Hi Bill

Bill I'll try to make things a little simpler, that works just fine for
everyday approximation, like a circle is about 3/4 the area of a
square, some will cringe but yes thats good enough for what we need in
most cases.

Also with vacuum, take all air away and you would have 15 # pressure
per inch square

The 15 # is the same as 30" mercury, or 75 cm mercury and that is also
1 bar.

To get 1# of pressure per square inch you'd need 2" mercury or 5 cm of
mercury.

So now you have a let's say 10" Diameter bowl, that would be 10x10=100,
and we take 3/4 of that for the area of a circle and we have 75 square
inches.

Now if you had a vacuum of just 4" or 10 cm of mercury and no leakage
you'd have 150 pounds holding your bowl down, that should be lots,
but.............

Next you have a 6"D bowl, that would be 6x6=36, and 3/4 of that is 27"
square with no leakage and 4" or 10 cm of mercury vacuum and 54#
holding it down should also be enough to clean up the bottom

Now we take a 3"D bowl and 3x3=9, 3/4 of that gives us 6.75" square,
no leakage, and 4" mercury vacuum 10 cm or 100 mm same as before, and
now we'd have only 13# holding it down, I would suggest that's not
enough.

If you now increase the vacuum to let's say 20" of mercury, you'd have
10# of pressure per square inch, and on that 3"D bowl the hold down
would be 67.5# and that would be lots IMO.

So do you need all that high vacuum ??, I would say no, but you do need
enough volume though, as there is always leakage and if you don't have
the volume you'd never get any vacuum.

A good shopvac will give you enough vacuum for a larger bowl, however
you better keep in mind that the vacuum motor needs air to flow by to
cool it, and if you have very little air going past, like with using it
as a vacuum holding pump your shopvac's motor will burn out, period.

Hope this helps you and anybody else that has some questions about all
of this.

Have fun and take care
Leo Van Der Loo



Bill in Detroit wrote:
How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

with inches of mercury so as to make a comparison between various makes
of vacuum pumps. I'm not in the market today but I'm reasonably
confident that I eventually will be ... so I'm trying to understand the
lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

Bill


--
When the rich wage war it's the poor who die.
Jean-Paul Sartre, The Devil and the Good Lord (1951) act 1


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