"I have just finished a semi-repair on my tiny mech pump. I have measured a vacuum of -993.2 mm/Hg with an electronic portable multimeter..."
Daniele,
I don't understand the "-993.2 mmHg" measurement. If your reading is a negative number that would imply to me that your gauge is referenced to atmospheric pressure (how many mmHg below the prevailing ambient pressure).
Atmospheric pressure at sea level is 760 mmHg give or take depending upon meteorological conditions. Given that you shouldn't be able to get an atm. referenced reading significanty exceeding -760 mmHg.
Are you sure that the reading isn't in mbar? 1000 mbar is about 750 mmHg.
In either case, an atmospheric pressure referenced reading won't tell you much about the pump. What you need is an absolute pressure measurement (i.e. measured relative to a perfect vacuum or 0 mmHg). You are trying to determine the performance of the pump (which should be in a fraction of a mmHg) using a wavering reference that can be plus or minus many mmHg.
Hope this helps.
Steve
Created on Thursday, March 22, 2001 8:20 PM EDT by Steve Hansen