Pressure is a measurement of force per unit area. The metric system uses kilopascals (kPa) and bar. The Imperial system uses pounds per square inch (PSI) and atmospheres. Vacuum is a term given to a pressure that is less than atmospheric pressure. The Imperial system measures vacuum in inches of mercury (“Hg) or inches of water. The metric system measures vacuum in millimeters of mercury (mm Hg). You’ll need to understand pressure conversions when filling tires and replacing air conditioning refrigerants or using a vacuum gauge.
| Pressure Conversions | |
| 14.7 PSI = 1 atmosphere | 100 kPa = 1 bar |
| 1 PSI = 6.89 kPa | 1” Hg = 25.4 mm Hg |
| 1 atmosphere = 101.3 kPa | 1” Hg = 14”H20 |
| 1 atmosphere = 1.013 bar | 0: Hg = 1 atmosphere |
