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A large vacuum pump which uses a 25 horsepower (about 20 kiloWatt) motor?!!  I hope that anyone designing an exhibit using such a powerful vacuum pump to evacuate a sizable chamber is cognizant of the hazards in dealing with evacuating large volumes.  Most school demonstration systems use glass bell jars which have a rather small volume (a few liters or less) and strong, thick-walled glass bell jars whose shape as well as thickness enable them to withstand large inward force.  But when the air inside is evacuated the atmospheric pressure pushing inward on the glass walls is close to one ton per square foot (or about 10 metric tonnes per square meter), arrived at by multiplying the familiar 14.7 pounds per square inch by 144 square inches in 1 square foot.   As a familiar example consider a television picture tube, which has a near-perfect vacuum inside, and the size of the face is typically two or three square feet.  The inward force exerted by atmospheric pressure on the face of the tube is therefore two or three tons, and the glass must be thick enough to have the strength to withstand this much inward push.  Should the glass break, this much force acting on the fragments will accelerate them to great speeds, and could cause disastrous damage to anything, living or inanimate, which happens to be nearby.

When I was an undergraduate my physics professor told about  an episode he experienced when he was working in a commercial research laboratory in which a vacuum system included a large glass bottle, probably about five or ten gallon.  The bottle failed (fortunately at night when the laboratory was unoccupied) and peppered the concrete block walls with shards of glass which imbedded themselves into the concrete.  One can imagine the injuries or worse which would be inflicted on any person who happened to be in the lab when the bottle imploded.  

Albert J. Read, Director, Science Discovery Center of Oneonta (NY)





-----Original Message-----
From:   Informal Science Education Network on behalf of Mike Levad
Sent:   Mon 2/2/2004 9:46 AM
To:     [log in to unmask]
Cc:	
Subject:        Re: What To Do With A Big Vacuum Pump? An Exhibit?

ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related institutions.
*****************************************************************************

I would set up a chamber with a miniature kite inside.  You can could
attach the kitestring to the end of an arm that is spun by a motor.
When you remove the air no matter how fast you turn the arm the kite
will fall below the arm.  As soon as you let the air back in the kite
will fly inside the chamber above the arm.  The kite itself can be as
small as 1".  If you need more kite information let me know.

Mike

--
Mike Levad
Exhibit/Program Developer
The Franklin Institute Science Museum
222 N. 20th St. Philadelphia, PA 19103
Voice:215.448.1309
Fax:215.448.1188
E-mail: [log in to unmask]

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