L-1011 ECS: Cabin Altitude And Differential Pressure Indicator (21-23)

The main function of the cabin altitude and differential pressure indicator is to monitor the cabin altitude and cabin differential pressure throughout the flight.

The indicator has two scales (or two instruments in case of a TWA or Eastern panel configuration), the outer scale and larger pointer represents cabin altitude in thousands of feet. The barometric knob and window are used to set the correct barometric pressure in inches of mercury to correct for non-standard day conditions. Only this gauge is affected by the setting.

The inner scale and small pointer indicate cabin differential pressure. The amber range represents the maximum differential pressure controlled by the outflow valves. The red range represents the maximum  differential pressure which is limited by the positive pressure safety valves. Note that the maximum differential pressure for takeoff and landing is 0.15 psi and is placarded above the indicator. A pressure lower than 0.15 psi will allow all exits to be able to open in case of an emergency evacuation.