Boat Forum / Building / October 2003

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.  :-)

let's see.  vacuum is an absence of atmosphere, or a presence of nothing but
room for there to be something.  the earth's atmosphere applies a pressure
of approx 14.5 psi at sea level, with the pressure varying a little
depending on the weather.   thus, an absolute vacuum has approx 14.5psi less
pressure than one feels at the surface of the earth.  a vacuum bag with zero
pressure inside is 14.5psi lower than outside.  thus it is impossible - in
earth terms at least - to talk of 25psi of vacuum.  if one lived on jupiter,
one could talk about 25psi of vacuum and if the guy with the claim lives on
jupiter and is making composites there then i'll stop laughing and be
amazed.

as for laughing, the reason the engineering school profs got so upset with
this mistake is that they regarded it as something fundamental that should
have been learned at high school.  so do i.  it's kinda like wandering
around the world without knowing that light travels faster than sound or
that shakespeare was english.  you know - basic stuff.

hugh

p.s. and i wasn't rude.  i was laughing.