Absolute Pressure: Pressure that is measured relative to (referenced to) absolute zero pressure. Gauge Pressure: Pressure that is measured relative to (referenced to) atmospheric pressure. Barometric Pressure: An absolute pressure that measures atmospheric pressure. Vacuum Pressure: A gauge pressure that is expressed as a positive value below atmospheric pressure. Example: 29 inches of mercury vacuum. Bidirectional Pressure: A gauge pressure with a span below and above atmospheric pressure. Example: -15 to +15 psi, -10 to 100 psi etc. Sealed Gauge Pressure: A pressure that is referenced to a fixed sealed reference, usually standard atmospheric pressure. Differential Pressure: Pressure that represents the difference between two arbitrary pressures.