Pressure is exerted upon a surface due to the random molecular motion and the momentum exchange when the molecules bounce on the surface. In the limit of zero molecular motion of the molecules at absolute temperature of 0 K or perfect vacuum, there would be zero pressure. Some of the commonly used units of pressures are Pa (= N/m2), psi (= lbf/in.2), bar (1 bar = 100 kPa), and atm (1 atm = 101.325 kPa). A distinction is made in pressure measurements whether the pressure is measured relative to the atmospheric or zero pressure (vacuum). The pressure measured with respect to the atmospheric pressure is called the gauge pressure, and therefore the gauge pressure (pg) is equal to the absolute pressure (pabs) minus the atmospheric pressure (po).