Carl:

You are absolutely right. The pump inlet will see cabin pressure NOT outside pressure. My brain must not have been up to operating temperature...

What I should have said was that at a CABIN altitude of 12,000 the pump will only  be able to make 2/3 the pressure it makes at sea level so the switch pressure should be set to the lesser of manufacturers maximum pressure for the seal OR 1/2 of the pumps maximum pressure at sea level. 15 to 20 PSI seems like the right range.

Regards
Brent Regan

Carl Cadwell wrote:

Brent,

I understand from the factory that the pump is to be set for 20#. 
As I recall it will pump a maximum of about 30#.  Setting this for ¼ of the sea level pressure is 7-8#.

 

I am also not clear what this has to do with the FL280 being 1/3 of sea level pressure as the pump is on the inside of the pressure vessel.

 

Am I missing something?