa.
To summarize, the units used in science for pressure are
1 atm = 760 mm Hg (exactly) = 101.325 kilopascals (kPa) = 1.01325 bar
or
1 bar = 1 × 10 5 Pa (exactly) = 1 × 102 kPa = 0.9872 atm
You may have used a tire gauge to check the pressure in your car or bike tires. In the United States, such gauges usually indicate the pressure in pounds per square inch (psi) where
1 atm = 14.7 psi.