Originally Posted by
BOAC
Holds are pressurised but I am never convinced they will be as airtight as the cabin, so diff pressure will be at least the same if not higher.
Holds are absolutely the same pressure, there are quite significant vents between cabin (above) and hold (below) to ensure this.
The loss of the THY DC-10 near Paris in 1973 was caused by this. On climbout an underfloor cargo hatch blew out due to incorrect latching, and the hold depressurised immediately. Because there was an inadequate air path from the cabin above (poor design) the pressure in the cabin distorted and ruptured the floor itself, which is of course not designed to handle any pressurisation loads but was acting as a barrier. Control runs through the floor to the elevators were distorted and the aircraft became uncontrollable. It was a poor affair because it was not the first time this had happened on the DC-10, an American Airlines one had it happen just the same and was saved by skillful handling, and the lesson should have been immediately learned.
Originally Posted by BelArgUSA
Remember also the old "fountain pens" with ink, that were leaking...
Ah, you and me both
I presume most FOs nowadays don't even know what a fountain pen is, let alone have had them leak all over the place at altitude !
Originally Posted by VivaTheBeaver
what would the pressure be in the bladder when the aircraft has been pressurised and would this change at different altitudes
You may not realise it but this is fundamentally how altimeters work. There is a sealed unit with a flexible section (usually thin metal) and this flexes in and out dependent on the outside air pressure force. A geared mechanism resting on this drives the altimeter display. It is surprising the accuracy you get. Aircraft altimeters are set to today's initial ground pressure, which varies with the weather, before you start, known (in the US) as Altimeter Setting (which is understandable) and everywhere else in the world as QNH (whose origin you will have to look up).
Barometers at home work in exactly the same way.
On a pressurised aircraft there are two altimeters, one for the outside air pressure to show wat altitude you are at, and one for the inside cabin pressure to show what the pressurisation is doing.
will the temperature also make a difference?
It does, to a very marginal extent. It is taken into account in some aircraft performance calculations but can be ignored for your situation here.
would there be a difference in pressurisation between cargo aircraft or passenger aircraft
Well no, because the crew still have to breathe ! However I have heard that crew of aircraft conveying livestock, which has a tendency to go wild from time to time, will go onto oxygen and depressurise just a bit more than normal, and the animals just calm down and go to sleep.