Pressue differential

[ranklein]

I'm trying to go over some questions about pressure diferential:
Crusing at 300MB level, cabin altitude is 10000ft, what's the differential pressure in psi?
I have a note here that the cab.alt is 693mb, but I'm not sure if that's part of the answer, or I have to have this number? Any suggestions?

Cheers

[Three Blades]

Cabin pressure will be very roughly 1000mb - 330mb = 660mb (or 693mb more accurately).
So if you are at 300mb equivalent altitude then the differential is 693-300 = 393.

I think !

[ranklein]

So what's the pressure differential in psi?

[Three Blades]

psi = mb x 0.0145
so 5.7 psi

[ranklein]

o.k, that's the right answer!
now, why have u subtracted 330mb from 1000mb?

[Three Blades]

For a very rough answer only:
1) Sea level is 1000mb
2) Pressure falls by 1mb for every 30'
So for 10k' the pressure will be 330 (to 2 sig figs) less than 1000 = 670

[ranklein]

o.k, 670mb, but I guess a known number of 693mb for 10K is the number to work with, hence 693-300=393 * 0.0145= 5.7psi

that's exactly what I was looking for!!


Thanks for the speedy help!

[barit1]

This is a classic units conversion problem that we face every day. It would be so much neater if we used only one pressure standard...

:zzz: (dreaming yet again)

[john_tullamarine]

barit1, my friend ....

you and I had to sweat through getting on top of units conversions ... ought we not encourage our younger brethren to go through the same nonsense ... ?