"Stormin Mormon" wrote in message
...
You need a micron gage, not an inch gage. To tell if you've got a
good
vacuum.

Christopher A. Young

A 'full' vacuum reading on an inch scale would be the same as the
barometer reading, assuming both gauges are accurate, which is very
unlikely outside a lab.

My barometer shows 30.56" right now so 28-29" of vacuum wouldn't be
very good.

The inch and micron numbers are heights of a column of mercury
balanced against gas pressure, relative to either normal air pressure
(29" lower) or a perfect vacuum (400 microns higher) depending on how
the gauge is made. 400 microns is 0.01".

jsw