Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Thu, 30 May 2002 13:12:40 -0400
Message-ID: <redirect-1251912@logan.com>
From: Brent Regan <Brent@regandesigns.com>
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Subject: Free standby vacuum
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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