Gauge pressures are 14.7 psi higher the Absolute pressures. This can make a big difference in sizing the control valve at low pressures. For Absolute pressures, 0 equals a perfect vacuum. For Gauge pressures, 0 equals atmospheric pressure. If you state the pressure is 5 psi we do not know if you are working at a vacuum (5 psiA) or slightly above atmosphere (5 psiG). Each condition requires a different size control valve.