"khobar" wrote in message
news:c4QIb.16727$7D3.2605@fed1read02...
Ralph Nesbitt wrote in message
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"Robert Moore" wrote in message
...
"Ralph Nesbitt" wrote

Hopefully all realize when referring to "Air Pressure" at altitude
this
is
an "absolute" pressure value inside the fuselage irrespective of
ambient.
When referring to air pressures at ground level the pressure reading
is
above unadjusted ambient barometric pressure.

In the Boeing aircraft that I flew (B-727,B-707,B-720) there were two
gages on the FE's panel. One was a simple altimeter that indicated
the
cabin altitude at all times and the other, a differential pressure
gage
that indicated the difference in pressure between outside and inside.
The maximum differential for those aircraft was around 8.6 psi. The
only
way to determine the absolute pressure inside the aircraft would be to
use
a graph to convert the altimeter indication to pressure.

Bob Moore

The protocols you reiterate above will translate to a ~ constant
pressure
of
12 PSI Gauge inside the cabin as the A/C moves through it's flight
profile
above ~ 11,000'.
Ralph Nesbitt
Professional FD/CFR/ARFF Type

http://books.nap.edu/books/030908289...6.html#pagetop

http://print.nap.edu/pdf/0309082897/pdf_image/36.pdf


Paul Nixon

Thanks for detailed "Technical Spec's" for A/C pressurization.


My number of ~12 PSI Gauge was based on rule of thumb used by FD/CFR/ARFF
for off the cuff calculation of A/C pressurization & apparatus pump flow
rates; re 75% of 15.8 lbs (11.85 to 12) or 31.6 inches of mercury barometric
pressure rounded to closest number with 100% humidity at sea level. In
winter 75% of 14.4 lbs (10.8 to 11) or 28.7 inches of mercury barometric
pressure rounded to closest number with 100% humidity. These values are
"Rule of Thumb" ranges "Normally Encountered". Unusual atmospheric
conditions can cause higher or lower Barometric pressure values.

To determine exact PSIG it is necessary to use "unadjusted barometric
pressure values at any given location".

The variance in range of barometric pressures encountered is why
pressurization systems are based on "Relative Values" instead of absolute
values".
Ralph Nesbitt
Professional FD/CFR/ARFF Type