Standard atmospheric pressure is a bit over 14 PSI so that is as much
pressure as a vacuum can produce. The first proven vacuum was produced
by Evalgelista Torricelli, Galileo's assistant, by filling a glass tube
with mercury and turning it upside down. The mercury dropped until the
weight of the mercury in the tube equalled the pressure of the air
outside. That was about 30". Torricelli almost got in a lot of trouble
about this because the Pope had decreed that there was no such thing as
a vacuum so he sent the idea to a Frenchman named Pascal to work on. For
many years we have happily used the height of the mercury to describe a
vacuum. We and the English used inches and everyone else used
milimeters. Then the French and Itallians conspired to change
everything at an international standards conference and we ended up with
Torr as the official measure. Being Itallian the unit had to be totally
irrational so a Torr is 1/760th of an atmosphere.

To add to the confusion the "Bar", (short for barometer) was used to
describe one standard unit of atmospheric pressure so vacuums are
measured in fractions of a bar called millibars. Also the French did
not like for an Itallian to get any credit so in an evil pact with the
British, they came up with the Pascal to describe one atmosphere of
pressure. To get the British to go along the Pascal was based on a unit
named after an Englishman, the Newton, but to keep the English in their
proper place they made the Pascal equal to a million Newtons.

The thing is, the measurement of "standard Atomspheric pressure" that
they used to set the actual values was off a bit so all of these units
are not quite accurate. That is why I still use inches of Mercury. :-)


Ron Thornton wrote:

Hugh,

How about laughing to yourself next time and explain to the guys that
didn't go to engineering school why it's wrong. No need to be rude.

Regards, Ron


--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
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