Passenger cabin pressurization

Date:         08 Sep 97 02:03:53 
From:         jf mezei <"[non-spam]jfmezei">
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I have seen mentioned often that modern airliners maintain a cabin
pressure of 8000feet when flying at cruise altitude of 30-40k feet.

Is this an urban myth or actual precise figure that applies to all
airliners ?

Also, since most flights (even 1 hour hops)  on jets tend to go to 30k
feet and above, is it fair to assume that on just about all jet flights,
the cabin pressure will go down to the 8000ft limit ?

Or is the cabin pressure really determined by a pressure differential
maximum which would mean, for axample, that Airplane X, able to maintain
an 8000ft cabin pressure while flying at 40k feet, would maintain
a higher cabin pressure (lower alttude) if it were to fly at only at 30k
feet ?

Reason for question: for scuba diving, the actual cabin pressure
experienced by passengers can make a big difference for Decompression
Sickness following dives.