Pressure problems...

[Engineer_aus]

Trying to work out what the Diff P would be for a aircraft when it is pressurised at -300ft ie The aircraft is pressurised before takeoff and landing, and I seriously can not find the PSI figure in any of the aircraft manuals that we have at work. I spent over 2 hours trying to look for something. Anyone have any mathematical idea?

for example the aircraft is flying at 20,000ft and the pressure is 7psi in the cabin.

[SNS3Guppy]

http://www.sensorsone.co.uk/altitude-pressure-units-conversion.html

Your first question appears to involve an aircraft sitting on the ground, pressurized, with a cabin pressure of -300'. If this is the case, and you're asking what the differential pressure is, then you'll need to know what the outside pressure is.

If you are to assume a standard atmosphere outside the aircraft and the internal cabin pressure altimeter referenced to 29.92" (or 1013 mb, as your pleasure takes), then the differential pressure is .16 psid, reference the chart cited above for standard atmosphere.

With a cabin pressure of 7 psi, you're holding a cabin of approximately 17,500', again reference the chart cited above. This, regardless of the actual aircraft altitude. Your differential pressure at is approximately .7 psid.

If instead your cabin differential is 7 psid, then your cabin at 20,000' is going to be approximately 1,800'.

Someone correct those figures if you will; I'm listening to four yelling kids and doing it in my head....far from exact.

[BOAC]

I recall the 737 pressurises at 0.125psi on the ground.

[Engineer_aus]

It doesn't seem to be much at all (PSID on the ground), but thanks for the link. Will be doing some maths tonight at work.

[enicalyth]

Try online calculators? Do you really want the maths behind the ISA figures?

http://www.onlineconversion.com/pressure.htm
tells us:
30 inch of mercury = 1 015.92 hectopascal
1 000 hectopascal = 29.53 inch of mercury
30 inch of mercury = 14.73 pound/square inch
1 000 hectopascal = 14.50 pound/square inch

http://www.digitaldutch.com/atmoscalc
tells us:
minus 300ft ISA conditions = 1024.28 hectopascal
= 30.25 inch of mercury
=14.86 pound/square inch

So if the a/c is on the ground and the barometer is 30inches the differential pressure is 0.15 lb/square inch.

Now if the a/c is at 20000ft ISA, digitaldutch gives atmos pressure as 6.75lb/sqare inch but you tell us it is 7 pound/square inch in the cabin. So the differential pressure is 0.25pound/square inch.

If you wish to know more about density altitude rather than pressure altitude try
http://wahiduddin.net/calc and the discussions he includes. From memory he supplies all the underlying maths for pressure and density altitudes and is a fun guy to boot.

I hope this helps

Best Rgds

The "E"

[MarkerInbound]

I thought the limit for pressurization on the ground was .125 so that you could open the doors during an evac. Maybe aircraft specific. We set 200 below to get it.

[SNS3Guppy]

We do zero differential on the ground, and for takeoff or landing. Most places I've always set field elevation plus five hundred or a thousand, but here the FE sets field elevation.