They _are_ being driven by air pressure. The air is sucked in from the cabin
and through an air filter by the vacuum generated by the pump (or venturi).
The filter is inside the cabin. This setup comes from the fact that when
faced with the choice of air from the outside (or from the engine
compartment), or air sucked in from the cabin using the vacuum generated by
a venturi or pump, the latter is considered more suitable to contribute to a
longer instrument working life.

"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!