Regarding a free standby vacuum source I designed for a pressurized IV I wrote:
<<I configured it with a vacuum regulator, filter and a small
sonic venturi so that the system used filtered cabin air and discharged
into the engine compartment. The advantage to this configuration was
that when the vacuum pump failed (not IF) the cabin pressure would run
the system. Free standby vacuum, works great.>>

Gary asks:

<<
Could you elaborate?  I assume this is a "standby" system with a second regulator.  Where is the regulator mounted? Outside the cabin or inside?  I initially wanted to mount my regulator and filter inside the cabin on my main system, thinking the inside pressure would "help" the pump, but was told all that would do was make the pump work harder, and everything has to be outside the cabin.  I know I'm missing some concept here.  Can you help?  Thanks.>>

Gary, the trick to making the system work is the sonic venturi. Typically, instruments need about a 5" H2O (0.18 PSI) differential pressure. This is 1/25 of the available pressure across the firewall. The sonic venturi limits the flow through the system so as not to overwhelm the vacuum pump. Inside the cabin is the filter, instruments and vacuum regulator. The sonic venturi is in the line that penetrates the firewall (I machined it into the penetrating fitting). Once on the engine side the system doesn't care if the vacuum pump is sucking or the cabin pressure is blowing. It is assumed that the vacuum pump will fail "open".

The sonic venturi must be sized to meet the flow requirements of all the instruments. A typical gyro will require 1.5 SCFM. Calculating venturi throat diameter I got 0.084" for MSL and 0.101" for FL 290. Adding a safety margin of 50% and you get a throat diameter of 0.125" (for one instrument).

Without the flow limiting effect of the sonic venturi the vacuum pump would be overwhelmed by the flow from the cabin and likely fail in short order.

Regards
Brent Regan