See below. The “Diff Press PSI” gauge…that is not a measurement of the pressure altitute difference between outside and the cabin, correct?
If it were, the gauge would show a number closer to 10,000. Since the cabin altitude in this picture is near zero, and yet the altitute of the aircraft is 10,100
It’s literally in the description of the gauge you observed - the outer scale is in PSI differential and the digits you read are not multiplied, the differential is literally read straight off the digits on the scale.
How it works:
Standard atmosphere at your cabin pressure altitude of zero (sea level) is 14.7 PSI.
Standard atmosphere at your (outside) pressure altitude of 10,000’ (the 100’ is negligible) is 10.1 PSI.
Inside the plane is 14.7, outside is 10.1 and the higher pressure inside will want to move/expand toward the lower pressure outside.
To get the differential, we simply find the difference (subtract) between the two: 14.7-10.1 is 4.6, and is about what your gauge is showing, which is within limits. Get into the red zone of the gauge and there’s too much pressure inside relative to outside, which could theoretically burst the pressure vessel, if not for operable relief valves, even if the pressurization systems or main outflow valve fails.
