Absolute Pressure is measured relative to a full vacuum (in the empty, air-free space), and uses absolute zero as its reference pressure. Pressure that is measured against Atmospheric Pressure (also known as Barometric Pressure) is called Gauge Pressure.
According to two picture,we can find the Formula for the Absolute Pressure:
Absolute Pressure = Atmospheric Pressure + Gauge Pressure
Atmospheric (Barometric) Pressure Sensor
The correct reading for the Atmospheric Pressure Sensor is between 50 kPa (7.3 psi) and 100 kPa (15 psi).
The engine may be derated due to high elevation. Use Cat ET to check for an active Engine Derate on the Status Flags screen.
Note: There are no event codes that are associated with the Altitude Derate.