Vacuum pumps, circuit breakers, and windows

[E_Dawg]

Kind of off the wall, but here goes:

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-What exactly does the suction guage measure?

It can't be the actual suction a la a manifold pressure guage because lower values would mean more suction, not less. When you say 'the vacuum's low' that means it's something less than 4.4" Hg, but that would actually mean more suction right? My guess: it measures the difference in pressure from ambient to the pressure in the vacuum. 4.4" Hg on the guage at sea level would = a pressure of 25.52" Hg in the pump. Right?

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-This is from Jepp's 'Aircraft Systems for Pilots': "Aircraft circuit breakers are of the trip-free type which means that they will open the circuit irrespective of the position of the operating control. With this type of breaker, it is impossible to hold the circuit closed, if an actual fault exists."

If this is true, why's everyone tell you to never hold the breakers in?

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This one's just out of curiosity:
-If transport aircraft fly up where the OAT is -something *C, why doesn't condensation constantly build on the inside panel of the windows?

 

[sixpack]

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-What exactly does the suction guage measure?

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The vacuum guage measures the pressure differential between the cabin (usually) and the internal lines of the vacuum system. For example, if you're on the ground on a standard day at sea level, the cabin will be about 29.92. The vacuum system be at typically 24.92. The vacuum guage will indicate the difference. (29.92 - 24.92 = 5.0).
Note, the system tries to hold 5.0 (not 24.92).

If you plane really sucks, the vacuum system might be read something like 5.5, meaning more vacuum and less absolute pressure in the vacuum system.

[ QUOTE ]
-This is from Jepp's 'Aircraft Systems for Pilots': "Aircraft circuit breakers are of the trip-free type which means that they will open the circuit irrespective of the position of the operating control. With this type of breaker, it is impossible to hold the circuit closed, if an actual fault exists."

If this is true, why's everyone tell you to never hold the breakers in?

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Just a guess, but perhaps pushing and holding the breaker in, will allow the current draw to remain at the trip point (or continually resetting). Although your wiring would probably be protected from fire, the faulty device might still be drawing high currents and be a fire hazard.