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On Sun, 10 Feb 2002, Herbert H Gottlieb wrote:
... why does Macy's fill their Thanksgiving Day parade
balloons with helium under a pressure that is greater than the
atmospheric pressure?
In order to give them some rigidity. Fortunately it only takes a
tiny bit of overpressure ("gauge pressure") to do that *secondary*
job. As a result the helium is not appreciably more dense than it
is at atmospheric pressure and it still easily manages to do its
*primary* job which is to displace air without weighing very much.
John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm