Vacuum Gyro Question

This is a fantastic question. I sure can't think of any good reason.

"Jim Carriere" wrote in message
...
This is one of those questions where there must be a simple answer. This
is probably the forum to get the best explanation.

Why aren't vacuum driven gyro instruments driven by pressure instead? The
reason I'm wondering is because about 2" of suction is sufficient to power
a turn needle, and while attitude gyros require more, about 5", either of
those amounts is still much less than standard atmospheric pressure,
meaning that the absolute pressure inside the instrument is still close to
ambient pressure.

Note other than seeing one on an instrument panel, I've never actually
taken apart or looked at the inside of any of these instruments.

In other words, what am I missing and what don't I understand? Thanks!