Inches of water
Inches of water (inH₂O) is a pressure unit from the Imperial system. An inch of water is defined as the pressure from a one inch column of water at 4°C (39.2°F) (which is the temperature at which water is densest).
pound force per square inch
Pound force per square inch (psi) is the pressure from a force of one pound-force on an area of one square inch, which is about 6,894.76 pascals. Car tires in the United States are typically inflated to 30–35 psi.