Converting between Units of Pressure Gases are different from solids and liquids. In a sample of gas, the molecules are far apart. The gas molecules also move around and collide with each other as well as with the walls of the container. These collisions generate pressure. The pressure of a gas can be measured in different units. One convenient unit of measure is called the atmosphere (atm) because it is based on atmospheric pressure. At sea level, the average pressure is 1 atm. As you get higher in altitude, the pressure steadily drops until you leave the atmosphere, where the pressure is very close to 0 atm. The table below shows the different commonly used units of measuring gas pressure. Use this table in the pressure unit conversions. Unit Abbreviation 1 atm equivalent atmosphere atm 1.00 atm (exact) millimeters of mercury mmHg 760 mmHg torr torr 760 torr inches of mercury in.Hg 29.9 in.Hg pounds per square inch (psi) lb/in.2 14.7 lb/in.2 pascal Pa 101,325 Pa The pressure in car tires is often measured in pounds per square inch (lb/in.2), with the recommended pressure being in the range of 25 to 45 lb/in.2. Suppose a tire has a pressure of 31.0lb/in.2 . Convert 31.0lb/in.2 to its equivalent in atmospheres. Express the pressure numerically in atmospheres.